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Speed up IrrBaumClausen (used for computing character tables of supersolvable groups, including $p$-groups) #4905

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merged 1 commit into from
Jun 24, 2022

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For the non-linear characters fewer multiplications of monomial matrices
are needed.
Also improved further details: multiplication of monomial matrices,
avoiding arithmetic with cyclotomics.

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This should never be slower than the previous version.
On my computer in the following example IrrBaumClausen (used to compute the irreducible characters of the group) becomes about 6 times faster:

gap> s:=SylowSubgroup(Sp(4,25),5);
ps<group of 4x4 matrices of size 390625 over GF(5^2)>
gap> ps:=Image(IsomorphismPcGroup(s));
Group([ f1, f2, f3, f4, f5, f6, f7, f8 ])
gap> ConjugacyClasses(ps);;time;
310
gap> BaumClausenInfo(ps);;time;
41
gap> ir:=IrrBaumClausen(ps);;time;
3608

Text for release notes

Speedup of IrrBaumClausen which is used by GAP to compute the character table of supersolvable (in particular p-) groups.

For the non-linear characters fewer multiplications of monomial matrices
are needed.
Also improved further details: multiplication of monomial matrices,
avoiding arithmetic with cyclotomics.
@ThomasBreuer ThomasBreuer self-requested a review June 24, 2022 08:09
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This looks good.

  • Avoiding arithmetic operations with cyclotomics is always a good idea; here this is done by creating the character values with CycList.
  • Using {} instead of a for loop yields a speedup (in spite of the additional intermediate list).
  • Memorizing partial products of the evaluation of a class representative and using the common initial part for the next representative is a good idea. (Since the lengths of the exponent vectors are in general much larger than the number of conjugacy classes, there are enough cases where common initial parts occur.)

@fingolfin fingolfin added kind: enhancement Label for issues suggesting enhancements; and for pull requests implementing enhancements topic: performance bugs or enhancements related to performance (improvements or regressions) release notes: to be added PRs introducing changes that should be (but have not yet been) mentioned in the release notes labels Jun 24, 2022
@fingolfin fingolfin merged commit 5061063 into gap-system:master Jun 24, 2022
@fingolfin fingolfin changed the title Speedup IrrBaumClausen Speed up of IrrBaumClausen which is used to compute the character table of supersolvable groups (in particular p-groups) Aug 17, 2022
@fingolfin fingolfin changed the title Speed up of IrrBaumClausen which is used to compute the character table of supersolvable groups (in particular p-groups) Speed up of IrrBaumClausen which is used to compute character tables of supersolvable groups (in particular p-groups) Aug 17, 2022
@fingolfin fingolfin changed the title Speed up of IrrBaumClausen which is used to compute character tables of supersolvable groups (in particular p-groups) Speed up of IrrBaumClausen (used for computing character tables of supersolvable groups, including $p$-groups) Aug 17, 2022
@fingolfin fingolfin added release notes: use title For PRs: the title of this PR is suitable for direct use in the release notes and removed release notes: to be added PRs introducing changes that should be (but have not yet been) mentioned in the release notes labels Aug 17, 2022
@fingolfin fingolfin changed the title Speed up of IrrBaumClausen (used for computing character tables of supersolvable groups, including $p$-groups) Speed up IrrBaumClausen (used for computing character tables of supersolvable groups, including $p$-groups) Aug 17, 2022
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