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<div class="ChapSects" ><a href="chap14.html#X810FFB1C8035C8BE" >14 <span class="Heading" >Utility functions</span ></a>
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap14.html#X7C9734B880042C73" >14 .1 <span class="Heading" >Mappings</span ></a>
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap14.html#X7F8E297F7C84DE51" >14 .1 -1 InclusionMappingGroups</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap14.html#X81D29E737F3D4878" >14 .1 -2 InnerAutomorphismsByNormalSubgroup</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap14.html#X7FC631B786C1DC8B" >14 .1 -3 IsGroupOfAutomorphisms</a></span >
</div ></div >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap14.html#X852BD9CA84C2AFF0" >14 .2 <span class="Heading" >Abelian Modules</span ></a>
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap14.html#X806DEFCC859BB4F1" >14 .2 -1 AbelianModuleObject</a></span >
</div ></div >
</div >
<h3>14 <span class="Heading" >Utility functions</span ></h3>
<p>By a utility function we mean a <strong class="pkg" >GAP</strong > function which is</p>
<ul >
<li ><p>needed by other functions in this package,</p>
</li >
<li ><p>not (as far as we know) provided by the standard <strong class="pkg" >GAP</strong > library,</p>
</li >
<li ><p>more suitable for inclusion in the main library than in this package.</p>
</li >
</ul >
<p>Sections on <em >Printing Lists</em > and <em >Distinct and Common Representatives</em > were moved to the <strong class="pkg" >Utils</strong > package with version 2 .56 .</p>
<p><a id="X7C9734B880042C73" name="X7C9734B880042C73" ></a></p>
<h4>14 .1 <span class="Heading" >Mappings</span ></h4>
<p>The following two functions have been moved to the <strong class="pkg" >gpd</strong > package, but are still documented here.</p>
<p><a id="X7F8E297F7C84DE51" name="X7F8E297F7C84DE51" ></a></p>
<h5>14 .1 -1 InclusionMappingGroups</h5>
<div class="func" ><table class="func" width="100%" ><tr ><td class="tdleft" ><code class="func" >an style='color: green'>8227; InclusionMappingGroups</code >( <var class="Arg" >G</var >, <var class="Arg" >H</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" >an style='color: green'>8227; MappingToOne</code >( <var class="Arg" >G</var >, <var class="Arg" >H</var > )</td ><td class="tdright" >( operation )</td ></tr ></table ></div >
<p>This set of utilities concerns mappings. The map <code class="code" >incd8</code > is the inclusion of <code class="code" >d8</code > in <code class="code" >d16</code > used in Section <a href="chap3.html#X7B09A28579707CAF" ><span class="RefLink" >3 .4 </span ></a>. <code class="code" >MappingToOne(G,H)</code > maps the whole of <span class="SimpleMath" >G</span > to the identity element in <span class="SimpleMath" >H</span >.</p>
<div class="example" ><pre >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >Print( incd8, "\n" );</span >
[ (11 ,13 ,15 ,17 )(12 ,14 ,16 ,18 ), (11 ,18 )(12 ,17 )(13 ,16 )(14 ,15 ) ] ->
[ (11 ,13 ,15 ,17 )(12 ,14 ,16 ,18 ), (11 ,18 )(12 ,17 )(13 ,16 )(14 ,15 ) ]
<span class="GAPprompt" >gap></span > <span class="GAPinput" >imd8 := Image( incd8 );;</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >MappingToOne( c4, imd8 );</span >
[ (11 ,13 ,15 ,17 )(12 ,14 ,16 ,18 ) ] -> [ () ]
</pre ></div >
<p><a id="X81D29E737F3D4878" name="X81D29E737F3D4878" ></a></p>
<h5>14 .1 -2 InnerAutomorphismsByNormalSubgroup</h5>
<div class="func" ><table class="func" width="100%" ><tr ><td class="tdleft" ><code class="func" >an style='color: green'>8227; InnerAutomorphismsByNormalSubgroup</code >( <var class="Arg" >G</var >, <var class="Arg" >N</var > )</td ><td class="tdright" >( operation )</td ></tr ></table ></div >
<p>Inner automorphisms of a group <code class="code" >G</code > by the elements of a normal subgroup <code class="code" >N</code > are calculated, often with <code class="code" >G</code > = <code class="code" >N</code >.</p>
<div class="example" ><pre >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >autd8 := AutomorphismGroup( d8 );;</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >innd8 := InnerAutomorphismsByNormalSubgroup( d8, d8 );;</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >GeneratorsOfGroup( innd8 );</span >
[ ^(1 ,2 ,3 ,4 ), ^(1 ,3 ) ]
</pre ></div >
<p><a id="X7FC631B786C1DC8B" name="X7FC631B786C1DC8B" ></a></p>
<h5>14 .1 -3 IsGroupOfAutomorphisms</h5>
<div class="func" ><table class="func" width="100%" ><tr ><td class="tdleft" ><code class="func" >an style='color: green'>8227; IsGroupOfAutomorphisms</code >( <var class="Arg" >A</var > )</td ><td class="tdright" >( property )</td ></tr ></table ></div >
<p>Tests whether the elements of a group are automorphisms.</p>
<div class="example" ><pre >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >IsGroupOfAutomorphisms( innd8 );</span >
true
</pre ></div >
<p><a id="X852BD9CA84C2AFF0" name="X852BD9CA84C2AFF0" ></a></p>
<h4>14 .2 <span class="Heading" >Abelian Modules</span ></h4>
<p><a id="X806DEFCC859BB4F1" name="X806DEFCC859BB4F1" ></a></p>
<h5>14 .2 -1 AbelianModuleObject</h5>
<div class="func" ><table class="func" width="100%" ><tr ><td class="tdleft" ><code class="func" >an style='color: green'>8227; AbelianModuleObject</code >( <var class="Arg" >grp</var >, <var class="Arg" >act</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" >an style='color: green'>8227; IsAbelianModule</code >( <var class="Arg" >obj</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" >an style='color: green'>8227; AbelianModuleGroup</code >( <var class="Arg" >obj</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" >an style='color: green'>8227; AbelianModuleAction</code >( <var class="Arg" >obj</var > )</td ><td class="tdright" >( attribute )</td ></tr ></table ></div >
<p>An abelian module is an abelian group together with a group action. These are used by the crossed module constructor <code class="func" >XModByAbelianModule</code > (<a href="chap2.html#X824631577864961E" ><span class="RefLink" >2 .1 -7 </span ></a>).</p>
<p>The resulting <code class="code" >Xabmod</code > is isomorphic to the output from <code class="code" >XModByAutomorphismGroup( k4 );</code >.</p>
<div class="example" ><pre >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >x := (6 ,7 )(8 ,9 );; y := (6 ,8 )(7 ,9 );; z := (6 ,9 )(7 ,8 );;</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >k4a := Group( x, y );; SetName( k4a, "k4a" );</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >gens3a := [ (1 ,2 ), (2 ,3 ) ];;</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >s3a := Group( gens3a );; SetName( s3a, "s3a" );</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >alpha := GroupHomomorphismByImages( k4a, k4a, [x,y], [y,x] );;</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >beta := GroupHomomorphismByImages( k4a, k4a, [x,y], [x,z] );;</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >auta := Group( alpha, beta );;</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >acta := GroupHomomorphismByImages( s3a, auta, gens3a, [alpha,beta] );;</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >abmod := AbelianModuleObject( k4a, acta );;</span >
<span class="GAPprompt" >gap></span > <span class="GAPinput" >Xabmod := XModByAbelianModule( abmod );</span >
[k4a->s3a]
<span class="GAPprompt" >gap></span > <span class="GAPinput" >Display( Xabmod );</span >
Crossed module [k4a->s3a] :-
: Source group k4a has generators:
[ (6 ,7 )(8 ,9 ), (6 ,8 )(7 ,9 ) ]
: Range group s3a has generators:
[ (1 ,2 ), (2 ,3 ) ]
: Boundary homomorphism maps source generators to:
[ (), () ]
: Action homomorphism maps range generators to automorphisms:
(1 ,2 ) --> { source gens --> [ (6 ,8 )(7 ,9 ), (6 ,7 )(8 ,9 ) ] }
(2 ,3 ) --> { source gens --> [ (6 ,7 )(8 ,9 ), (6 ,9 )(7 ,8 ) ] }
These 2 automorphisms generate the group of automorphisms.
</pre ></div >
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