<h4>2.1 <span class="Heading">The main functions</span></h4>
<p>Let <span class="SimpleMath">\(G\)</span> be a <span class="SimpleMath">\(T\)</span>-group and <span class="SimpleMath">\(G^\)</span> its <span class="SimpleMath">\(\Q\)</span>-powered hull. In this chapter we describe functionality for setting up the Mal'cev correspondence between \(G^\) and the Lie algebra \(L(G)\). The data structures needed for computations with \(G^\) and \(L(G)\) are stored in a so-called Mal'cev object. Computational representations of elements of <span class="SimpleMath">\(G^\)</span>, respectively <span class="SimpleMath">\(L(G)\)</span>, will be called Mal'cev group elements, respectively Mal'cev Lie elements.</p>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ MalcevObjectByTGroup</code>( <var class="Arg">N</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>If <var class="Arg">N</var> is a a <var class="Arg">T</var>-group (i.e. a finitely generated torsion-free nilpotent group), given by a polycyclic presentation with respect to a Mal'cev basis, then this function computes the Mal'cev correspondence for <var class="Arg">N</var> and stores the result in a so-called Mal'cev object. Otherwise this function returns `fail'. In the moment this function is restricted to groups <var class="Arg">N</var> of nilpotency class at most 9.</p>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ UnderlyingGroup</code>( <var class="Arg">mo</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For a Mal'cev object mo this function returns the T-group, which was used to build mo.
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ UnderlyingLieAlgebra</code>( <var class="Arg">mo</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For a Mal'cev object mo this function returns the Lie algebra, which underlies the correspondence described by mo.
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ Dimension</code>( <var class="Arg">mo</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>Returns the dimension of the Lie algebra that underlies the Mal'cev object mo.
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ MalcevGrpElementByExponents</code>( <var class="Arg">mo</var>, <var class="Arg">exps</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For a Mal'cev object mo and an exponent vector exps with rational entries, this functions returns the Mal'cev group element, which has exponents <var class="Arg">exps</var> with respect to the Mal'cev basis of the underlying group of mo.
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ MalcevLieElementByCoefficients</code>( <var class="Arg">mo</var>, <var class="Arg">coeffs</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For a Mal'cev object mo and a coefficient vector coeffs with rational entries, this functions returns the Mal'cev Lie element, which has coefficients <var class="Arg">coeffs</var> with respect to the basis of the underlying Lie algebra of <var class="Arg">mo</var>.</p>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ RandomGrpElm</code>( <var class="Arg">mo</var>, <var class="Arg">range</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For a Mal'cev object mo this function returns the output of MalcevGrpElementByExponents( mo, exps ), where exps is an exponent vector whose entries are randomly chosen integers between -range and range.
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ RandomLieElm</code>( <var class="Arg">mo</var>, <var class="Arg">range</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For a Mal'cev object mo this function returns the output of MalcevLieElementByExponents( mo, coeffs ), where coeffs is a coefficient vector whose entries are randomly chosen integers between -range and range.
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ Log</code>( <var class="Arg">g</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For Mal'cev group element g this function returns the corresponding Mal'cev Lie element.</p>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ Exp</code>( <var class="Arg">x</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For Mal'cev Lie element x this function returns the corresponding Mal'cev group element.</p>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ Comm</code>( <var class="Arg">x</var>, <var class="Arg">y</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>If <var class="Arg">x</var>,<var class="Arg">y</var> are Mal'cev group elements, then this function returns the group theoretic commutator of x and y. If x,y are Mal'cev Lie elements, then this function returns the Lie commutator of <var class="Arg">x</var> and <var class="Arg">y</var>.</p>
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ MalcevSymbolicGrpElementByExponents</code>( <var class="Arg">mo</var>, <var class="Arg">exps</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For a Mal'cev object mo and an exponent vector exps with rational indeterminates as entries, this functions returns the Mal'cev group element, which has exponents <var class="Arg">exps</var> with respect to the Mal'cev basis of the underlying group of mo.
<div class="func"><table class="func" width="100%"><tr><td class="tdleft"><code class="func">‣ MalcevLieElementByCoefficients</code>( <var class="Arg">mo</var>, <var class="Arg">coeffs</var> )</td><td class="tdright">( function )</td></tr></table></div>
<p>For a Mal'cev object mo and a coefficient vector coeffs with rational indeterminates as entries, this functions returns the Mal'cev Lie element, which has coefficients <var class="Arg">coeffs</var> with respect to the basis of the underlying Lie algebra of <var class="Arg">mo</var>.</p>
<h4>2.2 <span class="Heading">An example application</span></h4>
<div class="example"><pre>
gap> n := 2;
2
gap> F := FreeGroup( n );
<free group on the generators [ f1, f2 ]>
gap> c := 3;
3
gap> N := NilpotentQuotient( F, c );
Pcp-group with orders [ 0, 0, 0, 0, 0 ]
gap> mo := MalcevObjectByTGroup( N );
<<Malcev object of dimension 5>>
gap> dim := Dimension( mo );
5
gap> UnderlyingGroup( mo );
Pcp-group with orders [ 0, 0, 0, 0, 0 ]
gap> UnderlyingLieAlgebra( mo );
<Lie algebra of dimension 5 over Rationals>
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