GaloisCohomology: ERROR: Cannot promote to common finite field (doctest, julia 1.11)

[benlorenz]

Since this morning all runs of Run tests / doctest (1.11-nightly, ubuntu-latest) are failing in the experimental/GModule/src/GaloisCohomology.jl:1610-1632 doctest.
For example a re-run on master:
https://github.com/oscar-system/Oscar.jl/actions/runs/10560179378/job/29298281640#step:8:4424
The same happened on several different PRs, e.g:
https://github.com/oscar-system/Oscar.jl/actions/runs/10575052818/job/29302018540?pr=3996#step:8:4395
https://github.com/oscar-system/Oscar.jl/actions/runs/10567160335/job/29291853079#step:8:4391

Error output:

Error: doctest failure in ~/work/Oscar.jl/Oscar.jl/experimental/GModule/src/GaloisCohomology.jl:1610-1632

```jldoctest
julia> G = SL(2,5)
SL(2,5)

julia> T = character_table(G);

julia> R = gmodule(T[9])
G-module for G acting on vector space of dimension 6 over abelian closure of Q

julia> S = gmodule(CyclotomicField, R)
G-module for G acting on vector space of dimension 6 over cyclotomic field of order 5

julia> B, mB = relative_brauer_group(base_ring(S), character_field(S));

julia> B
Relative Brauer group for cyclotomic field of order 5 over number field of degree 1 over QQ

julia> b = B(S)
Element of relative Brauer group of number field of degree 1 over QQ
  <2, 2> -> 1//2 + Z
  <5, 5> -> 0 + Z
  Complex embedding of number field -> 1//2 + Z
```

Subexpression:

b = B(S)

Evaluated output:

ERROR: Cannot promote to common finite field
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:35
  [2] _promote(a::FqFieldElem, b::FqFieldElem)
    @ Nemo ~/.julia/packages/Nemo/QNyoV/src/flint/fq_default_extended.jl:889
  [3] *(x::FqFieldElem, y::FqFieldElem)
    @ Nemo ~/.julia/packages/Nemo/QNyoV/src/flint/fq_default.jl:443
  [4] dot(x::FqFieldElem, y::FqFieldElem)
    @ AbstractAlgebra ~/.julia/packages/AbstractAlgebra/zhruP/src/NCRings.jl:294
  [5] dot(x::Vector{FqFieldElem}, y::Vector{FqFieldElem})
    @ LinearAlgebra /opt/hostedtoolcache/julia/1.11-nightly/x64/share/julia/stdlib/v1.11/LinearAlgebra/src/generic.jl:894
  [6] frobenius_equation(X::Hecke.ArtinSchreierSolveCtx{FqMatrix, Vector{FqFieldElem}}, c::FqFieldElem)
    @ Hecke ~/.julia/packages/Hecke/LCgP9/src/LocalField/neq.jl:515
  [7] frobenius_equation2(c::Hecke.LocalFieldElem{Hecke.LocalFieldElem{QadicFieldElem, Hecke.EisensteinLocalField}, Hecke.UnramifiedLocalField}, F::Hecke.LocalField{QadicFieldElem, Hecke.EisensteinLocalField}; frobenius::Hecke.MapEvalCtx, start::Nothing)
    @ Hecke ~/.julia/packages/Hecke/LCgP9/src/LocalField/neq.jl:664
  [8] (::Hecke.var"#5238#5239"{Hecke.LocalField{QadicFieldElem, Hecke.EisensteinLocalField}, Int64, Hecke.LocalField{Hecke.LocalFieldElem{QadicFieldElem, Hecke.EisensteinLocalField}, Hecke.UnramifiedLocalField}})()
    @ Hecke ~/.julia/packages/Hecke/LCgP9/src/LocalField/neq.jl:649
  [9] with_precision(f::Hecke.var"#5238#5239"{Hecke.LocalField{QadicFieldElem, Hecke.EisensteinLocalField}, Int64, Hecke.LocalField{Hecke.LocalFieldElem{QadicFieldElem, Hecke.EisensteinLocalField}, Hecke.UnramifiedLocalField}}, K::QadicField, n::Int64)
    @ Nemo ~/.julia/packages/Nemo/QNyoV/src/flint/qadic.jl:969
 [10] setprecision(f::Function, K::QadicField, n::Int64)
    @ Nemo ~/.julia/packages/Nemo/QNyoV/src/flint/qadic.jl:977
 [11] setprecision(f::Function, K::Hecke.LocalField{QadicFieldElem, Hecke.EisensteinLocalField}, n::Int64)
    @ Hecke ~/.julia/packages/Hecke/LCgP9/src/LocalField/LocalField.jl:403
 [12] setprecision(f::Function, K::Hecke.LocalField{Hecke.LocalFieldElem{QadicFieldElem, Hecke.EisensteinLocalField}, Hecke.UnramifiedLocalField}, n::Int64)
    @ Hecke ~/.julia/packages/Hecke/LCgP9/src/LocalField/LocalField.jl:403
 [13] frobenius_equation(c::Hecke.LocalFieldElem{Hecke.LocalFieldElem{QadicFieldElem, Hecke.EisensteinLocalField}, Hecke.UnramifiedLocalField}, F::Hecke.LocalField{QadicFieldElem, Hecke.EisensteinLocalField}; frobenius::Hecke.MapEvalCtx)
    @ Hecke ~/.julia/packages/Hecke/LCgP9/src/LocalField/neq.jl:620
 [14] local_fundamental_class_serre(mKL::Hecke.LocalFieldMor{Hecke.LocalField{QadicFieldElem, Hecke.EisensteinLocalField}, Hecke.LocalField{QadicFieldElem, Hecke.EisensteinLocalField}, Hecke.MapDataFromLocalField{Hecke.LocalFieldElem{QadicFieldElem, Hecke.EisensteinLocalField}, Hecke.MapDataFromQadicField{Hecke.LocalFieldElem{QadicFieldElem, Hecke.EisensteinLocalField}}}, Hecke.MapDataFromLocalField{Hecke.LocalFieldElem{QadicFieldElem, Hecke.EisensteinLocalField}, Hecke.MapDataFromQadicField{Hecke.LocalFieldElem{QadicFieldElem, Hecke.EisensteinLocalField}}}, Hecke.LocalFieldElem{QadicFieldElem, Hecke.EisensteinLocalField}})
    @ Hecke ~/.julia/packages/Hecke/LCgP9/src/LocalField/neq.jl:873
 [15] local_index(CC::Vector{Oscar.GrpCoh.CoChain{2, PermGroupElem, Oscar.GrpCoh.MultGrpElem{AbsSimpleNumFieldElem}}}, P::AbsSimpleNumFieldOrderIdeal, mG::AbstractAlgebra.Generic.CompositeMap{PermGroup, Hecke.NfMorSet{AbsSimpleNumField}}; B::Oscar.GaloisCohomology_Mod.RelativeBrauerGroup, index_only::Bool)
    @ Oscar.GaloisCohomology_Mod ~/work/Oscar.jl/Oscar.jl/experimental/GModule/src/GaloisCohomology.jl:1365
 [16] local_index
    @ ~/work/Oscar.jl/Oscar.jl/experimental/GModule/src/GaloisCohomology.jl:1300 [inlined]
 [17] local_index(CC::Oscar.GrpCoh.CoChain{2, PermGroupElem, Oscar.GrpCoh.MultGrpElem{AbsSimpleNumFieldElem}}, P::AbsSimpleNumFieldOrderIdeal, mG::AbstractAlgebra.Generic.CompositeMap{PermGroup, Hecke.NfMorSet{AbsSimpleNumField}}; B::Oscar.GaloisCohomology_Mod.RelativeBrauerGroup, index_only::Bool)
    @ Oscar.GaloisCohomology_Mod ~/work/Oscar.jl/Oscar.jl/experimental/GModule/src/GaloisCohomology.jl:1228
 [18] (::Oscar.GaloisCohomology_Mod.RelativeBrauerGroup)(CC::Oscar.GrpCoh.CoChain{2, PermGroupElem, Oscar.GrpCoh.MultGrpElem{AbsSimpleNumFieldElem}})
    @ Oscar.GaloisCohomology_Mod ~/work/Oscar.jl/Oscar.jl/experimental/GModule/src/GaloisCohomology.jl:1575
 [19] (::Oscar.GaloisCohomology_Mod.var"#cocycle_to_elem#291")(C::Oscar.GrpCoh.CoChain{2, PermGroupElem, Oscar.GrpCoh.MultGrpElem{AbsSimpleNumFieldElem}})
    @ Oscar.GaloisCohomology_Mod ~/work/Oscar.jl/Oscar.jl/experimental/GModule/src/GaloisCohomology.jl:1722
 [20] (::Oscar.GaloisCohomology_Mod.var"#276#293"{Oscar.GaloisCohomology_Mod.var"#cocycle_to_elem#291"})(y::Oscar.GrpCoh.CoChain{2, PermGroupElem, Oscar.GrpCoh.MultGrpElem{AbsSimpleNumFieldElem}})
    @ Oscar.GaloisCohomology_Mod ~/work/Oscar.jl/Oscar.jl/experimental/GModule/src/GaloisCohomology.jl:1728
 [21] preimage(f::MapFromFunc{Oscar.GaloisCohomology_Mod.RelativeBrauerGroup, Oscar.GrpCoh.AllCoChains{2, PermGroupElem, Oscar.GrpCoh.MultGrpElem{AbsSimpleNumFieldElem}}}, y::Oscar.GrpCoh.CoChain{2, PermGroupElem, Oscar.GrpCoh.MultGrpElem{AbsSimpleNumFieldElem}})
    @ Hecke ~/.julia/packages/Hecke/LCgP9/src/Map/MapType.jl:202
 [22] (::Oscar.GaloisCohomology_Mod.RelativeBrauerGroup)(M::GModule{MatrixGroup{FqFieldElem, FqMatrix}, AbstractAlgebra.Generic.FreeModule{AbsSimpleNumFieldElem}})
    @ Oscar.GaloisCohomology_Mod ~/work/Oscar.jl/Oscar.jl/experimental/GModule/src/GaloisCohomology.jl:1735
 [23] top-level scope
    @ none:1

I don't think we had any releases of our dependencies and I did not see any differences for the package version between the first run on master
https://github.com/oscar-system/Oscar.jl/actions/runs/10560179378/job/29253105967
and the rerun
https://github.com/oscar-system/Oscar.jl/actions/runs/10560179378/job/29298281640

Only the julia version changed from 1.11.0-rc2.60 (0e423987803) to 1.11.0-rc3 (616e45539db) but but the only difference between those two is the version name since 0e423987803 is the merge of the rc3 backports.

I was unable to reproduce this locally.

cc: @Sequenzer

[thofma]

We had new releases for Nemo and AA yesterday, but it seems to not be the cause, as the versions agree in both runs.

[thofma]

With the help of @benlorenz I could reproduce it locally. I don't quite now why it started to happen now, but it seems to come down to something like the following:

julia> K, a = quadratic_field(5);

julia> P = prime_decomposition(maximal_order(K), 5)[1][1];

julia> C, mC = completion(K, P);

julia> K, mK = residue_field(C);

julia> X = Hecke.ArtinSchreierSolveCtx(K, 5);

julia> Hecke.frobenius_equation(X, K(1))
ERROR: Cannot promote to common finite field
Stacktrace:

@fieker: This should work, right?

@benlorenz Do you have both and rc2/rc3 around? If so, can you try if this also throws an error? (I don't know why it should work, but since the julia version change is our only hint so far, we should try it.)

[fieker]

On Tue, Aug 27, 2024 at 01:26:57PM -0700, Tommy Hofmann wrote: With the help of @benlorenz I could reproduce it locally. I don't quite now why it started to happen now, but it seems to come down to something like the following: ``` julia> K, a = quadratic_field(5); julia> P = prime_decomposition(maximal_order(K), 5)[1][1]; julia> C, mC = completion(K, P); julia> K, mK = residue_field(C); julia> X = Hecke.ArtinSchreierSolveCtx(K, 5); julia> Hecke.frobenius_equation(X, K(1)) ERROR: Cannot promote to common finite field Stacktrace: ``` @fieker: This should work, right?

My guess, just glancing, is that base_field or coefficient field of a prime field changed. The code si using absolute_frobenius_matrix and absolute_basis of s.th. that prints as prime field: julia> prime_field(K) == K false julia> K Prime field of characteristic 5

…

@benlorenz Do you have both and rc2/rc3 around? If so, can you try if this also throws an error? (I don't know why it should work, but since the julia version change is our only hint so far, we should try it.) -- Reply to this email directly or view it on GitHub: #4047 (comment) You are receiving this because you were mentioned. Message ID: ***@***.***>

[benlorenz]

@benlorenz Do you have both and rc2/rc3 around? If so, can you try if this also throws an error? (I don't know why it should work, but since the julia version change is our only hint so far, we should try it.)

This code seems to fail for rc3, rc2 and also 1.10.4.

[thofma]

Thanks for checking. I am working on a fix.

[thofma]

Should be fixed with Nemocas/Nemo.jl#1845

[lgoettgens]

Nemocas/Nemo.jl#1845 is now available in CI as Nemo v0.46.2. I restarted the same CI job as in the initial comment to see if the fix works: https://github.com/oscar-system/Oscar.jl/actions/runs/10560179378/job/29379752468

[thofma]

Seems to be fixed, see for example https://github.com/oscar-system/Oscar.jl/actions/runs/10600630264.

[thofma]

Maybe we should bump the Nemo version in the Project.toml for good measure?