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Change IsomorphismPermGroupForMatrixGroup to always use NiceMonomorphism if available #6244

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Merged
fingolfin merged 1 commit into from
Mar 23, 2026
Merged

Change IsomorphismPermGroupForMatrixGroup to always use NiceMonomorphism if available #6244

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17 changes: 13 additions & 4 deletions lib/grpmat.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -536,14 +536,23 @@ end );
BindGlobal( "IsomorphismPermGroupForMatrixGroup",
function(G)
local map;
if not HasNiceMonomorphism(G) or not IsPermGroup(Range(NiceMonomorphism(G))) then
if not HasIsFinite(G) then
Info(InfoWarning,1,
"IsomorphismPermGroup: The group is not known to be finite");

if HasNiceMonomorphism( G ) then
map:= NiceMonomorphism( G );
if not IsPermGroup( Range( map ) ) then
# Trust GAP that this map is still useful.
map:= CompositionMapping( IsomorphismPermGroup( Image( map ) ), map );
fi;
else
# We cannot be sure about `IsHandledByNiceMonomorphism` for `G`.
if not HasIsFinite( G ) then
Info( InfoWarning,1,
"IsomorphismPermGroup: The group is not known to be finite" );
fi;
map:=NicomorphismOfGeneralMatrixGroup(G,false,false);
SetNiceMonomorphism(G,map);
fi;
# Now `G` stores a `NiceMonomorphism`.
if IsPermGroup(Range(NiceMonomorphism(G))) then
map:=RestrictedNiceMonomorphism(G);
fi;
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