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# HH^ZZ(SimplicialComplex,Ring) -- compute the reduced cohomology of an abstract simplicial complex

## Synopsis

• Function: cohomology
• Usage:
cohomology(k, Delta, R)
• Inputs:
• Optional inputs:
• Degree => an integer, default value 0, is ignored by this particular function
• Outputs:
• , that is reduced cohomology group of $\Delta$ with coefficients in $R$

## Description

Each abstract simplicial complex $\Delta$ determines a chain complex $\widetilde C(\Delta, k)$ of free modules over its coefficient ring $k$. For all integers $i$, the $i$-th term of $\widetilde C(\Delta, k)$ has a basis corresponding to the $i$-dimensional faces of $\Delta$. The reduced cohomology of $\Delta$ with coefficients in $R$ is, by definition, the cohomology of the chain complex $\operatorname{Hom}( \widetilde C(\Delta, k) \otimes R, R)$.

The 2-sphere has vanishing first cohomology, but non-trivial second cohomology.

 i1 : S = ZZ[x_0..x_4]; i2 : Δ = skeleton(2, simplexComplex(3,S)); i3 : prune cohomology(1, Δ) o3 = 0 o3 : ZZ-module i4 : prune cohomology(2, Δ) 1 o4 = ZZ o4 : ZZ-module, free

A figure eight has rank two first cohomology.

 i5 : figureEight = simplicialComplex {x_0*x_1, x_0*x_2, x_1*x_2, x_2*x_3, x_2*x_4, x_3*x_4} o5 = simplicialComplex | x_3x_4 x_2x_4 x_2x_3 x_1x_2 x_0x_2 x_0x_1 | o5 : SimplicialComplex i6 : prune cohomology(1, figureEight) 2 o6 = ZZ o6 : ZZ-module, free