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Faster high-level Meataxe functions for irreducible modules (e.g. MTX.IsomorphismModules, MTX.Indecomposition, MTX.BasisModuleEndomorphisms, MTX.BasisModuleHomomorphisms) #6276

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fingolfin merged 2 commits into from
Mar 23, 2026
+19 −1
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Faster high-level Meataxe functions for irreducible modules (e.g. MTX.IsomorphismModules, MTX.Indecomposition, MTX.BasisModuleEndomorphisms, MTX.BasisModuleHomomorphisms) #6276

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0e5e7c4
Faster high-level Meataxe functions for irreducible modules
fingolfin Mar 23, 2026
fa209b8
Update tst/testbugfix/2026-03-23-meataxe.tst
fingolfin Mar 23, 2026
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8 changes: 7 additions & 1 deletion lib/meatauto.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -1320,6 +1320,10 @@ local n, hc1, hc2, nc, b1, b2, map, remain, j, found, hom, i, k;
Length(SMTX.BasisEndomorphismsRadical(M2)) ) then
# different endomorphism algebra dimensions
return fail;
elif MTX.IsIrreducible(M1) and MTX.IsIrreducible(M2) then
return MTX.IsomorphismIrred(M1, M2);
elif MTX.IsIrreducible(M1) <> MTX.IsIrreducible(M2) then
return fail;
fi;

hc1:=SMTX.HomogeneousComponents(M1);
Expand Down Expand Up @@ -1637,7 +1641,9 @@ end;
SMTX.BasisModuleHomomorphisms:=function(m1,m2)
local b;
TestModulesFitTogether(m1,m2);
if m1.dimension>5 then
if MTX.IsIrreducible(m1) then
b:=MTX.Homomorphisms(m1,m2);
elif m1.dimension>5 then
b:= SpinHom(m1,m2);
Assert(1,Length(b)=Length(SmalldimHomomorphismsModules(m1,m2)));
else
Expand Down
12 changes: 12 additions & 0 deletions tst/testbugfix/2026-03-23-meataxe.tst
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@@ -0,0 +1,12 @@
# Handling of irreducible Meataxe modules with lots of generators was far
# slower than necessary at least via the "high level" Meataxe APIs.
# See also <https://github.com/gap-system/gap/issues/6271>.
#
# The main test here is for performance: before the fix for issue #6271 this
# would have run for minutes, with the fix it should take far less than a
# second on a modern computer.
gap> n:=200;; # increase this to make the effect even stronger
gap> G:=GL(56,GF(25));;
gap> H:=Subgroup(G, Concatenation(GeneratorsOfGroup(G),List([1..n],i->PseudoRandom(G))));;
gap> MTX.IsomorphismModules(NaturalGModule(H), NaturalGModule(H)) <> fail;
true
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