Skip to content

Fix wrong result of isomorphism(FPGroup, G, on_gens = true) for trivial G with more than 0 generators #4378

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
fingolfin merged 2 commits into from
Dec 3, 2024
Merged

Fix wrong result of isomorphism(FPGroup, G, on_gens = true) for trivial G with more than 0 generators #4378

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
1d7dc83
fix `isomorphism(FPGroup, G, on_gens = true)`
ThomasBreuer Dec 3, 2024
110a2ae
Skip workaround
fingolfin Dec 3, 2024
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
8 changes: 1 addition & 7 deletions src/Groups/homomorphisms.jl
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -570,13 +570,7 @@ function isomorphism(::Type{FPGroup}, G::GAPGroup; on_gens::Bool=false)
# Known isomorphisms are cached in the attribute `:isomorphisms`.
isos = get_attribute!(Dict{Tuple{Type, Bool}, Any}, G, :isomorphisms)::Dict{Tuple{Type, Bool}, Any}
return get!(isos, (FPGroup, on_gens)) do
if is_trivial(G)
# TODO: remove this special treatment as soon as the change from
# https://github.com/gap-system/gap/pull/5700 is available in Oscar
# (not yet in GAP 4.13.0)
f = GAP.Globals.GroupHomomorphismByImages(GapObj(G), GAP.Globals.FreeGroup(0), GAP.Obj([]), GAP.Obj([]))
GAP.Globals.SetIsBijective(f, true)
elseif on_gens
if on_gens
Ggens = GAPWrap.GeneratorsOfGroup(GapObj(G))
# The computations are easy if `Ggens` is a pcgs,
# otherwise GAP will call `CoKernel`.
Expand Down
8 changes: 7 additions & 1 deletion test/Groups/homomorphisms.jl
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -456,7 +456,13 @@ end
@test [preimage(f2, x) for x in gens(codomain(f2))] == gens(G)
@test [preimage(f, x) for x in gens(codomain(f))] != gens(G)

@test is_bijective(isomorphism(FPGroup, symmetric_group(1), on_gens = true))
G = symmetric_group(1)
iso = @inferred isomorphism(FPGroup, G, on_gens = true)
@test ngens(G) == ngens(codomain(iso))
@test is_bijective(iso)
G = sub(G, [one(G)])[1]
iso = @inferred isomorphism(FPGroup, G, on_gens = true)
@test ngens(G) == ngens(codomain(iso))

G = abelian_group(PermGroup, [2, 2])
f = @inferred isomorphism(FinGenAbGroup, G)
Expand Down