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Recognise SL2 in naming #457

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e50ee94
Recognise SL2 in naming
fingolfin Apr 15, 2026
7bec1f8
replace call to RECOG.IsThisSL2Natural
fingolfin May 27, 2026
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23 changes: 23 additions & 0 deletions gap/matrix/classical.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -1136,6 +1136,27 @@ function(recognise)
fi;
end);

BindRecogMethod("FindHomMethodsClassical", "Degree2SL",
"tests whether a degree-2 group contains SL",
function(recognise)
if recognise.d <> 2 then
return NeverApplicable;
fi;

if RECOG.IsThisSL2Natural(GeneratorsOfGroup(recognise.grp),
recognise.field) then
recognise.isGeneric := false;
recognise.isReducible := false;
recognise.isNotExt := true;
recognise.isSLContained := true;
recognise.isSpContained := true;
Info(InfoClassical,2,"The group contains SL(2, ", recognise.q, ");");
return Success;
fi;

return TemporaryFailure;
end);

## Main function to test whether group contains SL
BindRecogMethod("FindHomMethodsClassical", "IsSLContained",
"tests whether group contains SL",
Expand Down Expand Up @@ -2436,6 +2457,8 @@ AddMethod(ClassicalMethDb, FindHomMethodsClassical.NonGenericOrthogonalMinus, 14

AddMethod(ClassicalMethDb, FindHomMethodsClassical.NonGenericOrthogonalCircle, 15);

AddMethod(ClassicalMethDb, FindHomMethodsClassical.Degree2SL, 16);

AddMethod(ClassicalMethDb, FindHomMethodsClassical.IsSLContained, 16);

AddMethod(ClassicalMethDb, FindHomMethodsClassical.IsSpContained, 17);
Expand Down
15 changes: 4 additions & 11 deletions gap/projective/classicalnatural.gi
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Expand Up @@ -854,17 +854,10 @@ function(ri)
RECOG.SetPseudoRandomStamp(g,"ClassicalNatural");

# First check whether we are applicable:
if d = 2 then
if not RECOG.IsThisSL2Natural(GeneratorsOfGroup(g),f) then
Info(InfoRecog,2,"ClassicalNatural: Is not PSL_2.");
return TemporaryFailure; # FIXME: TemporaryFailure here really correct?
fi;
else
classical := RecogniseClassical(g);
if classical.isSLContained <> true then
Info(InfoRecog,2,"ClassicalNatural: Is not PSL.");
return TemporaryFailure; # FIXME: TemporaryFailure here really correct?
fi;
classical := RecogniseClassical(g);
if classical.isSLContained <> true then
Info(InfoRecog,2,"ClassicalNatural: Is not PSL.");
return TemporaryFailure; # FIXME: TemporaryFailure here really correct?
fi;

# Now get rid of nasty determinants:
Expand Down
8 changes: 8 additions & 0 deletions tst/working/quick/classical.tst
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Expand Up @@ -18,3 +18,11 @@ gap> SetX(G, G, function(g,h)
> return gh = false or (IsEqualProjective(gh[1], g) and IsEqualProjective(gh[2], h));
> end);
[ true ]

# issue #345: degree-2 linear groups should still report containment of SL
gap> ri := RecogniseClassical(SL(2,37));;
gap> ri.isSLContained;
true
gap> ri := RecogniseClassical(SL(2,25));;
gap> ri.isSLContained;
true