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<div class="ChapSects"><a href="chap13.html#X85A9B66278AF63D9">13. <span class="Heading"> Cocycles</span></a>
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<h3>13. <span class="Heading"> Cocycles</span></h3>
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<td class="tdleft"><code class="code"> CcGroup(A,f) </code></p>
<p>Inputs a G-module A (i.e. an abelian G-outer group) and a standard 2-cocycle f G x G ---> A. It returns the extension group determined by the cocycle. The group is returned as a CcGroup.</p>
<p>This is a HAPcocyclic function and thus only works when HAPcocyclic is loaded.</td>
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<td class="tdleft"><code class="code"> CocycleCondition(R,n) </code></p>
<p>Inputs a resolution R and an integer n>0. It returns an integer matrix M with the following property. Suppose d=R.dimension(n). An integer vector f=[f_1, ... , f_d] then represents a ZG-homomorphism R_n --> Z_q which sends the ith generator of R_n to the integer f_i in the trivial ZG-module Z_q (where possibly q=0 ). The homomorphism f is a cocycle if and only if M^tf=0 mod q.</td>
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<td class="tdleft"><code class="code"> StandardCocycle(R,f,n) </code> <br /> <code class="code"> StandardCocycle(R,f,n,q) </code></p>
<p>Inputs a ZG-resolution R (with contracting homotopy), a positive integer n and an integer vector f representing an n-cocycle R_n --> Z_q where G acts trivially on Z_q. It is assumed q=0 unless a value for q is entered. The command returns a function F(g_1, ..., g_n) which is the standard cocycle G_n --> Z_q corresponding to f. At present the command is implemented only for n=2 or 3.</td>
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<td class="tdleft"><code class="code"> Syzygy(R,g) </code></p>
<p>Inputs a ZG-resolution R (with contracting homotopy) and a list g = [g[1], ..., g[n]] of elements in G. It returns a word w in R_n. The word w is the image of the n-simplex in the standard bar resolution corresponding to the n-tuple g. This function can be used to construct explicit standard n-cocycles. (Currently implemented only for n<4.)</td>
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