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Use TryNextMethod instead of Error in Intersection2 if at least one group is finite #113

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Merged
fingolfin merged 1 commit into from
Mar 31, 2026
Merged

Use TryNextMethod instead of Error in Intersection2 if at least one group is finite #113

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3 changes: 3 additions & 0 deletions gap/pcpgrp/inters.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -151,5 +151,8 @@ function( U, V )
return NormalIntersection( V, U );
fi;

if IsFinite( U ) or IsFinite( V ) then
TryNextMethod();
fi;
Error("sorry: intersection for non-normal groups not yet installed");
end );
14 changes: 12 additions & 2 deletions tst/inters.tst
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Expand Up @@ -94,7 +94,7 @@ Pcp-group with orders [ 5, 3, 3, 3, 2 ]
gap> H2 := Subgroup(H,[h[1]*h[7], h[4]*h[5]]);
Pcp-group with orders [ 3, 3, 3, 3 ]
gap> J := Intersection(H1,H2);
Error, sorry: intersection for non-normal groups not yet installed
Pcp-group with orders [ 3, 3, 3 ]
gap> G1 := PreImagesNC(iso,H1);
Group([ f2, f3*f4, f6 ])
gap> G2 := PreImagesNC(iso,H2);
Expand All @@ -114,7 +114,7 @@ Pcp-group with orders [ 2, 3, 3, 3 ]
gap> H2 := Subgroup(H,[h[1]*h[7], h[4]*h[5]^2]);
Pcp-group with orders [ 2, 3, 3, 3 ]
gap> Intersection(H1,H2);
Error, sorry: intersection for non-normal groups not yet installed
Pcp-group with orders [ 3, 3 ]
gap> G1 := PreImagesNC(iso,H1);
Group([ f2, f3*f4, f6^2 ])
gap> G2 := PreImagesNC(iso,H2);
Expand All @@ -124,5 +124,15 @@ Group([ f6^2, f7^2 ])
gap> Image(iso,I);
Pcp-group with orders [ 3, 3 ]

# finite - infinite combination example where the intersection isn't impl. when represented as a pcp-group (non-normalizing case)
gap> G := DirectProduct(ExamplesOfSomePcpGroups(8), PcGroupToPcpGroup(PcGroupCode(2835879971,72)));;
gap> g := GeneratorsOfGroup(G);;
gap> H1 := Subgroup(G,[g[6]*g[9]^2]);
Pcp-group with orders [ 2, 2 ]
gap> H2 := Subgroup(G,[g[6],g[2]*g[7]]);
Pcp-group with orders [ 0, 2, 2 ]
gap> Intersection(H1,H2);
Pcp-group with orders [ 2 ]

#
gap> STOP_TEST( "inters.tst", 10000000);
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