GAP (HAP) - Chapter 11: Poincare series

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11 Poincare series

### 11 Poincare series

`EfficientNormalSubgroups(G)` `EfficientNormalSubgroups(G,k)` Inputs a prime-power group G and, optionally, a positive integer k. The default is k=4. The function returns a list of normal subgroups N in G such that the Poincare series for G equals the Poincare series for the direct product (N × (G/N)) up to degree k.
`ExpansionOfRationalFunction(f,n)` Inputs a positive integer n and a rational function f(x)=p(x)/q(x) where the degree of the polynomial p(x) is less than that of q(x). It returns a list [a_0 , a_1 , a_2 , a_3 , ... ,a_n] of the first n+1 coefficients of the infinite expansion f(x) = a_0 + a_1x + a_2x^2 + a_3x^3 + ... .
`PoincareSeries(G,n)` `PoincareSeries(R,n)` `PoincareSeries(L,n)` `PoincareSeries(G)` Inputs a finite p-group G and a positive integer n. It returns a quotient of polynomials f(x)=P(x)/Q(x) whose coefficient of x^k equals the rank of the vector space H_k(G,Z_p) for all k in the range k=1 to k=n. (The second input variable can be omitted, in which case the function tries to choose a "reasonable" value for n. For 2-groups the function PoincareSeriesLHS(G) can be used to produce an f(x) that is correct in all degrees.) In place of the group G the function can also input (at least n terms of) a minimal mod p resolution R for G. Alternatively, the first input variable can be a list L of integers. In this case the coefficient of x^k in f(x) is equal to the (k+1)st term in the list.
`PoincareSeriesPrimePart(G,p,n)` Inputs a finite group G, a prime p, and a positive integer n. It returns a quotient of polynomials f(x)=P(x)/Q(x) whose coefficient of x^k equals the rank of the vector space H_k(G,Z_p) for all k in the range k=1 to k=n. The efficiency of this function needs to be improved.
`PoincareSeriesLHS(G)` Inputs a finite 2-group G and returns a quotient of polynomials f(x)=P(x)/Q(x) whose coefficient of x^k equals the rank of the vector space H_k(G,Z_2) for all k. This function was written by Paul Smith. It use the Singular system for commutative algebra.
`Prank(G)` Inputs a p-group G and returns the rank of the largest elementary abelian subgroup.

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