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<a id="X7D5919C182B1A462" name="X7D5919C182B1A462"></a>
<div class="ChapSects"><a href="chap4_mj.html#X7D5919C182B1A462">4 GAP Computations Concerning Hamiltonian Cycles in the Generating Graphs of Finite Groups</a>
<div class="ContSect"> <a href="chap4_mj.html#X8389AD927B74BA4A">4.1 Overview</a>

</div>
<div class="ContSect"> <a href="chap4_mj.html#X7B6AEBDF7B857E2E">4.2 Theoretical Background</a>

<div class="ContSSBlock">
 <a href="chap4_mj.html#X7AD3962D7AE4ADFB">4.2-1 Character-Theoretic Lower Bounds for Vertex Degrees</a>

 <a href="chap4_mj.html#X825776BA8687E475">4.2-2 Checking the Criteria</a>

</div></div>
<div class="ContSect"> <a href="chap4_mj.html#X7B56BE5384BAD54E">4.3 GAP Functions for the Computations</a>

<div class="ContSSBlock">
 <a href="chap4_mj.html#X802B2ED2802334B0">4.3-1 Computing Vertex Degrees from the Group</a>

 <a href="chap4_mj.html#X87FE2DDD7F086D2F">4.3-2 Computing Lower Bounds for Vertex Degrees</a>

 <a href="chap4_mj.html#X8677A8B1788ACD2C">4.3-3 Evaluating the (Lower Bounds for the) Vertex Degrees</a>

</div></div>
<div class="ContSect"> <a href="chap4_mj.html#X7A221012861440E2">4.4 Character-Theoretic Computations</a>

<div class="ContSSBlock">
 <a href="chap4_mj.html#X86CE51E180A3D4ED">4.4-1 Sporadic Simple Groups</a>

 <a href="chap4_mj.html#X867D338F7F453092">4.4-2 The Monster</a>

 <a href="chap4_mj.html#X7DC6DFCC83502CC3">4.4-3 Nonsimple Automorphism Groups of Sporadic Simple Groups</a>

 <a href="chap4_mj.html#X8130C9CB7A33140F">4.4-4 Alternating and Symmetric Groups $A_n$, $S_n$,
for $5 \leq n \leq 13$</a>

</div></div>
<div class="ContSect"> <a href="chap4_mj.html#X83DACCF07EF62FAE">4.5 Computations With Groups</a>

<div class="ContSSBlock">
 <a href="chap4_mj.html#X7B9ADC91802EE09F">4.5-1 Nonabelian Simple Groups of Order up to $10^7$</a>

 <a href="chap4_mj.html#X8033892B7FD6E62B">4.5-2 Nonsimple Groups with Nonsolvable Socle of Order at most $10^6$</a>

</div></div>
<div class="ContSect"> <a href="chap4_mj.html#X84E62545802FAB30">4.6 The Groups $PSL(2,q)$</a>

</div>
</div>

<h3>4 GAP Computations Concerning Hamiltonian Cycles in the Generating Graphs of Finite Groups</h3>

Date: April 24th, 2012

This is a collection of examples showing how the GAP system <a href="chapBib_mj.html#biBGAP">[GAP24]</a> can be used to compute information about the generating graphs of finite groups. It includes all examples that were needed for the computational results in <a href="chapBib_mj.html#biBGMN">[BGL+10]</a>.

The purpose of this writeup is twofold. On the one hand, the computations are documented this way. On the other hand, the GAP code shown for the examples can be used as test input for automatic checking of the data and the functions used.

A first version of this document, which was based on GAP 4.4.12, is available in the arXiv at <a href="http://arxiv.org/abs/0911.5589v1">http://arxiv.org/abs/0911.5589v1</a> since November 2009. The differences between this file and the current document are as follows.

<ul>
<li>The format of the GAP output was adjusted to the changed behaviour of GAP 4.5.

</li>
<li>The records returned by <code class="func">IsomorphismTypeInfoFiniteSimpleGroup</code> (<a href="../../../doc/ref/chap39_mj.html#X7C6AA6897C4409AC">Reference: IsomorphismTypeInfoFiniteSimpleGroup</a>) contain a component <code class="code">"shortname"</code> since GAP 4.11.

</li>
<li>The lower bounds computed for the sporadic simple Monster group have been improved in three steps.

First, the existence of exactly one class of maximal subgroups of the type $PSL(2, 41)$ (see <a href="chapBib_mj.html#biBNW12">[NW13]</a>) and the nonexistence of maximal subgroups with socle $PSL(2, 27)$ (see <a href="chapBib_mj.html#biBWil10">[Wil10]</a>) have been incorporated.

Second, the classification of classes of maximal subgroups of the Monster has been completed in <a href="chapBib_mj.html#biBDLP25">[DLP25]</a>. As a consequence, the nonexistence of maximal subgroups with socle Sz$(8)$ and $PSU(3, 8)$ and the existence of exactly one class of maximal subgroups with the isomorphism types $PSL(2, 13).2$ and $PSU(3, 4).4$ have been proved, and the proof of the nonexistence of the previously claimed subgroups of the type $L_2(59)$ yields that the Monster has maximal subgroups of the type $59:29$. Moreover, meanwhile also all class fusions of the maximal subgroups are known; previously, we got only candidates for some primitive permutation characters.

Third, all character tables of maximal subgroups of the Monster group and their class fusions are available since version 1.3.10 of CTblLib, hence no special treatment for the Monster group is needed anymore in order to compute its primitive permutation characters. In particular, the data file <code class="file">data/prim_perm_M.json</code> of CTblLib is no longer needed. In earlier versions, that file had been used to collect information about primitive permutation characters for which the character table was not yet available.

See Section <a href="chap4_mj.html#X867D338F7F453092">4.4-2</a> for comments on earlier versions, and for the bounds that had been computed from the partial information that was available at that time.

</li>
</ul>
<a id="X8389AD927B74BA4A" name="X8389AD927B74BA4A"></a>

<h4>4.1 Overview</h4>

The purpose of this note is to document the GAP computations that were carried out in order to obtain the computational results in <a href="chapBib_mj.html#biBGMN">[BGL+10]</a>.

In order to keep this note self-contained, we first describe the theory needed, in Section <a href="chap4_mj.html#X7B6AEBDF7B857E2E">4.2</a>. The translation of the relevant formulae into GAP functions can be found in Section <a href="chap4_mj.html#X7B56BE5384BAD54E">4.3</a>. Then Section <a href="chap4_mj.html#X7A221012861440E2">4.4</a> describes the computations that only require (ordinary) character tables in the GAP Character Table Library <a href="chapBib_mj.html#biBCTblLib">[Bre25]</a>. Computations using also the groups are shown in Section <a href="chap4_mj.html#X83DACCF07EF62FAE">4.5</a>.

The examples use the GAP Character Table Library and the GAP Library of Tables of Marks, so we first load these packages in the required versions.

<div class="example"><pre>
gap> if not CompareVersionNumbers( GAPInfo.Version, "4.5" ) then
> Error( "need GAP in version at least 4.5" );
> fi;
gap> LoadPackage( "ctbllib", "1.2", false );
true
gap> LoadPackage( "tomlib", "1.1.1", false );
true
</pre></div>

<a id="X7B6AEBDF7B857E2E" name="X7B6AEBDF7B857E2E"></a>

<h4>4.2 Theoretical Background</h4>

Let $G$ be a finite noncyclic group and denote by $G^{\times}$ the set of nonidentity elements in $G$. We define the generating graph $\Gamma(G)$ as the undirected graph on the vertex set $G^{\times}$ by joining two elements $x, y \in G^{\times}$ by an edge if and only if $\langle x, y \rangle = G$ holds. For $x \in G^{\times}$, the vertex degree $d(\Gamma, x)$ is $|\{ y \in G^{\times}; \langle x, y \rangle = G \}|$. The closure $cl(\Gamma)$ of the graph $\Gamma$ with $m$ vertices is defined as the graph with the same vertex set as $\Gamma$, where the vertices $x, y$ are joined by an edge if they are joined by an edge in $\Gamma$ or if $d(\Gamma, x) + d(\Gamma, y) \geq m$. We denote iterated closures by $cl^{(i)}(\Gamma) = cl(cl^{(i-1)}(\Gamma))$, where $cl^{(0)}(\Gamma) = \Gamma$.

In the following, we will show that the generating graphs of the following groups contain a Hamiltonian cycle:

<ul>
<li>Nonabelian simple groups of orders at most $10^7$,

</li>
<li>groups $G$ containing a unique minimal normal subgroup $N$ such that $N$ has order at most $10^6$, $N$ is nonsolvable, and $G/N$ is cyclic,

</li>
<li>sporadic simple groups and their automorphism groups.

</li>
</ul>
Clearly the condition that $G/N$ is cyclic for all nontrivial normal subgroups $N$ of $G$ is necessary for $\Gamma(G)$ being connected, and <a href="chapBib_mj.html#biBGMN">[BGL+10, Conjecture 1.6]</a> states that this condition is also sufficient. By <a href="chapBib_mj.html#biBGMN">[BGL+10, Proposition 1.1]</a>, this conjecture is true for all solvable groups, and the second entry in the above list implies that this conjecture holds for all nonsolvable groups of order up to $10^6$.

The question whether a graph $\Gamma$ contains a Hamiltonian cycle (i. e., a closed path in $\Gamma$ that visits each vertex exactly once) can be answered using the following sufficient criteria (see <a href="chapBib_mj.html#biBGMN">[BGL+10]</a>). Let $d_1 \leq d_2 \leq \cdots \leq d_m$ be the vertex degrees in $\Gamma$.

<dl>
<dt>Pósa's criterion:</dt>
<dd>If $d_k \geq k+1$ holds for $1 \leq k < m/2$ then $\Gamma$ contains a Hamiltonian cycle.

</dd>
<dt>Chvátal's criterion:</dt>
<dd>If $d_k \geq k+1$ or $d_{m-k} \geq m-k$ holds for $1 \leq k < m/2$ then $\Gamma$ contains a Hamiltonian cycle.

</dd>
<dt>Closure criterion:</dt>
<dd>A graph contains a Hamiltonian cycle if and only if its closure contains a Hamiltonian cycle.

</dd>
</dl>
<a id="X7AD3962D7AE4ADFB" name="X7AD3962D7AE4ADFB"></a>

<h5>4.2-1 Character-Theoretic Lower Bounds for Vertex Degrees</h5>

Using character-theoretic methods similar to those used to obtain the results in <a href="chapBib_mj.html#biBBGK">[BGK08]</a> (the computations for that paper are shown in <a href="chapBib_mj.html#biBProbGenArxiv">[Breb]</a>), we can compute lower bounds for the vertex degrees in generating graphs, as follows.

Let $R$ be a set of representatives of conjugacy classes of nonidentity elements in $G$, fix $s \in G^{\times}$, let $��(G,s)$ denote the set of those maximal subgroups of $G$ that contain $s$, let $��(G,s)/\sim$ denote a set of representatives in $��(G,s)$ w. r. t. conjugacy in $G$. For a subgroup $M$ of $G$, the permutation character $1_M^G$ is defined by

\[
 1_M^G(g):= (|G| \cdot |g^G \cap M|) / (|M| \cdot |g^G|),
\]

where $g^G = \{ g^x; x \in G \}$, with $g^x = x^{-1} g x$, denotes the conjugacy class of $g$ in $G$. So we have $1_M^G(1) = |G|/|M|$ and thus $|g^G \cap M| = |g^G| \cdot 1_M^G(g) / 1_M^G(1)$.

Doubly counting the set $\{ (s^x, M^y); x, y \in G, s^x \in M^y \}$ yields $|M^G| \cdot |s^G \cap M| = |s^G| \cdot |\{ M^x; x \in G, s \in M^x \}|$ and thus $|\{ M^x; x \in G, s \in M^x \}| = |M^G| \cdot 1_M^G(s) / 1_M^G(1) \leq 1_M^G(s)$. (If $M$ is a maximal subgroup of $G$ then either $M$ is normal in $G$ or self-normalizing, and in the latter case the inequality is in fact an equality.)

Let $\Pi$ denote the multiset of primitive permutation characters of $G$, i. e., of the permutation characters $1_M^G$ where $M$ ranges over representatives of the conjugacy classes of maximal subgroups of $G$.

Define

\[
 \delta(s, g^G):= |g^G| \cdot \max\left\{ 0, 1 - \sum_{{\pi \in \Pi}}
 \pi(g) \cdot \pi(s) / \pi(1) \right\}
\]

and $d(s, g^G):= |\{ x \in g^G; \langle s, x \rangle = G \}|$, the contribution of the class $g^G$ to the vertex degree of $s$. Then we have $d(\Gamma(G), s) = \sum_{{x \in R}} d(s, x^G)$ and

<div class="pcenter"><table> <tr> <td class="tdright">d(s, g^G)</td> <td class="tdcenter">=</td> <td class="tdleft">|g^G| - |⋃_{M ∈ M(G,s)} { x ∈ g^G; ⟨ x, s ⟩ ⊆ M }|</td> </tr> <td class="tdright"> </td> <td class="tdcenter">≥</td> <td class="tdleft">|g^G| ⋅ max{ 0, 1 - Σ_{M ∈ M(G,s)} 1_M^G(g) / 1_M^G(1) }</td> <tr> <td class="tdright"> </td> <td class="tdcenter">=</td> <td class="tdcenter">|g^G| ⋅ max{ 0, 1 - Σ_{M ∈ M(G,s)} 1_M^G(g) / 1_M^G(1) }</td> </tr> <tr> <td class="tdright"> </td> <td class="tdcenter">≥</td> <td class="tdcenter">|g^G| ⋅ max{ 0, 1 - Σ_{M ∈ M(G,s)/∼} 1_M^G(g) ⋅ 1_M^G(s) / 1_M^G(1) }</td> </tr> <tr> <td class="tdright"> </td> <td class="tdcenter">=</td> <td class="tdcenter">δ(s, g^G)</td> </tr> </table> </div>

So $\delta(s):= \sum_{x \in R} \delta(s, x^G)$ is a lower bound for the vertex degree of $s$; this bound can be computed if $\Pi$ is known.

For computing the vertex degrees of the iterated closures of $\Gamma(G)$, we define $d^{(0)}(s, g^G):= d(s, g^G)$ and

d^(i+1)(s,g^G):= |g^G| if d^(i)(Г(G),s) + d^(i)(Г(G),g) ≥ |G|-1, and d^(i+1)(s,g^G):= d^(i)(s,g^G) otherwise. </Alt>  Then <M>d(&cl;^{(i)}(\Gamma(G)), s) = \sum_{{x \in R}} d^{(i)}(s, g^G)</M> holds. Analogously, we set <M>\delta^{(0)}(s, g^G):= \delta(s, g^G)</M>, <Alt Only="LaTeX"><![CDATA[ \[ \delta^{(i+1)}(s, g^G):= \left\{ \begin{array}{lcl} |g^G| & ;; & \delta^{(i)}(s) + \delta^{(i)}(g) \geq |G|-1 \\ \delta^{(i)}(s, g^G) & ; & \mbox{\rm otherwise} \end{array} \right. \]

δ^(i+1)(s, g^G):= |g^G| if δ^(i)(s) + δ^(i)(g) ≥ |G|-1, δ^(i+1)(s, g^G):= δ^(i)(s, g^G) otherwise, and $\delta^{(i)}(s):= \sum_{{x \in R}} \delta^{(i)}(s, x^G)$, a lower bound for $d(cl^{(i)}(\Gamma(G)), s)$ that can be computed if $\Pi$ is known.

<a id="X825776BA8687E475" name="X825776BA8687E475"></a>

<h5>4.2-2 Checking the Criteria</h5>

Let us assume that we know lower bounds $\beta(s)$ for the vertex degrees $d(cl^{(i)}(\Gamma(G)), s)$, for some fixed $i$, and let us choose representatives $s_1, s_2, \ldots, s_l$ of the nonidentity conjugacy classes of $G$ such that $\beta(s_1) \leq \beta(s_2) \leq \cdots \leq \beta(s_l)$ holds. Let $c_k = |s_k^G|$ be the class lengths of these representatives.

Then the first $c_1$ vertex degrees, ordered by increasing size, are larger than or equal to $\beta(s_1)$, the next $c_2$ vertex degrees are larger than or equal to $\beta(s_2)$, and so on.

Then the set of indices in the $k$-th nonidentity class of $G$ for which Pósa's criterion is not guaranteed by the given bounds is

\[
 \{ x; c_1 + c_2 + \cdots + c_{k-1} < x \leq c_1 + c_2 + \cdots c_k,
 x < (|G| - 1) / 2, \beta(s_k) < x+1 \}.
\]

This is an interval $\{ L_k, L_k + 1, \ldots, U_k \}$ with

\[
 L_k = \max\left\{ 1 + c_1 + c_2 + \cdots + c_{k-1},
 \beta(s_k) \right\}
\]

and

\[
 U_k = \min\left\{ c_1 + c_2 + \cdots + c_k,
 \left\lfloor |G|/2 \right\rfloor - 1 \right\} .
\]

(Note that the generating graph has $m = |G|-1$ vertices, and that $x < m/2$ is equivalent to $x \leq \left\lfloor |G|/2 \right\rfloor - 1$.)

The generating graph $\Gamma(G)$ satisfies Pósa's criterion if all these intervals are empty, i. e., if $L_k > U_k$ holds for $1 \leq k \leq l$.

The set of indices for which Chvátal's criterion is not guaranteed is the intersection of

\[
 \{ m-k; 1 \leq m-k < m/2, d_k < k \}
\]

with the set of indices for which Pósa's criterion is not guaranteed.

Analogously to the above considerations, the set of indices $m-x$ in the former set for which Chvátal's criterion is not guaranteed by the given bounds and such that $x$ is an index in the $k$-th nonidentity class of $G$ is

\[
 \{ m-x; c_1 + c_2 + \cdots + c_{k-1} < x \leq c_1 + c_2 + \cdots c_k,
 1 \leq m-x < (|G| - 1) / 2, \beta(s_k) < x \}.
\]

This is again an interval $\{ L^{\prime}_k, L^{\prime}_k + 1, \ldots, U^{\prime}_k \}$ with

\[
 L^{\prime}_k = \max\left\{ 1, m - ( c_1 + c_2 + \cdots + c_k ) \right\}
\]

and

\[
 U^{\prime}_k = \min\left\{ m - ( c_1 + c_2 + \cdots + c_{k-1} ) - 1,
 \left\lfloor |G|/2 \right\rfloor - 1,
 m-1 - \beta(s_k) \right\} .
\]

The generating graph $\Gamma(G)$ satisfies Chvátal's criterion if the union of the intervals $\{ L^{\prime}_k, L^{\prime}_k + 1, \ldots, U^{\prime}_k \}$, for $1 \leq k \leq l$ is disjoint to the union of the intervals $\{ L_k, L_k + 1, \ldots, U_k \}$, for $1 \leq k \leq l$.

<a id="X7B56BE5384BAD54E" name="X7B56BE5384BAD54E"></a>

<h4>4.3 GAP Functions for the Computations</h4>

We describe two approaches to compute, for a given group $G$, vertex degrees for the generating graph of $G$ or lower bounds for them, by calculating exact vertex degrees from $G$ itself (see Section <a href="chap4_mj.html#X802B2ED2802334B0">4.3-1</a>) or by deriving lower bounds for the vertex degrees using just character-theoretic information about $G$ and its subgroups (see Section <a href="chap4_mj.html#X87FE2DDD7F086D2F">4.3-2</a>). Finally, Section <a href="chap4_mj.html#X8677A8B1788ACD2C">4.3-3</a> deals with deriving lower bounds of vertex degrees of iterated closures.

<a id="X802B2ED2802334B0" name="X802B2ED2802334B0"></a>

<h5>4.3-1 Computing Vertex Degrees from the Group</h5>

In this section, the task is to compute the vertex degrees $d(s,g^G)$ using explicit computations with the group $G$.

The function <code class="code">IsGeneratorsOfTransPermGroup</code> checks whether the permutations in the list <code class="code">list</code> generate the permutation group <code class="code">G</code>, provided that <code class="code">G</code> is transitive on its moved points. (Note that testing the necessary condition that the elements in <code class="code">list</code> generate a transitive group is usually much faster than testing generation.) This function has been used already in <a href="chapBib_mj.html#biBProbGenArxiv">[Breb]</a>.

<div class="example"><pre>
gap> IsGeneratorsOfTransPermGroup:= function( G, list )
> local S;
> 
> if not IsTransitive( G ) then
> Error( "<G> must be transitive on its moved points" );
> fi;
> S:= SubgroupNC( G, list );
> 
> return IsTransitive( S, MovedPoints( G ) )
> and Size( S ) = Size( G );
> end;;
</pre></div>

The function <code class="code">VertexDegreesGeneratingGraph</code> takes a transitive permutation group $G$ (in order to be allowed to use <code class="code">IsGeneratorsOfTransPermGroup</code>), the list <code class="code">classes</code> of conjugacy classes of $G$ (in order to prescribe an ordering of the classes), and a list <code class="code">normalsubgroups</code> of proper normal subgroups of $G$, and returns the matrix $[ d(s, g^G) ]_{s, g}$ of vertex degrees, with rows and columns indexed by nonidentity class representatives ordered as in the list <code class="code">classes</code>. (The class containing the identity element may be contained in <code class="code">classes</code>.)

The following criteria are used in this function.

<ul>
<li>The function tests the (non)generation only for representatives of $C_G(g)$-$C_G(s)$-double cosets, where $C_G(g):= \{ x \in G; g x = x g \}$ denotes the centralizer of $g$ in $G$. Note that for $c_1 \in C_G(g)$, $c_2 \in C_G(s)$, and a representative $r \in G$, we have $\langle s, g^{c_1 r c_2} \rangle = \langle s, g^r \rangle^{c_2}$. If $\langle s, g^r \rangle = G$ then the double coset $D = C_G(g) r C_G(s)$ contributes $|D|/|C_G(g)|$ to the vertex degree $d(s, g^G)$, otherwise the contribution is zero.

</li>
<li>We have $d(s, g^G) \cdot |C_G(g)| = d(g, s^G) \cdot |C_G(s)|$. (To see this, either establish a bijection of the above double cosets, or doubly count the edges between elements of the conjugacy classes of $s$ and $g$.)

</li>
<li>If $\langle s_1 \rangle = \langle s_2 \rangle$ and $\langle g_1 \rangle = \langle g_2 \rangle$ hold then we have $d(s_1, g_1^G) = d(s_2, g_1^G) = d(s_1, g_2^G) = d(s_2, g_2^G)$, so only one of these values must be computed.

</li>
<li>If both $s$ and $g$ are contained in one of the normal subgroups given then $d(s, g^G)$ is zero.

</li>
<li>If $G$ is not a dihedral group and both $s$ and $g$ are involutions then $d(s, g^G)$ is zero.

</li>
</ul>

<div class="example"><pre>
gap> BindGlobal( "VertexDegreesGeneratingGraph",
> function( G, classes, normalsubgroups )
> local nccl, matrix, cents, powers, normalsubgroupspos, i, j, g_i,
> nsg, g_j, gen, pair, d, pow;
> 
> if not IsTransitive( G ) then
> Error( "<G> must be transitive on its moved points" );
> fi;
> 
> classes:= Filtered( classes,
> C -> Order( Representative( C ) ) <> 1 );
> nccl:= Length( classes );
> matrix:= [];
> cents:= [];
> powers:= [];
> normalsubgroupspos:= [];
> for i in [ 1 .. nccl ] do
> matrix[i]:= [];
> if IsBound( powers[i] ) then
> # The i-th row equals the earlier row 'powers[i]'.
> for j in [ 1 .. i ] do
> matrix[i][j]:= matrix[ powers[i] ][j];
> matrix[j][i]:= matrix[j][ powers[i] ];
> od;
> else
> # We have to compute the values.
> g_i:= Representative( classes[i] );
> nsg:= Filtered( [ 1 .. Length( normalsubgroups ) ],
> i -> g_i in normalsubgroups[i] );
> normalsubgroupspos[i]:= nsg;
> cents[i]:= Centralizer( G, g_i );
> for j in [ 1 .. i ] do
> g_j:= Representative( classes[j] );
> if IsBound( powers[j] ) then
> matrix[i][j]:= matrix[i][ powers[j] ];
> matrix[j][i]:= matrix[ powers[j] ][i];
> elif not IsEmpty( Intersection( nsg, normalsubgroupspos[j] ) )
> or ( Order( g_i ) = 2 and Order( g_j ) = 2
> and not IsDihedralGroup( G ) ) then
> matrix[i][j]:= 0;
> matrix[j][i]:= 0;
> else
> # Compute $d(g_i, g_j^G)$.
> gen:= 0;
> for pair in DoubleCosetRepsAndSizes( G, cents[j],
> cents[i] ) do
> if IsGeneratorsOfTransPermGroup( G,
> [ g_i, g_j^pair[1] ] ) then
> gen:= gen + pair[2];
> fi;
> od;
> matrix[i][j]:= gen / Size( cents[j] );
> if i <> j then
> matrix[j][i]:= gen / Size( cents[i] );
> fi;
> fi;
> od;
> 
> # For later, provide information about algebraic conjugacy.
> for d in Difference( PrimeResidues( Order( g_i ) ), [ 1 ] ) do
> pow:= g_i^d;
> for j in [ i+1 .. nccl ] do
> if not IsBound( powers[j] ) and pow in classes[j] then
> powers[j]:= i;
> break;
> fi;
> od;
> od;
> fi;
> od;
> 
> return matrix;
> end );
</pre></div>

<a id="X87FE2DDD7F086D2F" name="X87FE2DDD7F086D2F"></a>

<h5>4.3-2 Computing Lower Bounds for Vertex Degrees</h5>

In this section, the task is to compute the lower bounds $\delta(s, g^G)$ for the vertex degrees $d(s, g^G)$ using character-theoretic methods.

We provide GAP functions for computing the multiset $\Pi$ of the primitive permutation characters of a given group $G$ and for computing the lower bounds $\delta(s, g^G)$ from $\Pi$.

For many almost simple groups, the GAP libraries of character tables and of tables of marks contain information for quickly computing the primitive permutation characters of the group in question. Therefore, the function <code class="code">PrimitivePermutationCharacters</code> takes as its argument not the group $G$ but its character table $T$, say. (This function is shown already in <a href="chapBib_mj.html#biBProbGenArxiv">[Breb]</a>.)

If $T$ is contained in the GAP Character Table Library (see <a href="chapBib_mj.html#biBCTblLib">[Bre25]</a>) then the complete set of primitive permutation characters can be easily computed if the character tables of all maximal subgroups and their class fusions into $T$ are known (in this case, we check whether the attribute <code class="func">Maxes</code> (<a href="../doc/chap3_mj.html#X8150E63F7DBDF252">CTblLib: Maxes</a>) of $T$ is bound) or if the table of marks of $G$ and the class fusion from $T$ into this table of marks are known (in this case, we check whether the attribute <code class="func">FusionToTom</code> (<a href="../doc/chap3_mj.html#X7B1AAED68753B1BE">CTblLib: FusionToTom</a>) of $T$ is bound). If the attribute <code class="func">UnderlyingGroup</code> (<a href="../../../doc/ref/chap70_mj.html#X81E41D3880FA6C4C">Reference: UnderlyingGroup for tables of marks</a>) of $T$ is bound then the group stored as the value of this attribute can be used to compute the primitive permutation characters. The latter happens if $T$ was computed from the group $G$; for tables in the GAP Character Table Library, this is not the case by default.

The GAP function <code class="code">PrimitivePermutationCharacters</code> tries to compute the primitive permutation characters of a group using this information; it returns the required list of characters if this can be computed this way, otherwise <code class="keyw">fail</code> is returned. (For convenience, we use the GAP mechanism of attributes in order to store the permutation characters in the character table object once they have been computed.)

<div class="example"><pre>
gap> DeclareAttribute( "PrimitivePermutationCharacters",
> IsCharacterTable );
gap> InstallOtherMethod( PrimitivePermutationCharacters,
> [ IsCharacterTable ],
> function( tbl )
> local maxes, i, fus, poss, tom, G;
> 
> if HasMaxes( tbl ) then
> maxes:= List( Maxes( tbl ), CharacterTable );
> for i in [ 1 .. Length( maxes ) ] do
> fus:= GetFusionMap( maxes[i], tbl );
> if fus = fail then
> fus:= PossibleClassFusions( maxes[i], tbl );
> poss:= Set( fus,
> map -> InducedClassFunctionsByFusionMap(
> maxes[i], tbl,
> [ TrivialCharacter( maxes[i] ) ], map )[1] );
> if Length( poss ) = 1 then
> maxes[i]:= poss[1];
> else
> return fail;
> fi;
> else
> maxes[i]:= TrivialCharacter( maxes[i] )^tbl;
> fi;
> od;
> return maxes;
> elif HasFusionToTom( tbl ) then
> tom:= TableOfMarks( tbl );
> maxes:= MaximalSubgroupsTom( tom );
> return PermCharsTom( tbl, tom ){ maxes[1] };
> elif HasUnderlyingGroup( tbl ) then
> G:= UnderlyingGroup( tbl );
> return List( MaximalSubgroupClassReps( G ),
> M -> TrivialCharacter( M )^tbl );
> fi;
> 
> return fail;
> end );
</pre></div>

The next function computes the lower bounds $\delta(s, g^G)$ from the two lists <code class="code">classlengths</code> of conjugacy class lengths of the group $G$ and <code class="code">prim</code> of all primitive permutation characters of $G$. (The first entry in <code class="code">classlengths</code> is assumed to represent the class containing the identity element of $G$.) The return value is the matrix that contains in row $i$ and column $j$ the value $\delta(s, g^G)$, where $s$ and $g$ are in the conjugacy classes represented by the $(i+1)$-st and $(j+1)$-st column of <code class="code">tbl</code>, respectively. So the row sums of this matrix are the values $\delta(s)$.

<div class="example"><pre>
gap> LowerBoundsVertexDegrees:= function( classlengths, prim )
> local sizes, nccl;
> 
> nccl:= Length( classlengths );
> return List( [ 2 .. nccl ],
> i -> List( [ 2 .. nccl ],
> j -> Maximum( 0, classlengths[j] - Sum( prim,
> pi -> classlengths[j] * pi[j] * pi[i]
> / pi[1] ) ) ) );
> end;;
</pre></div>

<a id="X8677A8B1788ACD2C" name="X8677A8B1788ACD2C"></a>

<h5>4.3-3 Evaluating the (Lower Bounds for the) Vertex Degrees</h5>

In this section, the task is to compute (lower bounds for) the vertex degrees of iterated closures of a generating graph from (lower bounds for) the vertex degrees of the graph itself, and then to check the criteria of Pósa and Chvátal.

The arguments of all functions defined in this section are the list <code class="code">classlengths</code> of conjugacy class lengths for the group $G$ (including the class of the identity element, in the first position) and a matrix <code class="code">bounds</code> of the values $d^{(i)}(s, g^G)$ or $\delta^{(i)}(s, g^G)$, with rows and columns indexed by nonidentity class representatives $s$ and $g$, respectively. Such a matrix is returned by the functions <code class="code">VertexDegreesGeneratingGraph</code> or <code class="code">LowerBoundsVertexDegrees</code>, respectively.

The function <code class="code">LowerBoundsVertexDegreesOfClosure</code> returns the corresponding matrix of the values $d^{(i+1)}(s, g^G)$ or $\delta^{(i+1)}(s, g^G)$, respectively.

<div class="example"><pre>
gap> LowerBoundsVertexDegreesOfClosure:= function( classlengths, bounds )
> local delta, newbounds, size, i, j;
> 
> delta:= List( bounds, Sum );
> newbounds:= List( bounds, ShallowCopy );
> size:= Sum( classlengths );
> for i in [ 1 .. Length( bounds ) ] do
> for j in [ 1 .. Length( bounds ) ] do
> if delta[i] + delta[j] >= size - 1 then
> newbounds[i][j]:= classlengths[ j+1 ];
> fi;
> od;
> od;
> 
> return newbounds;
> end;;
</pre></div>

Once the values $d^{(i)}(s, g^G)$ or $\delta^{(i)}(s, g^G)$ are known, we can check whether Pósa's or Chvátal's criterion is satisfied for the graph $cl^{(i)}(\Gamma(G))$, using the function <code class="code">CheckCriteriaOfPosaAndChvatal</code> shown below. (Of course a negative result is meaningless in the case that only lower bounds for the vertex degrees are used.)

The idea is to compute the row sums of the given matrix, and to compute the intervals $\{ L_k, L_k + 1, \ldots, U_k \}$ and $\{ L^{\prime}_k, L^{\prime}_k + 1, \ldots, U^{\prime}_k \}$ that were introduced in Section <a href="chap4_mj.html#X825776BA8687E475">4.2-2</a>.

The function <code class="code">CheckCriteriaOfPosaAndChvatal</code> returns, given the list of class lengths of $G$ and the matrix of (bounds for the) vertex degrees, a record with the components <code class="code">badForPosa</code> (the list of those pairs $[ L_k, U_k ]$ with the property $L_k \leq U_k$), <code class="code">badForChvatal</code> (the list of pairs of lower and upper bounds of nonempty intervals where Chvátal's criterion may be violated), and <code class="code">data</code> (the sorted list of triples $[ \delta(g_k), |g_k^G|, \iota(k) ]$, where $\iota(k)$ is the row and column position of $g_k$ in the matrix <code class="code">bounds</code>). The ordering of class lengths must of course be compatible with the ordering of rows and columns of the matrix, and the identity element of $G$ must belong to the first entry in the list of class lengths.

<div class="example"><pre>
gap> CheckCriteriaOfPosaAndChvatal:= function( classlengths, bounds )
> local size, degs, addinterval, badForPosa, badForChvatal1, pos,
> half, i, low1, upp2, upp1, low2, badForChvatal, interval1,
> interval2;
> 
> size:= Sum( classlengths );
> degs:= List( [ 2 .. Length( classlengths ) ],
> i -> [ Sum( bounds[ i-1 ] ), classlengths[i], i ] );
> Sort( degs );
> 
> addinterval:= function( intervals, low, upp )
> if low <= upp then
> Add( intervals, [ low, upp ] );
> fi;
> end;
> 
> badForPosa:= [];
> badForChvatal1:= [];
> pos:= 1;
> half:= Int( size / 2 ) - 1;
> for i in [ 1 .. Length( degs ) ] do
> # We have pos = c_1 + c_2 + \cdots + c_{i-1} + 1
> low1:= Maximum( pos, degs[i][1] ); # L_i
> upp2:= Minimum( half, size-1-pos, size-1-degs[i][1] ); # U'_i
> pos:= pos + degs[i][2];
> upp1:= Minimum( half, pos-1 ); # U_i
> low2:= Maximum( 1, size-pos ); # L'_i
> addinterval( badForPosa, low1, upp1 );
> addinterval( badForChvatal1, low2, upp2 );
> od;
> 
> # Intersect intervals.
> badForChvatal:= [];
> for interval1 in badForPosa do
> for interval2 in badForChvatal1 do
> addinterval( badForChvatal,
> Maximum( interval1[1], interval2[1] ),
> Minimum( interval1[2], interval2[2] ) );
> od;
> od;
> 
> return rec( badForPosa:= badForPosa,
> badForChvatal:= Set( badForChvatal ),
> data:= degs );
> end;;
</pre></div>

Finally, the function <code class="code">HamiltonianCycleInfo</code> assumes that the matrix <code class="code">bounds</code> contains lower bounds for the vertex degrees in the generating graph $\Gamma$, and returns a string that describes the minimal $i$ with the property that the given bounds suffice to show that $cl^{(i)}(\Gamma)$ satisfies Pósa's or Chvátal's criterion, if such a closure exists. If no closure has this property, the string <code class="code">"no decision"</code> is returned.

<div class="example"><pre>
gap> HamiltonianCycleInfo:= function( classlengths, bounds )
> local i, result, res, oldbounds;
> 
> i:= 0;
> result:= rec( Posa:= fail, Chvatal:= fail );
> repeat
> res:= CheckCriteriaOfPosaAndChvatal( classlengths, bounds );
> if result.Posa = fail and IsEmpty( res.badForPosa ) then
> result.Posa:= i;
> fi;
> if result.Chvatal = fail and IsEmpty( res.badForChvatal ) then
> result.Chvatal:= i;
> fi;
> i:= i+1;
> oldbounds:= bounds;
> bounds:= LowerBoundsVertexDegreesOfClosure( classlengths,
> bounds );
> until oldbounds = bounds;
> 
> if result.Posa <> fail then
> if result.Posa <> result.Chvatal then
> return Concatenation(
> "Chvatal for ", Ordinal( result.Chvatal ), " closure, ",
> "Posa for ", Ordinal( result.Posa ), " closure" );
> else
> return Concatenation( "Posa for ", Ordinal( result.Posa ),
> " closure" );
> fi;
> elif result.Chvatal <> fail then
> return Concatenation( "Chvatal for ", Ordinal( result.Chvatal ),
> " closure" );
> else
> return "no decision";
> fi;
> end;;
</pre></div>

<a id="X7A221012861440E2" name="X7A221012861440E2"></a>

<h4>4.4 Character-Theoretic Computations</h4>

In this section, we apply the functions introduced in Section <a href="chap4_mj.html#X7B56BE5384BAD54E">4.3</a> to character tables of almost simple groups that are available in the GAP Character Table Library.

Our first examples are the sporadic simple groups, in Section <a href="chap4_mj.html#X86CE51E180A3D4ED">4.4-1</a>, then their automorphism groups are considered in Section <a href="chap4_mj.html#X7DC6DFCC83502CC3">4.4-3</a>. Small alternating and symmetric groups are treated in Section <a href="chap4_mj.html#X8130C9CB7A33140F">4.4-4</a>.

For our convenience, we provide a small function that takes as its argument only the character table in question, and returns a string, either <code class="code">"no prim. perm. characters"</code> or the return value of <code class="code">HamiltonianCycleInfo</code> for the bounds computed from the primitive permutation characters.

<div class="example"><pre>
gap> HamiltonianCycleInfoFromCharacterTable:= function( tbl )
> local prim, classlengths, bounds;
> 
> prim:= PrimitivePermutationCharacters( tbl );
> if prim = fail then
> return "no prim. perm. characters";
> fi;
> classlengths:= SizesConjugacyClasses( tbl );
> bounds:= LowerBoundsVertexDegrees( classlengths, prim );
> return HamiltonianCycleInfo( classlengths, bounds );
> end;;
</pre></div>

<a id="X86CE51E180A3D4ED" name="X86CE51E180A3D4ED"></a>

<h5>4.4-1 Sporadic Simple Groups</h5>

Since version 1.3.10 of CTblLib, the GAP Character Table Library contains the tables of maximal subgroups of all sporadic simple groups.

So the function <code class="code">PrimitivePermutationCharacters</code> can be used to compute all their primitive permutation characters.

<div class="example"><pre>
gap> spornames:= AllCharacterTableNames( IsSporadicSimple, true,
> IsDuplicateTable, false );
[ "B", "Co1", "Co2", "Co3", "F3+", "Fi22", "Fi23", "HN", "HS", "He",
 "J1", "J2", "J3", "J4", "Ly", "M", "M11", "M12", "M22", "M23",
 "M24", "McL", "ON", "Ru", "Suz", "Th" ]
gap> for tbl in List( spornames, CharacterTable ) do
> info:= HamiltonianCycleInfoFromCharacterTable( tbl );
> if info <> "Posa for 0th closure" then
> Print( Identifier( tbl ), ": ", info, "\n" );
> fi;
> od;
</pre></div>

It turns out that the information available in the GAP Character Table Library is sufficient to prove that the generating graph contains a Hamiltonian cycle.

<a id="X867D338F7F453092" name="X867D338F7F453092"></a>

<h5>4.4-2 The Monster</h5>

In the original version of this file, a special treatment of the Monster group had been necessary because not all character tabes of its maximal subgroups were available, and also not all class fusions from some maximal subgroups were known whose character tables were available. This section shows the computations that were used at that time.

Meanwhile the maximal subgroups of the Monster are classified, see <a href="chapBib_mj.html#biBDLP25">[DLP25]</a>, their character tables are available, and also all class fusions from the maximal subgroups are known. Thus we can easily compute the primitive permutation characters of the Monster group. The computations shown in this section are not needed to prove the claims from <a href="chapBib_mj.html#biBGMN">[BGL+10]</a>, they are kept just for historical reasons. Thus this section is an example how the availability of the full list of primitive permutation characters of the Monster simplifies proofs.

The situation for the following arguments is as follows. We do not assume the knowledge of all primitive permutation characters of the Monster group $M$, and we use only some of the character tables of the maximal subgroups of $M$ and only partial information on their class fusions. Note that for some maximal subgroups of $M$, the permutation character is not uniquely determined by the character tables involved. However, we can compute upper bounds for the values of the primitive permutation characters $1_S^M$ from the possible class fusions from $S$ into $M$ if the character table of $S$ is known. For the other subgroups $S$, the permutation characters $1_S^M$ have been computed with other methods. Using this information, we will show that the generating graph of $M$ satisfies Pósa's criterion.

The list <code class="code">primdata</code> defined below has length $46$. The entry at position $i$ is a list of length one or two. If <code class="code">primdata[</code>$i$<code class="code">]</code> has length one then its unique entry is the identifier of the library character table of the $i$-th maximal subgroup of $M$. If <code class="code">primdata[</code>$i$<code class="code">]</code> has length two then its entries are a string describing the structure of the $i$-th maximal subgroup $S$ of $M$ and the permutation character $1_S^M$.

(The construction of the explicitly given characters in this list will be documented elsewhere. Some of the constructions can be found in Section <a href="chap8_mj.html#X8337F3C682B6BE63">8.16</a>.)

<div class="example"><pre>
gap> dir:= DirectoriesPackageLibrary( "ctbllib", "data" );;
gap> filename:= Filename( dir, "prim_perm_M.json" );;
gap> primdata:= EvalString( StringFile( filename ) )[2];;
gap> Length( primdata );
46
gap> m:= CharacterTable( "M" );;
</pre></div>

We compute upper bounds for the permutation character values in the cases where the characters are not given explicitly. (We could improve this by using additional information about the class fusions, but this will not be necessary.)

<div class="example"><pre>
gap> s:= "dummy";; # Avoid a message about an unbound variable ...
gap> poss:= "dummy";; # Avoid a message about an unbound variable ...
gap> for entry in primdata do
> if not IsBound( entry[2] ) then
> s:= CharacterTable( entry[1] );
> poss:= Set( PossibleClassFusions( s, m ),
> x -> InducedClassFunctionsByFusionMap( s, m,
> [ TrivialCharacter( s ) ], x )[1] );
> entry[2]:= List( [ 1 .. NrConjugacyClasses( m ) ],
> i -> Maximum( List( poss, x -> x[i] ) ) );
> fi;
> od;
</pre></div>

Now we estimate the lower bounds $\delta(s, g^G)$ introduced in Section <a href="chap4_mj.html#X87FE2DDD7F086D2F">4.3-2</a>. Let $��$ denote a set of representatives of the classes of maximal subgroups of $M$. Then

\[
 \delta(s, g^G) =
 |s^G| - |s^G| \cdot \sum_{{S \in ��}} 1_S^M(s) \cdot 1_S^M(g) / 1_S^M(1) ,
\]

hence $\delta(s)$ can be computed from the corresponding primitive permutation characters, and a lower bound for $\delta(s)$ can be computed from the upper bounds for the characters $1_S^G$ which are given by the list <code class="code">primdata</code>.

This means that modifying the output of <code class="code">LowerBoundsVertexDegrees</code> as follows really yields lower bounds for the vertex degrees.

<div class="example"><pre>
gap> prim:= List( primdata, x -> x[2] );;
gap> classlengths:= SizesConjugacyClasses( m );;
gap> bounds:= LowerBoundsVertexDegrees( classlengths, prim );;
</pre></div>

Now we sum up the bounds for the individual classes. It turns out that the minimal vertex degree is more than $99.99998$ percent of $|M|$. This proves that the generating graph of the Monster satisfies Pósa's criterion.

<div class="example"><pre>
gap> degs:= List( bounds, Sum );;
gap> Int( 100000000 * Minimum( degs ) / Size( m ) );
99999987
</pre></div>

Without the results from <a href="chapBib_mj.html#biBDLP25">[DLP25]</a>, we can argue as follows. (This was the situation in earlier versions of this example file.)

According to <a href="chapBib_mj.html#biBNW12">[NW13]</a>, any maximal subgroup of the Monster is either among the $44$ known classes from the above list except L$_2(13).2$ and U$_3(4).4$, or it is an almost simple group whose socle is one of L$_2(13)$, Sz$(8)$, U$_3(4)$, and U$_3(8)$.

We show that the elements of such subgroups are contained in the union of $55$ conjugacy classes of the Monster that cover less than one percent of the elements in the Monster. For that, we compute the possible class fusions from the abovementioned simple groups $S$ into the Monster, and then the possible class fusions from the automorphic extensions of $S$ into the Monster, using the possible class fusions of $S$. (This approach is faster than computing each class fusion from scratch.)

After the following computations, the list <code class="code">badclasses</code> will contain the positions of all those classes of $M$ that may contain elements in some of the hypothetical maximal subgroups.

For each simple group in question, we enter the identifiers of the character tables of the automorphic extensions that can occur. Note that the automorphism groups of the four groups have the structures L$_2(13).2$, Sz$(8).3$, U$_3(4).4$, and U$_3(8).(3 \times S_3)$, respectively. We need not consider the groups U$_3(8).3^2$ and U$_3(8).(3 \times S_3)$ because already U$_3(8).3_2$ does not admit an embedding into $M$, and we need not consider the group U$_3(8).S_3$ because its set of elements is covered by its subgroups of the types U$_3(8).2$ and U$_3(8).3_2$.

<div class="example"><pre>
gap> PossibleClassFusions( CharacterTable( "U3(8).3_2" ), m );
[ ]
gap> badclasses:= [];;
gap> names:= [
> [ "L2(13)", "L2(13).2" ],
> [ "Sz(8)", "Sz(8).3" ],
> [ "U3(4)", "U3(4).2", "U3(4).4" ],
> [ "U3(8)", "U3(8).2", "U3(8).3_1", "U3(8).3_2", "U3(8).3_3",
> "U3(8).6" ],
> ];;
gap> for list in names do
> t:= CharacterTable( list[1] );
> tfusm:= PossibleClassFusions( t, m );
> UniteSet( badclasses, Flat( tfusm ) );
> for nam in list{ [ 2 .. Length( list ) ] } do
> ext:= CharacterTable( nam );
> for map1 in PossibleClassFusions( t, ext ) do
> inv:= InverseMap( map1 );
> for map2 in tfusm do
> init:= CompositionMaps( map2, inv );
> UniteSet( badclasses, Flat( PossibleClassFusions( ext, m,
> rec( fusionmap:= init ) ) ) );
> od;
> od;
> od;
> od;
gap> badclasses;
[ 1, 3, 4, 5, 6, 7, 9, 10, 11, 12, 14, 15, 17, 18, 19, 20, 21, 22,
 24, 25, 27, 28, 30, 32, 33, 35, 36, 38, 39, 40, 42, 43, 44, 45, 46,
 48, 49, 50, 51, 52, 53, 54, 55, 56, 60, 61, 62, 63, 70, 72, 73, 78,
 82, 85, 86 ]
gap> Length( badclasses );
55
gap> bad:= Sum( classlengths{ badclasses } ) / Size( m );;
gap> Int( 10000 * bad ); 
97
</pre></div>

In the original version of this file, also hypothetical maximal subgroups with socle L$_2(27)$ had been considered. As a consequence, the list <code class="code">badclasses</code> computed above had length $59$ in the original version; the list contained also the classes at the positions $90, 94, 95$, and $96$, that is, the classes <code class="code">26B</code>, <code class="code">28B</code>, <code class="code">28C</code>, <code class="code">28D</code>. The proportion <code class="code">bad</code> of elements in the classes of $M$ described by <code class="code">badclasses</code> was about $2.05$ percent of $|M|$, compared to the about $0.98$ percent in the current version.

Now we estimate the lower bounds $\delta(s, g^G)$ introduced in Section <a href="chap4_mj.html#X87FE2DDD7F086D2F">4.3-2</a>. Let $B$ denote the union of the classes described by <code class="code">badclasses</code>, and let $��$ denote a set of representatives of the $44$ known classes of maximal subgroups of $M$.

If $s \notin B$ then

\[
 \delta(s, g^G) =
 |s^G| - |s^G| \cdot \sum_{{S \in ��}} 1_S^M(s) \cdot 1_S^M(g) / 1_S^M(1) ,
\]

hence $\delta(s)$ can be computed from the corresponding primitive permutation characters, and a lower bound for $\delta(s)$ can be computed from the upper bounds for the characters $1_S^G$ which are given by the list <code class="code">primdata</code>.

If $s \in B$ then the above equation for $\delta(s, g^G)$ holds at least for $g \notin B$, so $\sum_{{g \in R \setminus B}} \delta(s, g^G)$ is a lower bound for $\delta(s)$. So <code class="code">primdata</code> yields a lower bound for $\delta(s)$ also for $s \in B$, by ignoring the pairs $(s, g)$ where both $s$ and $g$ lie in $B$.

This means that modifying the output of <code class="code">LowerBoundsVertexDegrees</code> as follows really yields lower bounds for the vertex degrees. (Note that the row and column positions in the matrix returned by <code class="code">LowerBoundsVertexDegrees</code> are shifted by one, compared to <code class="code">badclasses</code>.)

<div class="example"><pre>
gap> prim:= List( primdata, x -> x[2] );;
gap> badpos:= Difference( badclasses, [ 1 ] ) - 1;;
gap> bounds:= LowerBoundsVertexDegrees( classlengths, prim );;
gap> for i in badpos do
> for j in badpos do
> bounds[i][j]:= 0;
> od;
> od;
</pre></div>

Now we sum up the bounds for the individual classes. It turns out that the minimal vertex degree is more than $99$ percent of $|M|$. This proves that the generating graph of the Monster satisfies Pósa's criterion.

(And the minimal vertex degree of elements outside $B$ is more than $99.99998$ percent of $|M|$.)

In the original version of this file, we got only $97.95$ percent of $|M|$ as the lower bound for the minimal vertex degree. The bound for elements outside $B$ was the same in the original version. The fact that the maximal subgroups of type L$_2(41)$ had been ignored in the original version did not affect the lower bound for the minimal vertex degree.

<div class="example"><pre>
gap> degs:= List( bounds, Sum );;
gap> Int( 10000 * Minimum( degs ) / Size( m ) );
9902
gap> goodpos:= Difference( [ 1 .. NrConjugacyClasses( m ) - 1 ],
> badpos );;
gap> Int( 100000000 * Minimum( degs{ goodpos } ) / Size( m ) );
99999987
</pre></div>

<a id="X7DC6DFCC83502CC3" name="X7DC6DFCC83502CC3"></a>

<h5>4.4-3 Nonsimple Automorphism Groups of Sporadic Simple Groups</h5>

Next we consider the nonsimple automorphism groups of the sporadic simple groups. Nontrivial outer automorphisms exist exactly in $12$ cases, and then the simple group has index $2$ in its automorphism group. The character tables of the groups and their maximal subgroups are available in GAP.

<div class="example"><pre>
gap> spornames:= AllCharacterTableNames( IsSporadicSimple, true,
> IsDuplicateTable, false );;
gap> sporautnames:= AllCharacterTableNames( IsSporadicSimple, true,
> IsDuplicateTable, false,
> OfThose, AutomorphismGroup );;
gap> sporautnames:= Difference( sporautnames, spornames );
[ "F3+.2", "Fi22.2", "HN.2", "HS.2", "He.2", "J2.2", "J3.2", "M12.2",
 "M22.2", "McL.2", "ON.2", "Suz.2" ]
gap> for tbl in List( sporautnames, CharacterTable ) do
> info:= HamiltonianCycleInfoFromCharacterTable( tbl );
> Print( Identifier( tbl ), ": ", info, "\n" );
> od;
F3+.2: Chvatal for 0th closure, Posa for 1st closure
Fi22.2: Chvatal for 0th closure, Posa for 1st closure
HN.2: Chvatal for 0th closure, Posa for 1st closure
HS.2: Chvatal for 1st closure, Posa for 2nd closure
He.2: Chvatal for 0th closure, Posa for 1st closure
J2.2: Chvatal for 0th closure, Posa for 1st closure
J3.2: Chvatal for 0th closure, Posa for 1st closure
M12.2: Chvatal for 0th closure, Posa for 1st closure
M22.2: Posa for 1st closure
McL.2: Chvatal for 0th closure, Posa for 1st closure
ON.2: Chvatal for 0th closure, Posa for 1st closure
Suz.2: Chvatal for 0th closure, Posa for 1st closure
</pre></div>

<a id="X8130C9CB7A33140F" name="X8130C9CB7A33140F"></a>

<h5>4.4-4 Alternating and Symmetric Groups $A_n$, $S_n$,
for $5 \leq n \leq 13$</h5>

For alternating and symmetric groups $A_n$ and $S_n$, respectively, with $5 \leq n \leq 13$, the table of marks or the character tables of the group and all its maximal subgroups are available in GAP. So we can compute the character-theoretic bounds for vertex degrees.

<div class="example"><pre>
gap> for tbl in List( [ 5 .. 13 ], i -> CharacterTable(
> Concatenation( "A", String( i ) ) ) ) do
> info:= HamiltonianCycleInfoFromCharacterTable( tbl );
> if info <> "Posa for 0th closure" then
> Print( Identifier( tbl ), ": ", info, "\n" );
> fi;
> od;
</pre></div>

No messages are printed, so the generating graphs of the alternating groups in question satisfy Pósa's criterion.

<div class="example"><pre>
gap> for tbl in List( [ 5 .. 13 ], i -> CharacterTable(
> Concatenation( "S", String( i ) ) ) ) do
> info:= HamiltonianCycleInfoFromCharacterTable( tbl );
> Print( Identifier( tbl ), ": ", info, "\n" );
> od;
A5.2: no decision
A6.2_1: Chvatal for 4th closure, Posa for 5th closure
A7.2: Posa for 1st closure
A8.2: Chvatal for 2nd closure, Posa for 3rd closure
A9.2: Chvatal for 2nd closure, Posa for 3rd closure
A10.2: Chvatal for 2nd closure, Posa for 3rd closure
A11.2: Posa for 1st closure
A12.2: Chvatal for 2nd closure, Posa for 3rd closure
A13.2: Posa for 1st closure
</pre></div>

We see that sufficiently large closures of the generating graphs of the symmetric groups in question satisfy Pósa's criterion, except that the bounds for the symmetric group $S_5$ are not sufficient for the proof. In Section <a href="chap4_mj.html#X8033892B7FD6E62B">4.5-2</a>, it is shown that the 2nd closure of the generating graph of $S_5$ satisfies Pósa's criterion.

(We could find slightly better bounds derived only from character tables which suffice to prove that the generating graph for $S_5$ contains a Hamiltonian cycle, but this seems to be not worth while.)

<a id="X83DACCF07EF62FAE" name="X83DACCF07EF62FAE"></a>

<h4>4.5 Computations With Groups</h4>

We prove in Section <a href="chap4_mj.html#X7B9ADC91802EE09F">4.5-1</a> that the generating graphs of the nonabelian simple groups of order up to $10^6$ satisfy Pósa's criterion, and that the same holds for those nonabelian simple groups of order between $10^6$ and $10^7$ that are not isomorphic with some $PSL(2,q)$. (In Section <a href="chap4_mj.html#X84E62545802FAB30">4.6</a>, it is shown that the generating graph of $PSL(2,q)$ satifies Pósa's criterion for any prime power $q$.) Nonsimple nonsolvable groups of order up to $10^6$ are treated in Section <a href="chap4_mj.html#X8033892B7FD6E62B">4.5-2</a>.

(We could increase the bounds $10^6$ and $10^7$ with more computations, using the same methods.)

For our convenience, we provide a small function that takes as its argument only the group in question, and returns a string, the return value of <code class="code">HamiltonianCycleInfo</code> for the vertex degrees computed from the group. (In order to speed up the computations, the function computes the proper normal subgroups that contain the derived subgroup of the given group, and enters the list of these groups as the third argument of <code class="code">VertexDegreesGeneratingGraph</code>.)

<div class="example"><pre>
gap> HamiltonianCycleInfoFromGroup:= function( G )
> local ccl, nsg, der, degrees, classlengths;
> ccl:= ConjugacyClasses( G );
> if IsPerfect( G ) then
> nsg:= [];
> else
> der:= DerivedSubgroup( G );
> nsg:= Concatenation( [ der ],
> IntermediateSubgroups( G, der ).subgroups );
> fi;
> degrees:= VertexDegreesGeneratingGraph( G, ccl, nsg );
> classlengths:= List( ccl, Size );
> return HamiltonianCycleInfo( classlengths, degrees ); 
> end;;
</pre></div>

<a id="X7B9ADC91802EE09F" name="X7B9ADC91802EE09F"></a>

<h5>4.5-1 Nonabelian Simple Groups of Order up to $10^7$</h5>

Representatives of the $56$ isomorphism types of nonabelian simple groups of order up to $10^6$ can be accessed in GAP with the function <code class="code">AllSmallNonabelianSimpleGroups</code>.

<div class="example"><pre>
gap> grps:= AllSmallNonabelianSimpleGroups( [ 1 .. 10^6 ] );; 
gap> Length( grps );
56
gap> List( grps, StructureDescription );
[ "A5", "PSL(3,2)", "A6", "PSL(2,8)", "PSL(2,11)", "PSL(2,13)",
 "PSL(2,17)", "A7", "PSL(2,19)", "PSL(2,16)", "PSL(3,3)",
 "PSU(3,3)", "PSL(2,23)", "PSL(2,25)", "M11", "PSL(2,27)",
 "PSL(2,29)", "PSL(2,31)", "A8", "PSL(3,4)", "PSL(2,37)", "O(5,3)",
 "Sz(8)", "PSL(2,32)", "PSL(2,41)", "PSL(2,43)", "PSL(2,47)",
 "PSL(2,49)", "PSU(3,4)", "PSL(2,53)", "M12", "PSL(2,59)",
 "PSL(2,61)", "PSU(3,5)", "PSL(2,67)", "J1", "PSL(2,71)", "A9",
 "PSL(2,73)", "PSL(2,79)", "PSL(2,64)", "PSL(2,81)", "PSL(2,83)",
 "PSL(2,89)", "PSL(3,5)", "M22", "PSL(2,97)", "PSL(2,101)",
 "PSL(2,103)", "HJ", "PSL(2,107)", "PSL(2,109)", "PSL(2,113)",
 "PSL(2,121)", "PSL(2,125)", "O(5,4)" ]
gap> for g in grps do 
> info:= HamiltonianCycleInfoFromGroup( g );
> if info <> "Posa for 0th closure" then
> Print( StructureDescription( g ), ": ", info, "\n" );
> fi;
> od;
</pre></div>

Nothing is printed during these computations, so the generating graphs of all processed groups satisfy Pósa's criterion.

(On my notebook, the above computations needed about $6300$ seconds of CPU time.)

For simple groups of order larger than $10^6$, there is not such an easy way (yet) to access representatives for each isomorphism type. Therefore, first we compute the orders of nonabelian simple groups between $10^6$ and $10^7$.

<div class="example"><pre>
gap> orders:= Filtered( [ 10^6+4, 10^6+8 .. 10^7 ],
> n -> IsomorphismTypeInfoFiniteSimpleGroup( n ) <> fail );
[ 1024128, 1123980, 1285608, 1342740, 1451520, 1653900, 1721400,
 1814400, 1876896, 1934868, 2097024, 2165292, 2328648, 2413320,
 2588772, 2867580, 2964780, 3265920, 3483840, 3594432, 3822588,
 3940200, 4245696, 4680000, 4696860, 5515776, 5544672, 5663616,
 5848428, 6004380, 6065280, 6324552, 6825840, 6998640, 7174332,
 7906500, 8487168, 9095592, 9732420, 9951120, 9999360 ]
gap> Length( orders );
41
gap> info:= List( orders, IsomorphismTypeInfoFiniteSimpleGroup );;
gap> Number( info, x -> IsBound( x.series ) and x.series = "L"
> and x.parameter[1] = 2 );
31
</pre></div>

We see that there are $31$ groups of the type $PSL(2,q)$ and $10$ other nonabelian simple groups with order in the range from $10^6$ to $10^7$. The former groups can be ignored because the generating graphs of any group $PSL(2,q)$ satisfies Pósa's criterion, see Section <a href="chap4_mj.html#X84E62545802FAB30">4.6</a>. For the latter groups, we can apply the character-theoretic method to prove that the generating graph satisfies Pósa's criterion.

<div class="example"><pre>
gap> info:= Filtered( info, x -> not IsBound( x.series ) or
> x.series <> "L" or x.parameter[1] <> 2 );
[ rec( name := "B(3,2) = O(7,2) ~ C(3,2) = S(6,2)",
 parameter := [ 3, 2 ], series := "B", shortname := "S6(2)" ),
 rec( name := "A(10)", parameter := 10, series := "A",
 shortname := "A10" ),
 rec( name := "A(2,7) = L(3,7) ", parameter := [ 3, 7 ],
 series := "L", shortname := "L3(7)" ),
 rec( name := "2A(3,3) = U(4,3) ~ 2D(3,3) = O-(6,3)",
 parameter := [ 3, 3 ], series := "2A", shortname := "U4(3)" ),
 rec( name := "G(2,3)", parameter := 3, series := "G",
 shortname := "G2(3)" ),
 rec( name := "B(2,5) = O(5,5) ~ C(2,5) = S(4,5)",
 parameter := [ 2, 5 ], series := "B", shortname := "S4(5)" ),
 rec( name := "2A(2,8) = U(3,8)", parameter := [ 2, 8 ],
 series := "2A", shortname := "U3(8)" ),
 rec( name := "2A(2,7) = U(3,7)", parameter := [ 2, 7 ],
 series := "2A", shortname := "U3(7)" ),
 rec( name := "A(3,3) = L(4,3) ~ D(3,3) = O+(6,3) ",
 parameter := [ 4, 3 ], series := "L", shortname := "L4(3)" ),
 rec( name := "A(4,2) = L(5,2) ", parameter := [ 5, 2 ],
 series := "L", shortname := "L5(2)" ) ]
gap> names:= [ "S6(2)", "A10", "L3(7)", "U4(3)", "G2(3)", "S4(5)",
> "U3(8)", "U3(7)", "L4(3)", "L5(2)" ];;
gap> for tbl in List( names, CharacterTable ) do
> info:= HamiltonianCycleInfoFromCharacterTable( tbl );
> if info <> "Posa for 0th closure" then
> Print( Identifier( tbl ), ": ", info, "\n" );
> fi;
> od;
</pre></div>

<a id="X8033892B7FD6E62B" name="X8033892B7FD6E62B"></a>

<h5>4.5-2 Nonsimple Groups with Nonsolvable Socle of Order at most $10^6$</h5>

Let $G$ be a nonsolvable group such that $G/N$ is cyclic for all nontrivial normal subgroups $N$ of $G$. Then the socle Soc$(G)$ of $G$ is the unique minimal normal subgroup. Moreover, Soc$(G)$ is nonsolvable and thus a direct product of isomorphic nonabelian simple groups, and $G$ is isomorphic to a subgroup of Aut$($Soc$(G))$.

In order to deal with all such groups $G$ for which additionally $|$Soc$(G)| \leq 10^6$ holds, it is sufficient to run over the simple groups $S$ of order up to $10^6$ and to consider those subgroups $G$ of Aut$(S^n)$, with $|S|^n \leq 10^6$, for which Inn$(G)$ is the unique minimal normal subgroup and $G / $Inn$(G)$ is cyclic.

We show that for each such group, a sufficient closure of the generating graph satisfies Pósa's criterion.

<div class="example"><pre>
gap> grps:= AllSmallNonabelianSimpleGroups( [ 1 .. 10^6 ] );; 
gap> epi:= "dummy";; # Avoid a message about an unbound variable ...
gap> for simple in grps do
> for n in [ 1 .. LogInt( 10^6, Size( simple ) ) ] do
> # Compute the n-fold direct product S^n.
> soc:= CallFuncList( DirectProduct,
> ListWithIdenticalEntries( n, simple ) );
> # Compute Aut(S^n) as a permutation group.
> aut:= Image( IsomorphismPermGroup( AutomorphismGroup( soc ) ) );
> aut:= Image( SmallerDegreePermutationRepresentation( aut ) );
> # Compute class representatives of subgroups of
> # Aut(S^n)/Inn(S^n).
> socle:= Socle( aut );
> epi:= NaturalHomomorphismByNormalSubgroup( aut, socle );
> # Compute the candidates for G. (By the above computations,
> # we need not consider simple groups.)
> reps:= List( ConjugacyClassesSubgroups( Image( epi ) ),
> Representative );
> reps:= Filtered( reps, x -> IsCyclic( x ) and Size( x ) <> 1 );
> greps:= Filtered( List( reps, x -> PreImages( epi, x ) ),
> x -> Length( MinimalNormalSubgroups( x ) ) = 1 );
> for g in greps do
> # We have to deal with a *transitive* permutation group.
> # (Each group in question acts faithfully on an orbit.)
> if not IsTransitive( g ) then
> g:= First( List( Orbits( g, MovedPoints( g ) ),
> x -> Action( g, x ) ),
> x -> Size( x ) = Size( g ) );
> fi;
> # Check this group G.
> info:= HamiltonianCycleInfoFromGroup( g );
> Print( Name( simple ), "^", n, ".", Size( g ) / Size( soc ),
> ": ", info, "\n" );
> od;
> od;
> od;
A5^1.2: Posa for 2nd closure
A5^2.2: Posa for 0th closure
A5^2.4: Posa for 0th closure
A5^3.3: Posa for 0th closure
A5^3.6: Chvatal for 1st closure, Posa for 2nd closure
PSL(2,7)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,7)^2.2: Posa for 0th closure
PSL(2,7)^2.4: Posa for 0th closure
A6^1.2: Chvatal for 0th closure, Posa for 1st closure
A6^1.2: Chvatal for 4th closure, Posa for 5th closure
A6^1.2: Chvatal for 0th closure, Posa for 1st closure
A6^2.2: Posa for 0th closure
A6^2.4: Posa for 0th closure
A6^2.4: Posa for 0th closure
A6^2.4: Posa for 0th closure
PSL(2,8)^1.3: Posa for 0th closure
PSL(2,8)^2.2: Posa for 0th closure
PSL(2,8)^2.6: Chvatal for 0th closure, Posa for 1st closure
PSL(2,11)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,11)^2.2: Posa for 0th closure
PSL(2,11)^2.4: Posa for 0th closure
PSL(2,13)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,17)^1.2: Chvatal for 0th closure, Posa for 1st closure
A7^1.2: Posa for 1st closure
PSL(2,19)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,16)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,16)^1.4: Chvatal for 0th closure, Posa for 1st closure
PSL(3,3)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSU(3,3)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,23)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,25)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,25)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,25)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,27)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,27)^1.3: Posa for 0th closure
PSL(2,27)^1.6: Chvatal for 0th closure, Posa for 1st closure
PSL(2,29)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,31)^1.2: Chvatal for 0th closure, Posa for 1st closure
A8^1.2: Chvatal for 2nd closure, Posa for 3rd closure
PSL(3,4)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(3,4)^1.2: Chvatal for 1st closure, Posa for 2nd closure
PSL(3,4)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(3,4)^1.3: Posa for 0th closure
PSL(3,4)^1.6: Chvatal for 0th closure, Posa for 1st closure
PSL(2,37)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSp(4,3)^1.2: Chvatal for 1st closure, Posa for 2nd closure
Sz(8)^1.3: Posa for 0th closure
PSL(2,32)^1.5: Posa for 0th closure
PSL(2,41)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,43)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,47)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,49)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,49)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,49)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSU(3,4)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSU(3,4)^1.4: Chvatal for 0th closure, Posa for 1st closure
PSL(2,53)^1.2: Chvatal for 0th closure, Posa for 1st closure
M12^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,59)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,61)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSU(3,5)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSU(3,5)^1.3: Posa for 0th closure
PSL(2,67)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,71)^1.2: Chvatal for 0th closure, Posa for 1st closure
A9^1.2: Chvatal for 2nd closure, Posa for 3rd closure
PSL(2,73)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,79)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,64)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,64)^1.3: Posa for 0th closure
PSL(2,64)^1.6: Chvatal for 0th closure, Posa for 1st closure
PSL(2,81)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,81)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,81)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,81)^1.4: Chvatal for 0th closure, Posa for 1st closure
PSL(2,81)^1.4: Chvatal for 0th closure, Posa for 1st closure
PSL(2,83)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,89)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(3,5)^1.2: Chvatal for 0th closure, Posa for 1st closure
M22^1.2: Posa for 1st closure
PSL(2,97)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,101)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,103)^1.2: Chvatal for 0th closure, Posa for 1st closure
J_2^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,107)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,109)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,113)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,121)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,121)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,121)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,125)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSL(2,125)^1.3: Posa for 0th closure
PSL(2,125)^1.6: Chvatal for 0th closure, Posa for 1st closure
PSp(4,4)^1.2: Chvatal for 0th closure, Posa for 1st closure
PSp(4,4)^1.4: Posa for 0th closure
</pre></div>

<a id="X84E62545802FAB30" name="X84E62545802FAB30"></a>

<h4>4.6 The Groups $PSL(2,q)$</h4>

We show that the generating graph of any group $PSL(2,q)$, for $q \geq 2$, satisfies Pósa's criterion. Throughout this section, let $q = p^f$ for a prime integer $p$, and $G = PSL(2,q)$. Set $k = \gcd(q-1, 2)$.

Lemma 1: (see <a href="chapBib_mj.html#biBHup67">[Hup67, II., § 8]</a>) The subgroups of $G$ are

<dl>
<dt>(1)</dt>
<dd>cyclic groups of order dividing $(q \pm 1)/k$, and their normalizers, which are dihedral groups of order $2 (q \pm 1)/k$,

</dd>
<dt>(2)</dt>
<dd>subgroups of Sylow $p$ normalizers, which are semidirect products of elementary abelian groups of order $q$ with cyclic groups of order $(q-1)/k$,

</dd>
<dt>(3)</dt>
<dd>subgroups isomorphic with $PSL(2, p^m)$ if $m$ divides $f$, and isomorphic with $PGL(2, p^m)$ if $2 m$ divides $f$,

</dd>
<dt>(4)</dt>
<dd>subgroups isomorphic with $A_4$, $S_4$, or $A_5$, for appropriate values of $q$.

</dd>
</dl>
$G$ contains exactly one conjugacy class of cyclic subgroups of each of the orders $(q-1)/k$ and $(q+1)/k$, and each nonidentity element of $G$ is contained in exactly one of these subgroups or in exactly one Sylow $p$ subgroup of $G$.

We estimate the number of elements that are contained in subgroups of type (3).

Lemma 2: Let $n_{sf}(q)$ denote the number of those nonidentity elements in $G$ that are contained in proper subgroups of type (3). Then $n_{sf}(q) \leq q^2 (2 p (\sqrt{q}-1) / (p-1) - 1)$. If $f$ is a prime then $n_{sf}(q) \leq (2p-1) q^2$ holds, and if $p = q$ then we have of course $n_{sf}(q) = 0$.

Proof: The group $PGL(2, p^m)$ is equal to $PSL(2, p^m)$ for $p = 2$, and contains $PSL(2, p^m)$ as a subgroup of index two if $p \ne 2$. So the largest element order in $PGL(2, p^m)$ is at most $p^m+1$. Let $C$ be a cyclic subgroup of order $(q + \epsilon)/k$ in $G$, for $\epsilon \in \{ \pm 1 \}$. The intersection of $C$ with any subgroup of $G$ isomorphic with $PGL(2, p^m)$ or $PSL(2, p^m)$ is contained in the union of the unique subgroups of the orders $\gcd(|C|, p^m + 1)$ and $\gcd(|C|, p^m - 1)$ in $C$. So $C$ contains at most $2 p^m - 2$ nonidentity elements that can lie inside subgroups isomorphic with $PGL(2, p^m)$ or $PSL(2, p^m)$. Hence $C$ contains at most $\sum_m (2 p^m - 2)$ nonidentity elements in proper subgroups of type (3), where $m$ runs over the proper divisors of $f$. This sum is bounded from above by $\sum_{{m=1}}^{{f/2}} (2 p^m - 2) \leq 2 p (\sqrt{{q}}-1) / (p-1) - 2$.

The numbers of cyclic subgroups of the orders $(q + \epsilon)/k$ in $G$ are $q (q - \epsilon) / 2$, so $G$ contains altogether $q^2$ such cyclic subgroups. They contain at most $q^2 (2 p (\sqrt{{q}}-1) / (p-1) - 2)$ elements inside proper subgroups of the type (3).

All elements of order $p$ in $G$ are contained in subgroups of type (3), and there are exactly $q^2 - 1$ such elements. This yields the claimed bound for $n_{sf}(q)$. The better bound for the case that $f$ is a prime follows from $\sum_m (2 p^m - 2) = 2 p - 2$ if $m$ ranges over the proper divisors of $f$. ▒

Using these bounds, we see that the vertex degree of any element in $G$ that does not lie in subgroups of type (4) is larger than $|G|/2$. (In fact we could use the calculations below to derive a better asymptotic bound, but this is not an issue here.)

Lemma 3: Let $s \in G$ be an element of order larger than $5$. Then $|\{ g \in G; \langle g, s \rangle = G \}| > |G|/2$.

Proof: First suppose that the order of $s$ divides $(q+1)/k$ or $(q-1)/k$. If $g \in G$ such that $U = \langle s, g \rangle$ is a proper subgroup of $G$ then $U \leq N_G(\langle s \rangle)$ or $U$ lies in a Sylow $p$ normalizer of $G$ or $U$ lies in a subgroup of type (3). Since $s$ is contained in at most two Sylow $p$ normalizers (each Sylow $p$ normalizer contains $q$ cyclic subgroups of order $(q-1)/k$, and $G$ contains $q+1$ Sylow normalizers and $q (q+1)/2$ cyclic subgroups of order $(q-1)/k$), the number of $g \in G$ with the property that $\langle s, g \rangle ≠ G$ is at most $N = 2(q+1)/k + 2 q(q-1)/k + n_{sf}(q) = 2(q^2+1)/k + n_{sf}(q)$; for $s$ of order equal to $(q+1)/k$ or $(q-1)/k$, we can set $N = 2(q^2+1)/k$.

Any element $s$ of order $p$ (larger than $5$), lies only in a unique Sylow $p$ normalizer and in subgroups of type (3), so the bound $N$ holds also in this case.

For $f = 1$, $N$ is smaller than $|G|/2 = q (q^2-1) / (2 k)$ if $q \geq 5$. (The statement of the lemma is trivially true for $q \leq 5$.)

For primes $f$, $N$ is smaller than $|G|/2$ if $q^2 (q-8p) > q+4$ holds, which is true for $p^f > 8p$. Only the following values of $p^f$ with prime $f$ do not satisfy this condition: $2^2$ and $3^2$ (where no element of order larger than $5$ exists), $2^3$ (where only elements of order equal to $q \pm 1$ must be considered), $5^2$ and $7^2$ (where $n_{sf}(q) < (p-1) q (q+1)$ because in these cases the cyclic subgroups of order $(q+1)/k$ cannot contain nonidentity elements in subgroups of type (3)).

Finally, if $f$ is not a prime then $N$ is smaller than $|G|/2$ if $q^2 (q - 8p (\sqrt{{q}}-1) / (p-1)) > q+4$ holds, which is true for $q \geq 256$. The only values of $p^f$ with non-prime $f$ that do not satisfy this condition are $2^4$, $2^6$, and $3^4$. In all three cases, we have in fact $N < |G|/2$, where we have to use the better bound $n_{sf}(q) < 16 q^2$ in the third case. ▒

In order to show that the generating graph of $G$ satisfies Pósa's criterion, it suffices to show that the vertex degrees of involutions is larger than the number of involutions, and that the vertex degrees of elements of orders $2$, $3$, $4$, and $5$ are larger than the number of elements whose order is at most $5$.

Lemma 4: Let $n(q, m)$ denote the number of elements of order $m$ in $G$, and let $\varphi(m)$ denote the number of prime residues modulo $m$.

<ul>
<li>We have $n(q, 2) = q^2 - 1$ if $q$ is even and $n(q, 2) \leq q (q+1)/2$ if $q$ is odd.

</li>
<li>For $m \in \{ 3, 4, 5 \}$, we have $n(q, m) \leq \varphi(m) q (q+1)/2$.

</li>
<li>We have $n(q, (q+1)/k) = \varphi((q+1)/k) q (q-1)/2$.

</li>
</ul>
Lemma 5: If $q > 11$ then each involution in $G$ has vertex degree larger than $n(q, 2)$.

If $\varphi((q+1)/k) \geq 12$ then each element of order $3$, $4$, or $5$ has vertex degree larger than $\sum_{{m=2}}^5 n(q, m)$.

Proof: Let $s \in G$ of order at most $5$. For each element $g \in G$ of order $(q+1)/k$, $U = \langle g, s \rangle$ is either $G$ or contained in the dihedral group of order $2(q+1)/k$ that normalizes $\langle g \rangle$.

If $s$ is an involution then the number of such dihedral groups that contain $s$ is at most $(q+3)/2$, and at least $n(q, (q+1)/k) - \varphi((q+1)/k) (q+3)/2 = \varphi((q+1)/k) (q^2-2q-3)/2$ elements of order $(q+1)/k$ contribute to the vertex degree of $s$. This number is larger than $q^2 - 1 \geq n(q, 2)$ if $q > 11$ (and hence $\varphi((q+1)/k) \geq 3$) holds.

If $s$ is an element of order $3$, $4$, or $5$ then $U ≠ G$ means that $s \in \langle g \rangle$, so at least $n(q, (q+1)/k) - 4$ elements of order $(q+1)/k$ contribute to the vertex degree of $s$. This number is larger than $5 q (q+1) > \sum_{{m=2}}^5 n(q, m)$ if $\varphi((q+1)/k) \geq 12$. ▒

It remains to deal with the values $q$ where $\varphi((q+1)/k) < 12$, that is, $(q+1)/k \leq 30$. We compute that the statement of Lemma 5 is true also for prime powers $q$ with $11 < q \leq 59$.

<div class="example"><pre>
gap> TestL2q:= function( t )
> local name, orders, nccl, cl, prim, bds, n, ord;
> 
> name:= Identifier( t );
> orders:= OrdersClassRepresentatives( t );
> nccl:= Length( orders );
> cl:= SizesConjugacyClasses( t );
> prim:= PrimitivePermutationCharacters( t );
> bds:= List( LowerBoundsVertexDegrees( cl, prim ), Sum );
> n:= List( [ 1 .. 5 ], i -> Sum( cl{ Filtered( [ 1 .. nccl ],
> x -> orders[x] = i ) } ) );
> if ForAny( Filtered( [ 1 .. nccl ], i -> orders[i] > 5 ),
> i -> bds[i-1] <= Size( t ) / 2 ) then
> Error( "problem with large orders for ", name );
> elif ForAny( Filtered( [ 1 .. nccl ], i -> orders[i] = 2 ),
> i -> bds[i-1] <= n[2] ) then
> Error( "problem with order 2 for ", name, "\n" );
> elif ForAny( Filtered( [ 1 .. nccl ],
> i -> orders[i] in [ 3 .. 5 ] ),
> i -> bds[i-1] <= Sum( n{ [ 2 .. 5 ] } ) ) then
> Error( "problem with order in [ 3 .. 5 ] for ", name );
> fi;
> end;;
gap> for q in Filtered( [ 13 .. 59 ], IsPrimePowerInt ) do
> TestL2q( CharacterTable(
> Concatenation( "L2(", String( q ), ")" ) ) );
> od;
</pre></div>

For $2 \leq q \leq 11$, the statement of Lemma 5 is not true but Pósa's criterion is satisfied for the generating graphs of the groups $PSL(2,q)$ with $2 \leq q \leq 11$.

<div class="example"><pre>
gap> for q in Filtered( [ 2 .. 11 ], IsPrimePowerInt ) do
> info:= HamiltonianCycleInfoFromGroup( PSL( 2, q ) );
> if info <> "Posa for 0th closure" then
> Print( q, ": ", info, "\n" );
> fi;
> od;
</pre></div>

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