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Fix size issue in SpStabilizerOfIsotropicSubspace #128

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Fix size issue in SpStabilizerOfIsotropicSubspace #128

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aa0c647
Fix Error and Comment
TristanPfersdorff Jul 7, 2024
0e94d98
Add Sp isotropic test
TristanPfersdorff Jul 7, 2024
0ada065
Fix typo
TristanPfersdorff Jul 7, 2024
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10 changes: 7 additions & 3 deletions gap/ReducibleMatrixGroups.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -425,8 +425,10 @@ function(d, q, k)

SpDim := d - 2 * k;
if SpDim <> 0 then
# These two generators are diagonal block matrices with 2x2 blocks
# that generate a subgroup of Sp(d, q) corresponding to Sp(d - 2 * k, q).
# These two generators are diagonal block matrices with a 3x3 arrangement
# of blocks with the 3 diagonal blocks being square of size k, d - 2k and
# k respectively and the other blocks being zero. They generate a subgroup
# of Sp(d, q) corresponding to Sp(d - 2 * k, q).
for Spgen in GeneratorsOfGroup(Sp(SpDim, q)) do
Yi := IdentityMat(d, field);
Yi{[k + 1 .. d - k]}{[k + 1 .. d - k]} := Spgen;
Expand All @@ -442,7 +444,9 @@ function(d, q, k)

# This generator is in Sp(d, q) and stabilises the group generated by
# the first k unit vectors.
Add(gens, I + SquareSingleEntryMatrix(field, d, d, d - k) - SquareSingleEntryMatrix(field, d, k + 1, 1));
if SpDim <> 0 then
Add(gens, I + SquareSingleEntryMatrix(field, d, d, d - k) - SquareSingleEntryMatrix(field, d, k + 1, 1));
fi;

# Size according to Table 2.3 of [BHR13]
result := MatrixGroupWithSize(field, gens, q ^ (k * d + QuoInt(k - 3 * k * k, 2)) * SizeGL(k, q) * SizeSp(d - 2 * k, q));
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1 change: 1 addition & 0 deletions tst/standard/ReducibleMatrixGroups.tst
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Expand Up @@ -46,6 +46,7 @@ gap> TestSpStabilizerOfIsotropicSubspace := function(n, q, k)
> CheckSize(G);
> end;;
gap> TestSpStabilizerOfIsotropicSubspace(4, 2, 1);
gap> TestSpStabilizerOfIsotropicSubspace(4, 5, 2);
gap> TestSpStabilizerOfIsotropicSubspace(4, 9, 1);
gap> TestSpStabilizerOfIsotropicSubspace(6, 4, 1);
gap> TestSpStabilizerOfIsotropicSubspace(6, 7, 2);
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