<h3>9 <span class="Heading">Cat<span class="SimpleMath">^3</span>-groups and Crossed cubes</span></h3>
<p>The term <em>4d-group</em> refers to a set of equivalent categories of which the ones we are most interested are the categories of <em>crossed cubes</em> and <em>cat<span class="SimpleMath">^3</span>-groups</em>. A <em>4d-mapping</em> is a function between two 4d-groups which preserves all the structure.</p>
<p>The material in this chapter should be considered very experimental. As yet there are no functions for crossed cubes.</p>
<h4>9.1 <span class="Heading">Functions for (pre-)cat<span class="SimpleMath">^3</span>-groups</span></h4>
<p>We shall use the following standard orientation of a cat<span class="SimpleMath">^3</span>-group <span class="SimpleMath">calE</span> on a group <span class="SimpleMath">G</span>. <span class="SimpleMath">calE</span> contains <span class="SimpleMath">8</span> groups; <span class="SimpleMath">12</span> cat<span class="SimpleMath">^1</span>-groups and <span class="SimpleMath">6</span> cat<span class="SimpleMath">^2</span>-groups forming the vertices; edges and faces of a cube, as shown in the following diagram.</p>
<p>By definition, <span class="SimpleMath">calE</span> is generated by three commuting cat<span class="SimpleMath">^1</span>-groups <span class="SimpleMath">(G ⇒ R), (G ⇒ Q)</span> and <span class="SimpleMath">(G ⇒ H)</span>, but it is more convenient to think of <span class="SimpleMath">calE</span> as generated by two cat<span class="SimpleMath">^2</span>-groups</p>
<ul>
<li><p><em>front</em><span class="SimpleMath">(calE)</span>, generated by <span class="SimpleMath">(G ⇒ R)</span> and <span class="SimpleMath">(G ⇒ Q)</span>;</p>
</li>
<li><p><em>left</em><span class="SimpleMath">(calE)</span>, generated by <span class="SimpleMath">(G ⇒ Q)</span> and <span class="SimpleMath">(G ⇒ H)</span>.</p>
</li>
</ul>
<p>Because the tail, head and embedding maps all commute, it follows that <em>up</em><span class="SimpleMath">(calE)</span>, generated by <span class="SimpleMath">(G ⇒ H)</span> and <span class="SimpleMath">(G ⇒ R)</span>, is a third cat<span class="SimpleMath">^2</span>-group. The three remaining faces (cat<span class="SimpleMath">^2</span>-groups) <em>right</em><span class="SimpleMath">(calE)</span>, <em>down</em><span class="SimpleMath">(calE)</span> and <em>back</em><span class="SimpleMath">(calE)</span> are then easily constructed. We shall always use the order [<em>front, left, up, right, down, back</em>] for the six faces.</p>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ Cat3Group</code>( <var class="Arg">args</var> )</td><td class="tdright">( function )</td></tr></table></div>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ PreCat3Group</code>( <var class="Arg">args</var> )</td><td class="tdright">( function )</td></tr></table></div>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ IsCat3Group</code>( <var class="Arg">C</var> )</td><td class="tdright">( property )</td></tr></table></div>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ PreCat3GroupByPreCat2Groups</code>( <var class="Arg">L</var> )</td><td class="tdright">( operation )</td></tr></table></div>
<p>The global functions <code class="code">Cat3Group</code> and <code class="code">PreCat3Group</code> normally take as arguments a pair of cat<span class="SimpleMath">^2</span>-groups [<em>front, left</em>], or a trio of cat<span class="SimpleMath">^1</span>-groups [<em>front-up, front-left = left-up, left-left</em>].</p>
<p>In subsection <code class="func">AllCat2GroupsIterator</code> (<a href="chap8.html#X7EFCF9697E845B2C"><span class="RefLink">8.6-4</span></a>) the list of pairs <code class="code">CatnGroupLists(d12).pairs</code> contains the three entries <code class="code">[6,8],[8,11]</code> and <code class="code">[6,11]</code>. It follows that the sixth, eighth and eleventh cat<span class="SimpleMath">^1</span>-groups for <code class="code">d12</code> generate a cat<span class="SimpleMath">^3</span>-group.</p>
<h4>9.2 <span class="Heading">Enumerating cat<span class="SimpleMath">^3</span>-groups with a given source</span></h4>
<p>Once the list <code class="code">CatnGroupLists(G).pairs</code> has been obtained we may seek all triples <span class="SimpleMath">[i,j],[j,k]</span> and <span class="SimpleMath">[k,i]</span> or <span class="SimpleMath">[i,k]</span> of pairs in this list and then, for each such triple, construct a cat<span class="SimpleMath">^3</span>-group generated by the <span class="SimpleMath">i</span>-th, <span class="SimpleMath">j</span>-th and <span class="SimpleMath">k</span>-th cat<span class="SimpleMath">^1</span>-group on <span class="SimpleMath">G</span>.</p>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ AllCat3GroupTriples</code>( <var class="Arg">G</var> )</td><td class="tdright">( operation )</td></tr></table></div>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ AllCat3GroupsNumber</code>( <var class="Arg">G</var> )</td><td class="tdright">( attribute )</td></tr></table></div>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ AllCat3Groups</code>( <var class="Arg">G</var> )</td><td class="tdright">( operation )</td></tr></table></div>
<p>The list of triples returned by the operation <code class="code">AllCat3GroupTriples</code> is saved as <code class="code">CatnGroupLists(G).cat3triples</code>. The length of this list is the number of cat<span class="SimpleMath">^3</span>-groups on <span class="SimpleMath">G</span>, and is saved as <code class="code">CatnGroupNumbers(G).cat3</code>.</p>
<p>As yet there is no operation <code class="code">AllCat3GroupsUpToIsomorphism(G)</code>.</p>
<h4>9.3 <span class="Heading">
Definition and constructions for cat<span class="SimpleMath">^n</span>-groups and their morphisms
</span></h4>
<p>In this chapter and the previous one we are interested in cat<span class="SimpleMath">^2</span>-groups and cat<span class="SimpleMath">^3</span>-groups, and it is convenient in this section to give the more general definition. There are three equivalent descriptions of a cat<span class="SimpleMath">^n</span>-group.</p>
<p>A <em>cat<span class="SimpleMath">^n</span>-group</em> consists of the following.</p>
<ul>
<li><p><span class="SimpleMath">2^n</span> groups <span class="SimpleMath">G_A</span>, one for each subset <span class="SimpleMath">A</span> of <span class="SimpleMath">[n]</span>, the <em>vertices</em> of an <span class="SimpleMath">n</span>-cube.</p>
<p class="pcenter">
\calC_{A,i} ~=~ (e_{A,i};\; t_{A,i},\; h_{A,i} \ :\
G_A \to G_{A \setminus \{i\}}),
\quad\mbox{for all} \quad A \subseteq [n],~ i \in A,
</p>
<p>the <em>edges</em> of the cube.</p>
</li>
<li><p>These cat<span class="SimpleMath">^1</span>-groups combine (in sets of <span class="SimpleMath">4</span>) to form <span class="SimpleMath">n(n-1)2^n-3</span> cat<span class="SimpleMath">^2</span>-groups <span class="SimpleMath">calC_A,{i,j}</span> for all <span class="SimpleMath">{i,j} ⊆ A ⊆ [n],~ i ≠ j</span>, the <em>faces</em> of the cube.</p>
</li>
</ul>
<p>Note that, since the <span class="SimpleMath">t_A,i, h_A,i</span> and <span class="SimpleMath">e_A,i</span> commute, composite homomorphisms <span class="SimpleMath">t_A,B, h_A,B : G_A -> G_A ∖ B</span> and <span class="SimpleMath">e_A,B : G_A ∖ B -> G_A</span> are well defined for all <span class="SimpleMath">B ⊆ A ⊆ [n]</span>.</p>
<p>Secondly, we give the simplest of the three descriptions, again adapted from Ellis-Steiner <a href="chapBib.html#biBell:st">[ES87]</a>.</p>
<p>A cat<span class="SimpleMath">^n</span>-group <span class="SimpleMath">calC</span> consists of <span class="SimpleMath">2^n</span> groups <span class="SimpleMath">G_A</span>, one for each subset <span class="SimpleMath">A</span> of <span class="SimpleMath">[n]</span>, and <span class="SimpleMath">3n</span> homomorphisms</p>
</li>
</ul>
<p>Our third description defines a cat<span class="SimpleMath">^n</span>-group as a "cat^1-group of cat^(n-1)-groups".</p>
<p>A <em>cat<span class="SimpleMath">^n</span>-group</em> <span class="SimpleMath">calC</span> consists of two cat<span class="SimpleMath">^(n-1)</span>-groups:</p>
<ul>
<li><p><span class="SimpleMath">calA</span> with groups <span class="SimpleMath">G_A, A ⊆ [n-1]</span>, and homomorphisms <span class="SimpleMath">ddott_A,i, ddoth_A,i, ddote_A,i</span>,</p>
</li>
<li><p><span class="SimpleMath">calB</span> with groups <span class="SimpleMath">H_B, B ⊆ [n-1]</span>, and homomorphisms <span class="SimpleMath">dott_B,i, doth_B,i, dote_B,i</span>, and</p>
</li>
<li><p>cat<span class="SimpleMath">^(n-1)</span>-morphisms <span class="SimpleMath">t,h : calA -> calB</span> and <span class="SimpleMath">e : calB -> calA</span> subject to the following conditions:</p>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ PreCatnGroup</code>( <var class="Arg">L</var> )</td><td class="tdright">( operation )</td></tr></table></div>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ CatnGroup</code>( <var class="Arg">L</var> )</td><td class="tdright">( operation )</td></tr></table></div>
<p>The operation <code class="code">(Pre)CatnGroup</code> expects as input a list of cat<span class="SimpleMath">^1</span>-groups. For our group <code class="code">d12</code> we may construct various cat<span class="SimpleMath">^4</span>-groups, and here is one of them.</p>
<p>For a cat<span class="SimpleMath">^5</span>-group we may start with the cyclic group whose order is the product of the first five primes. With this group we may form <span class="SimpleMath">32</span> cat<span class="SimpleMath">^1</span>-groups and <span class="SimpleMath">528</span> cat<span class="SimpleMath">^2</span>-groups.</p>
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