From aaf486ffbc4abf18dc3ce8a3a779e0ba8bb0c235 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Fri, 6 Sep 2024 13:10:47 +0200
Subject: [PATCH 01/17] Add `adjoint_matrix`

---
 experimental/LieAlgebras/src/LieAlgebra.jl    | 27 +++++++++++++++++++
 experimental/LieAlgebras/src/exports.jl       |  1 +
 .../LieAlgebras/test/LieAlgebra-test.jl       |  9 +++++++
 3 files changed, 37 insertions(+)

diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index 1ed5624f7324..49daa3aee9b3 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -432,6 +432,33 @@ Return `true` if `L` is solvable, i.e. the derived series of `L` terminates in $
   return dim(derived_series(L)[end]) == 0
 end
 
+###############################################################################
+#
+#   Adjoint elements
+#
+###############################################################################
+
+@attr Vector{dense_matrix_type(C)} function adjoint_matrices(
+  L::LieAlgebra{C}
+) where {C<:FieldElem}
+  return map(1:dim(L)) do i
+    x = basis(L, i)
+    A = zero_matrix(coefficient_ring(L), dim(L), dim(L))
+    for (j, bj) in enumerate(basis(L))
+      A[j, :] = _matrix(bracket(x, bj))
+    end
+    return A
+  end
+end
+
+function adjoint_matrix(x::LieAlgebraElem{C}) where {C<:FieldElem}
+  L = parent(x)
+  return sum(
+    c * g for (c, g) in zip(coefficients(x), adjoint_matrices(L));
+    init=zero_matrix(coefficient_ring(L), dim(L), dim(L)),
+  )
+end
+
 ###############################################################################
 #
 #   Root system getters
diff --git a/experimental/LieAlgebras/src/exports.jl b/experimental/LieAlgebras/src/exports.jl
index 18a021ab3bff..56ea2eca6632 100644
--- a/experimental/LieAlgebras/src/exports.jl
+++ b/experimental/LieAlgebras/src/exports.jl
@@ -20,6 +20,7 @@ export WeylOrbitIterator
 
 export abelian_lie_algebra
 export abstract_module
+export adjoint_matrix
 export base_lie_algebra
 export bilinear_form
 export bracket
diff --git a/experimental/LieAlgebras/test/LieAlgebra-test.jl b/experimental/LieAlgebras/test/LieAlgebra-test.jl
index 121d73446ccb..65aa0d63560b 100644
--- a/experimental/LieAlgebras/test/LieAlgebra-test.jl
+++ b/experimental/LieAlgebras/test/LieAlgebra-test.jl
@@ -93,4 +93,13 @@
     @test !is_nilpotent(L)
     @test !is_solvable(L)
   end
+
+  @testset "adjoint_matrix" begin
+    @testset for n in 2:4, F in [QQ, GF(2), GF(3)]
+      L = general_linear_lie_algebra(F, n)
+      x = L(rand(-10:10, dim(L)))
+      y = L(rand(-10:10, dim(L)))
+      @test coefficients(x * y) == coefficients(y) * adjoint_matrix(x)
+    end
+  end
 end

From 16d1a8bb3e9737a9431e962c686644aa68aac4da Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Fri, 6 Sep 2024 14:01:21 +0200
Subject: [PATCH 02/17] Add `killing_matrix`

---
 experimental/LieAlgebras/src/LieAlgebra.jl       | 13 +++++++++++++
 experimental/LieAlgebras/src/exports.jl          |  1 +
 experimental/LieAlgebras/test/LieAlgebra-test.jl | 12 ++++++++++++
 3 files changed, 26 insertions(+)

diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index 49daa3aee9b3..b570855983bc 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -459,6 +459,19 @@ function adjoint_matrix(x::LieAlgebraElem{C}) where {C<:FieldElem}
   )
 end
 
+@attr dense_matrix_type(C) function killing_matrix(L::LieAlgebra{C}) where {C<:FieldElem}
+  R = coefficient_ring(L)
+  A = zero_matrix(R, dim(L), dim(L))
+  for (i, adxi) in enumerate(adjoint_matrices(L))
+    for (j, adxj) in enumerate(adjoint_matrices(L))
+      i > j && continue # killing form is symmetric
+      val = tr(adxi * adxj)
+      A[j, i] = A[i, j] = val
+    end
+  end
+  return A
+end
+
 ###############################################################################
 #
 #   Root system getters
diff --git a/experimental/LieAlgebras/src/exports.jl b/experimental/LieAlgebras/src/exports.jl
index 56ea2eca6632..cb07ee4db216 100644
--- a/experimental/LieAlgebras/src/exports.jl
+++ b/experimental/LieAlgebras/src/exports.jl
@@ -66,6 +66,7 @@ export is_simple_coroot
 export is_simple_coroot_with_index
 export is_simple_root
 export is_simple_root_with_index
+export killing_matrix
 export lie_algebra
 export lmul, lmul!
 export longest_element
diff --git a/experimental/LieAlgebras/test/LieAlgebra-test.jl b/experimental/LieAlgebras/test/LieAlgebra-test.jl
index 65aa0d63560b..00d07c14bbf0 100644
--- a/experimental/LieAlgebras/test/LieAlgebra-test.jl
+++ b/experimental/LieAlgebras/test/LieAlgebra-test.jl
@@ -102,4 +102,16 @@
       @test coefficients(x * y) == coefficients(y) * adjoint_matrix(x)
     end
   end
+
+  @testset "Killing matrix" begin
+    # Example from Hum72, Ch. 5.1
+    F = QQ
+    L = lie_algebra(
+      F,
+      2,
+      [matrix(F, [0 1; 0 0]), matrix(F, [1 0; 0 -1]), matrix(F, [0 0; 1 0])],
+      [:x, :h, :y],
+    )
+    @test killing_matrix(L) == matrix(F, [0 0 4; 0 8 0; 4 0 0])
+  end
 end

From aee118ac4935699f4d311dfb9a993be75b50eb9c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Fri, 6 Sep 2024 18:02:58 +0200
Subject: [PATCH 03/17] Add `any_non_ad_nilpotent_element`

---
 Project.toml                                  |  2 +-
 docs/oscar_references.bib                     | 11 +++++
 experimental/LieAlgebras/src/LieAlgebra.jl    | 46 +++++++++++++++++++
 experimental/LieAlgebras/src/LieSubalgebra.jl | 12 +++++
 experimental/LieAlgebras/src/exports.jl       |  2 +
 5 files changed, 72 insertions(+), 1 deletion(-)

diff --git a/Project.toml b/Project.toml
index ad8dff36a42c..65d51ce667e6 100644
--- a/Project.toml
+++ b/Project.toml
@@ -25,7 +25,7 @@ UUIDs = "cf7118a7-6976-5b1a-9a39-7adc72f591a4"
 cohomCalg_jll = "5558cf25-a90e-53b0-b813-cadaa3ae7ade"
 
 [compat]
-AbstractAlgebra = "0.42.3"
+AbstractAlgebra = "0.42.5"
 AlgebraicSolving = "0.5.1"
 Distributed = "1.6"
 GAP = "0.11.3"
diff --git a/docs/oscar_references.bib b/docs/oscar_references.bib
index 3e4463c66139..068a9f9c2e78 100644
--- a/docs/oscar_references.bib
+++ b/docs/oscar_references.bib
@@ -1091,6 +1091,17 @@ @InProceedings{GHJ16
   year          = {2016}
 }
 
+@Article{GIR96,
+  author        = {de Graaf, Willem A. and Ivanyos, Gábor and Rónyai, Lajos},
+  title         = {Computing Cartan subalgebras of Lie algebras},
+  journal       = {Appl. Algebra Eng. Commun. Comput.},
+  volume        = {7},
+  number        = {5},
+  pages         = {339--349},
+  year          = {1996},
+  doi           = {10.1007/BF01293593}
+}
+
 @InCollection{GJ00,
   author        = {Gawrilow, Ewgenij and Joswig, Michael},
   title         = {polymake: a Framework for Analyzing Convex Polytopes},
diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index b570855983bc..2b44085d533f 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -472,6 +472,52 @@ end
   return A
 end
 
+@doc raw"""
+    is_ad_nilpotent(x::LieAlgebraElem{C}) -> Bool
+
+Return whether `x` is ad-nilpotent, i.e. whether the linear operator $\mathrm{ad}(x)$ is nilpotent.
+"""
+function is_ad_nilpotent(x::LieAlgebraElem{C}) where {C<:FieldElem}
+  return is_nilpotent(adjoint_matrix(x))
+end
+
+@doc raw"""
+    any_non_ad_nilpotent_element(L::LieAlgebra{C}) -> LieAlgebraElem{C}
+
+Return an element of `L` that is not ad-nilpotent, or the zero element if all elements are ad-nilpotent.
+
+The used algorithm is described in [GIR96; Ch. 3](@cite).
+"""
+function any_non_ad_nilpotent_element(L::LieAlgebra{C}) where {C<:FieldElem}
+  if dim(L) <= 1
+    # L is abelian and hence nilpotent
+  elseif characteristic(L) == 0
+    for x in basis(L)
+      !is_ad_nilpotent(x) && return x
+    end
+    for (i, x) in enumerate(basis(L))
+      for (j, y) in enumerate(basis(L))
+        i > j && continue
+        xy = x * y
+        !is_ad_nilpotent(xy) && return xy
+      end
+    end
+  else # characteristic > 0
+    x = basis(L, 1)
+    !is_ad_nilpotent(x) && return x
+    K = sub(L, [x]; is_basis=true)
+    while dim(K) < dim(L)
+      # find an element b in L \ K with [b,K]⊆K
+      N = normalizer(L, K)
+      b = basis(N, findfirst(b -> !(b in K), basis(N)))
+      !is_ad_nilpotent(b) && return b
+      K = sub(L, [basis(K); b]; is_basis=true)
+    end
+  end
+  set_attribute!(L, :is_nilpotent, true)
+  return zero(L)
+end
+
 ###############################################################################
 #
 #   Root system getters
diff --git a/experimental/LieAlgebras/src/LieSubalgebra.jl b/experimental/LieAlgebras/src/LieSubalgebra.jl
index d9267e1403b0..574aae2de7e6 100644
--- a/experimental/LieAlgebras/src/LieSubalgebra.jl
+++ b/experimental/LieAlgebras/src/LieSubalgebra.jl
@@ -211,6 +211,18 @@ function is_self_normalizing(S::LieSubalgebra)
   return normalizer(base_lie_algebra(S), S) == S
 end
 
+###############################################################################
+#
+#   More misc stuff
+#
+###############################################################################
+
+function any_non_ad_nilpotent_element(S::LieSubalgebra)
+  LS, emb = lie_algebra(S)
+  x = any_non_ad_nilpotent_element(LS)
+  return emb(x)
+end
+
 ###############################################################################
 #
 #   Conversion
diff --git a/experimental/LieAlgebras/src/exports.jl b/experimental/LieAlgebras/src/exports.jl
index cb07ee4db216..1c414f817eff 100644
--- a/experimental/LieAlgebras/src/exports.jl
+++ b/experimental/LieAlgebras/src/exports.jl
@@ -21,6 +21,7 @@ export WeylOrbitIterator
 export abelian_lie_algebra
 export abstract_module
 export adjoint_matrix
+export any_non_ad_nilpotent_element
 export base_lie_algebra
 export bilinear_form
 export bracket
@@ -46,6 +47,7 @@ export fundamental_weights
 export general_linear_lie_algebra
 export induced_map_on_symmetric_power
 export induced_map_on_tensor_power
+export is_ad_nilpotent
 export is_cartan_matrix
 export is_cartan_type
 export is_coroot

From 7af7e889808e7c9eb4ae0cf09d19325eb9348239 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Mon, 9 Sep 2024 11:24:12 +0200
Subject: [PATCH 04/17] Add `engel_subalgebra` and `cartan_subalgebra`

---
 docs/oscar_references.bib                  | 11 ++++
 experimental/LieAlgebras/src/LieAlgebra.jl | 69 ++++++++++++++++++++++
 experimental/LieAlgebras/src/exports.jl    |  2 +
 3 files changed, 82 insertions(+)

diff --git a/docs/oscar_references.bib b/docs/oscar_references.bib
index 068a9f9c2e78..1e9ada469139 100644
--- a/docs/oscar_references.bib
+++ b/docs/oscar_references.bib
@@ -1235,6 +1235,17 @@ @Article{Gat96
   doi           = {10.1007/BF01191379}
 }
 
+@Book{Gra00,
+  author        = {de Graaf, Willem A.},
+  title         = {Lie algebras: theory and algorithms},
+  series        = {North-Holland Mathematical Library},
+  volume        = {56},
+  publisher     = {North-Holland Publishing Co., Amsterdam},
+  pages         = {xii+393},
+  year          = {2000},
+  url           = {https://www.sciencedirect.com/bookseries/north-holland-mathematical-library/vol/56/}
+}
+
 @PhDThesis{Gro20,
   author        = {Gromada, Daniel},
   title         = {Compact matrix quantum groups and their representation categories},
diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index 2b44085d533f..1c017c7cecc9 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -481,6 +481,12 @@ function is_ad_nilpotent(x::LieAlgebraElem{C}) where {C<:FieldElem}
   return is_nilpotent(adjoint_matrix(x))
 end
 
+###############################################################################
+#
+#   Root system detection
+#
+###############################################################################
+
 @doc raw"""
     any_non_ad_nilpotent_element(L::LieAlgebra{C}) -> LieAlgebraElem{C}
 
@@ -518,6 +524,69 @@ function any_non_ad_nilpotent_element(L::LieAlgebra{C}) where {C<:FieldElem}
   return zero(L)
 end
 
+@doc raw"""
+    engel_subalgebra(x::LieAlgebraElem{C}) -> LieSubalgebra{C,elem_type(parent(x))}
+
+Return the Engel subalgebra of `x`, i.e. the generalized eigenspace of the linear operator $\mathrm{ad}(x)$.
+"""
+function engel_subalgebra(x::LieAlgebraElem{C}) where {C<:FieldElem}
+  L = parent(x)
+  n = dim(L)
+  A = adjoint_matrix(x)^n
+  ker = kernel(A; side=:left)
+  basis = [L(ker[i, :]) for i in 1:nrows(ker)]
+  L0adx = sub(L, basis; is_basis=true)
+  return L0adx
+end
+
+@doc raw"""
+    Oscar.LieAlgebras.cartan_subalgebra(L::LieAlgebra{C}) where {C<:FieldElem} -> LieSubalgebra{C,elem_type(L)}
+
+Return a Cartan subalgebra of `L`.
+
+If `L` knows its root system, this function uses the Chevalley basis to construct a Cartan subalgebra.
+Otherise, it uses the algorithm described in [Gra00; Ch. 3.2](@cite).
+"""
+function cartan_subalgebra(L::LieAlgebra{C}) where {C<:FieldElem}
+  if has_root_system(L)
+    return sub(L, chevalley_basis(L)[3]; is_basis=true)
+  else
+    return _cartan_subalgebra(L)
+  end
+end
+
+function _cartan_subalgebra(L::LieAlgebra{C}) where {C<:FieldElem}
+  F = coefficient_ring(L)
+  n = dim(L)
+  @req is_infinite(F) || length(F) > dim(L) "The implemented algorithm requires a large field"
+  x = any_non_ad_nilpotent_element(L)
+  if is_zero(x) # L is nilpotent
+    return sub(L)
+  end
+
+  L0adx = engel_subalgebra(x)
+  while true # decreasing variant is dim(L0adx)
+    y = any_non_ad_nilpotent_element(L0adx)
+    if is_zero(y) # L0adx is nilpotent
+      return L0adx
+    end
+
+    c_itr =
+      characteristic(F) == 0 ? (F(i) for i in 1:(n + 1)) : Iterators.filter(!iszero, F)
+    z = x
+    L0adz = L0adx
+    for c in c_itr # at most n+1 iterations
+      z = x + c * (y - x)
+      L0adz = engel_subalgebra(z)
+      if dim(L0adz) < dim(L0adx) && is_subset(L0adz, L0adx)
+        break
+      end
+    end
+    x = z
+    L0adx = L0adz
+  end
+end
+
 ###############################################################################
 #
 #   Root system getters
diff --git a/experimental/LieAlgebras/src/exports.jl b/experimental/LieAlgebras/src/exports.jl
index 1c414f817eff..8383f35fcc20 100644
--- a/experimental/LieAlgebras/src/exports.jl
+++ b/experimental/LieAlgebras/src/exports.jl
@@ -27,6 +27,7 @@ export bilinear_form
 export bracket
 export cartan_bilinear_form
 export cartan_matrix
+export cartan_subalgebra
 export cartan_symmetrizer
 export cartan_type
 export cartan_type_with_ordering
@@ -41,6 +42,7 @@ export derived_algebra
 export dim_of_simple_module
 export dominant_character
 export dominant_weights
+export engel_subalgebra
 export exterior_power
 export fundamental_weight
 export fundamental_weights

From d4d7611b41c911410de0c76788f6ff3ae73bf9b3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Wed, 11 Sep 2024 15:03:40 +0200
Subject: [PATCH 05/17] More functionality for ideals and subalgebras

---
 .../LieAlgebras/src/LieAlgebraIdeal.jl        | 69 ++++++++++++++++++-
 experimental/LieAlgebras/src/LieSubalgebra.jl | 67 +++++++++++++++++-
 2 files changed, 132 insertions(+), 4 deletions(-)

diff --git a/experimental/LieAlgebras/src/LieAlgebraIdeal.jl b/experimental/LieAlgebras/src/LieAlgebraIdeal.jl
index 0e622304d5c5..73d8db05890b 100644
--- a/experimental/LieAlgebras/src/LieAlgebraIdeal.jl
+++ b/experimental/LieAlgebras/src/LieAlgebraIdeal.jl
@@ -50,6 +50,8 @@ Return the dimension of the ideal `I`.
 """
 dim(I::LieAlgebraIdeal) = length(basis(I))
 
+coefficient_ring(I::LieAlgebraIdeal) = coefficient_ring(base_lie_algebra(I))
+
 ###############################################################################
 #
 #   String I/O
@@ -81,6 +83,55 @@ function Base.show(io::IO, I::LieAlgebraIdeal)
   end
 end
 
+###############################################################################
+#
+#   Parent object call overload
+#
+###############################################################################
+
+@doc raw"""
+    (I::LieAlgebraIdeal{C})() -> LieAlgebraElem{C}
+
+Return the zero element of the Lie algebra ideal `I`.
+"""
+function (I::LieAlgebraIdeal)()
+  return zero(base_lie_algebra(I))
+end
+
+@doc raw"""
+    (I::LieAlgebraIdeal{C})(v::Vector{Int}) -> LieAlgebraElem{C}
+
+Return the element of `I` with coefficient vector `v`.
+Fail, if `Int` cannot be coerced into the base ring of `I`.
+"""
+function (I::LieAlgebraIdeal)(v::Vector{Int})
+  return I(coefficient_ring(I).(v))
+end
+
+@doc raw"""
+    (I::LieAlgebraIdeal{C})(v::Vector{C}) -> LieAlgebraElem{C}
+
+Return the element of `I` with coefficient vector `v`.
+"""
+function (I::LieAlgebraIdeal{C})(v::Vector{C}) where {C<:FieldElem}
+  @req length(v) == dim(I) "Length of vector does not match dimension."
+  mat = matrix(coefficient_ring(I), 1, length(v), v)
+  L = base_lie_algebra(I)
+  return elem_type(L)(L, mat * basis_matrix(I))
+end
+
+@doc raw"""
+    (I::LieAlgebraIdeal{C})(mat::MatElem{C}) -> LieAlgebraElem{C}
+
+Return the element of `I` with coefficient vector equivalent to
+the $1 \times \dim(I)$ matrix `mat`.
+"""
+function (I::LieAlgebraIdeal{C})(mat::MatElem{C}) where {C<:FieldElem}
+  @req size(mat) == (1, dim(I)) "Invalid matrix dimensions."
+  L = base_lie_algebra(I)
+  return elem_type(L)(L, mat * basis_matrix(I))
+end
+
 ###############################################################################
 #
 #   Comparisons
@@ -116,14 +167,28 @@ end
 ###############################################################################
 
 @doc raw"""
-    in(x::LieAlgebraElem, I::LieAlgebraIdeal) -> Bool
+    in(x::LieAlgebraElem{C}, I::LieAlgebraIdeal{C}) -> Bool
 
 Return `true` if `x` is in the ideal `I`, `false` otherwise.
 """
-function Base.in(x::LieAlgebraElem, I::LieAlgebraIdeal)
+function Base.in(x::LieAlgebraElem{C}, I::LieAlgebraIdeal{C}) where {C<:FieldElem}
+  @req parent(x) === base_lie_algebra(I) "Incompatible Lie algebras"
   return can_solve(basis_matrix(I), _matrix(x); side=:left)
 end
 
+@doc raw"""
+    Oscar.LieAlgebras.coefficient_vector(x::LieAlgebraElem{C}, I::LieAlgebraIdeal{C}) -> Vector{C}
+
+Return the coefficient vector of `x` in the basis of `I`.
+This function will throw an error if `x` is not in `I`.
+"""
+function coefficient_vector(
+  x::LieAlgebraElem{C}, I::LieAlgebraIdeal{C}
+) where {C<:FieldElem}
+  @req parent(x) === base_lie_algebra(I) "Incompatible Lie algebras"
+  return solve(basis_matrix(I), _matrix(x); side=:left)
+end
+
 ###############################################################################
 #
 #   Constructions
diff --git a/experimental/LieAlgebras/src/LieSubalgebra.jl b/experimental/LieAlgebras/src/LieSubalgebra.jl
index 574aae2de7e6..f83919ff27cb 100644
--- a/experimental/LieAlgebras/src/LieSubalgebra.jl
+++ b/experimental/LieAlgebras/src/LieSubalgebra.jl
@@ -48,6 +48,8 @@ Return the dimension of the Lie subalgebra `S`.
 """
 dim(S::LieSubalgebra) = length(basis(S))
 
+coefficient_ring(S::LieSubalgebra) = coefficient_ring(base_lie_algebra(S))
+
 ###############################################################################
 #
 #   String I/O
@@ -79,6 +81,55 @@ function Base.show(io::IO, S::LieSubalgebra)
   end
 end
 
+###############################################################################
+#
+#   Parent object call overload
+#
+###############################################################################
+
+@doc raw"""
+    (S::LieSubalgebra{C})() -> LieAlgebraElem{C}
+
+Return the zero element of the Lie subalgebra `S`.
+"""
+function (S::LieSubalgebra)()
+  return zero(base_lie_algebra(S))
+end
+
+@doc raw"""
+    (S::LieSubalgebra{C})(v::Vector{Int}) -> LieAlgebraElem{C}
+
+Return the element of `S` with coefficient vector `v`.
+Fail, if `Int` cannot be coerced into the base ring of `S`.
+"""
+function (S::LieSubalgebra)(v::Vector{Int})
+  return S(coefficient_ring(S).(v))
+end
+
+@doc raw"""
+    (S::LieSubalgebra{C})(v::Vector{C}) -> LieAlgebraElem{C}
+
+Return the element of `S` with coefficient vector `v`.
+"""
+function (S::LieSubalgebra{C})(v::Vector{C}) where {C<:FieldElem}
+  @req length(v) == dim(S) "Length of vector does not match dimension."
+  mat = matrix(coefficient_ring(S), 1, length(v), v)
+  L = base_lie_algebra(S)
+  return elem_type(L)(L, mat * basis_matrix(S))
+end
+
+@doc raw"""
+    (S::LieSubalgebra{C})(mat::MatElem{C}) -> LieAlgebraElem{C}
+
+Return the element of `S` with coefficient vector equivalent to
+the $1 \times \dim(S)$ matrix `mat`.
+"""
+function (S::LieSubalgebra{C})(mat::MatElem{C}) where {C<:FieldElem}
+  @req size(mat) == (1, dim(S)) "Invalid matrix dimensions."
+  L = base_lie_algebra(S)
+  return elem_type(L)(L, mat * basis_matrix(S))
+end
+
 ###############################################################################
 #
 #   Comparisons
@@ -114,14 +165,26 @@ end
 ###############################################################################
 
 @doc raw"""
-    in(x::LieAlgebraElem, S::LieSubalgebra) -> Bool
+    in(x::LieAlgebraElem{C}, S::LieSubalgebra{C}) -> Bool
 
 Return `true` if `x` is in the Lie subalgebra `S`, `false` otherwise.
 """
-function Base.in(x::LieAlgebraElem, S::LieSubalgebra)
+function Base.in(x::LieAlgebraElem{C}, S::LieSubalgebra{C}) where {C<:FieldElem}
+  @req parent(x) === base_lie_algebra(S) "Incompatible Lie algebras"
   return can_solve(basis_matrix(S), _matrix(x); side=:left)
 end
 
+@doc raw"""
+    Oscar.LieAlgebras.coefficient_vector(x::LieAlgebraElem{C}, S::LieSubalgebra{C}) -> Vector{C}
+
+Return the coefficient vector of `x` in the basis of `S`.
+This function will throw an error if `x` is not in `S`.
+"""
+function coefficient_vector(x::LieAlgebraElem{C}, S::LieSubalgebra{C}) where {C<:FieldElem}
+  @req parent(x) === base_lie_algebra(S) "Incompatible Lie algebras"
+  return solve(basis_matrix(S), _matrix(x); side=:left)
+end
+
 ###############################################################################
 #
 #   Constructions

From 729308bbdec5bb39929fd56cb2029d3e5473c491 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Wed, 11 Sep 2024 15:03:57 +0200
Subject: [PATCH 06/17] Fix some generator printing

---
 experimental/LieAlgebras/src/AbstractLieAlgebra.jl | 2 +-
 experimental/LieAlgebras/src/LinearLieAlgebra.jl   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/experimental/LieAlgebras/src/AbstractLieAlgebra.jl b/experimental/LieAlgebras/src/AbstractLieAlgebra.jl
index 367779d6d872..afa9bb6660be 100644
--- a/experimental/LieAlgebras/src/AbstractLieAlgebra.jl
+++ b/experimental/LieAlgebras/src/AbstractLieAlgebra.jl
@@ -234,7 +234,7 @@ function lie_algebra(
     @req fl "Not closed under the bracket."
     struct_consts[i, j] = sparse_row(row)
   end
-  s = map(AbstractAlgebra.obj_to_string, basis)
+  s = map(AbstractAlgebra.obj_to_string_wrt_times, basis)
   return lie_algebra(R, struct_consts, s; check)
 end
 
diff --git a/experimental/LieAlgebras/src/LinearLieAlgebra.jl b/experimental/LieAlgebras/src/LinearLieAlgebra.jl
index 39328a3e17de..8e5dddba355d 100644
--- a/experimental/LieAlgebras/src/LinearLieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LinearLieAlgebra.jl
@@ -179,7 +179,7 @@ function lie_algebra(
   @req all(parent(x) === parent_L for x in basis) "Elements not compatible."
   R = coefficient_ring(parent_L)
   n = parent_L.n
-  s = map(AbstractAlgebra.obj_to_string, basis)
+  s = map(AbstractAlgebra.obj_to_string_wrt_times, basis)
   return lie_algebra(R, n, matrix_repr.(basis), s; check)
 end
 

From 6b2d101881e5252e558d4f136ccb38aab099f7e7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Wed, 11 Sep 2024 15:04:29 +0200
Subject: [PATCH 07/17] Add internal `_root_system_and_chevalley_basis`

---
 experimental/LieAlgebras/src/LieAlgebra.jl | 169 +++++++++++++++++++++
 1 file changed, 169 insertions(+)

diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index 1c017c7cecc9..5d7801a5136b 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -459,6 +459,16 @@ function adjoint_matrix(x::LieAlgebraElem{C}) where {C<:FieldElem}
   )
 end
 
+function _adjoint_matrix(S::LieSubalgebra{C}, x::LieAlgebraElem{C}) where {C<:FieldElem}
+  L = parent(x)
+  @req base_lie_algebra(S) === L "Incompatible Lie algebras"
+  A = zero_matrix(coefficient_ring(L), dim(S), dim(S))
+  for (i, bi) in enumerate(basis(S))
+    A[i, :] = coefficient_vector(bracket(x, bi), S)
+  end
+  return A
+end
+
 @attr dense_matrix_type(C) function killing_matrix(L::LieAlgebra{C}) where {C<:FieldElem}
   R = coefficient_ring(L)
   A = zero_matrix(R, dim(L), dim(L))
@@ -546,6 +556,8 @@ Return a Cartan subalgebra of `L`.
 
 If `L` knows its root system, this function uses the Chevalley basis to construct a Cartan subalgebra.
 Otherise, it uses the algorithm described in [Gra00; Ch. 3.2](@cite).
+The subalgebra object returned by this function may differ in the two cases, in particular, the bases may be different.
+However, their spans are guaranteed to be equal.
 """
 function cartan_subalgebra(L::LieAlgebra{C}) where {C<:FieldElem}
   if has_root_system(L)
@@ -587,6 +599,163 @@ function _cartan_subalgebra(L::LieAlgebra{C}) where {C<:FieldElem}
   end
 end
 
+function _root_system_and_chevalley_basis(
+  L::LieAlgebra{C}, H::LieSubalgebra{C}=_cartan_subalgebra(L)
+) where {C<:FieldElem}
+  @req base_lie_algebra(H) === L "Incompatible Lie algebras."
+  # we just assume that H is indeed a Cartan subalgebra
+
+  F = coefficient_ring(L)
+
+  # compute the common eigenspaces of the adjoint action of H.
+  # B is a list of subspaces of L that gets refined in each iteration.
+  # to exploit existing functionality, we use the LieSubalgebra type even
+  # though the subspaces are in general not Lie subalgebras. With setting
+  # is_basis=true, we ensure that the subspace generators do not get
+  # extended to a subalgebra.
+  B = [sub(L, basis(L); is_basis=true)]
+  for h in basis(H)
+    B_new = empty(B)
+    for B_j in B
+      A = _adjoint_matrix(B_j, h)
+      facs = factor(minimal_polynomial(A))
+      for (f, k) in facs
+        @assert k == 1 # TODO: is this always the case?
+        ker = kernel((f^k)(A); side=:left)
+        basis = [B_j(ker[i, :]) for i in 1:nrows(ker)]
+        push!(B_new, sub(L, basis; is_basis=true))
+      end
+    end
+    B = B_new
+  end
+  filter!(!=(H), B)
+  @req all(B_j -> dim(B_j) == 1, B) "The Cartan subalgebra is not split"
+
+  # compute the roots, i.e. the list of eigenvalues of basis(H) on each B_j
+  root_spaces = Dict(
+    begin
+      b = only(basis(B_j))
+      root = [only(solve(_matrix(b), _matrix(bracket(h, b)); side=:left)) for h in basis(H)]
+      root => B_j
+    end for B_j in B
+  )
+  roots = collect(keys(root_spaces))
+
+  # compute an R-basis of the root space, s.t. the corresponding co-roots are a basis of H
+  roots_basis = empty(roots)
+  basis_mat_H = zero_matrix(F, 0, dim(L))
+  for root in roots
+    nrows(basis_mat_H) == dim(H) && break
+    x_j = only(basis(root_spaces[root]))
+    y_j = only(basis(root_spaces[-root]))
+    h_j = bracket(x_j, y_j)
+    if !can_solve(basis_mat_H, _matrix(h_j); side=:left)
+      basis_mat_H = vcat(basis_mat_H, _matrix(h_j))
+      push!(roots_basis, root)
+    end
+  end
+
+  function CartanInt(
+    roots::Vector{Vector{QQFieldElem}}, a::Vector{QQFieldElem}, b::Vector{QQFieldElem}
+  )
+    # `a` and `b` are two roots in `roots`.
+    a == b && return 2
+    a == -b && return -2
+    # If a != ±b, the Cartan integer of `a` and `b` is `s-t`, where
+    # `s` and `t` are the largest integers such that `b-s*a` and `b+t*a` are still roots.
+    rt = b - a
+    s = 0
+    while rt in roots
+      s += 1
+      rt -= a
+    end
+    rt = b + a
+    t = 0
+    while rt in roots
+      t += 1
+      rt += a
+    end
+    return s - t
+  end
+
+  # we define a root to be positive if the first of its non-zero Cartan integers with the R-basis is positive
+  roots_positive = empty(roots)
+  for root in roots
+    2 * length(roots_positive) == length(roots) && break
+    root in roots_positive && continue
+    -root in roots_positive && continue
+    c = first(
+      Iterators.dropwhile(
+        iszero, Iterators.map(b_j -> CartanInt(roots, root, b_j), roots_basis)
+      ),
+    )
+    if c > 0
+      push!(roots_positive, root)
+    else
+      push!(roots_positive, -root)
+    end
+  end
+
+  # a positive root is simple if it is not the sum of two positive roots
+  roots_simple = empty(roots)
+  roots_positive_sums = Set(
+    alpha_i + alpha_j for (i, alpha_i) in enumerate(roots_positive) for
+    (j, alpha_j) in enumerate(roots_positive) if i < j
+  )
+  for root in roots_positive
+    if !(root in roots_positive_sums)
+      push!(roots_simple, root)
+    end
+  end
+  @assert length(roots_simple) == dim(H)
+
+  # compute the Cartan matrix and abstract root system
+  cm = matrix(
+    ZZ,
+    [
+      CartanInt(roots, alpha_i, alpha_j) for alpha_i in roots_simple,
+      alpha_j in roots_simple
+    ],
+  )
+  R = root_system(cm; check=true) # TODO: disable check
+
+  # reorder the simple roots according to the Dynkin diagram.
+  # users usually expect this ordering, but it is not necessary for the root system.
+  _, ordering = root_system_type_with_ordering(R)
+  permute!(roots_simple, ordering)
+  cm = matrix(
+    ZZ,
+    [
+      CartanInt(roots, alpha_i, alpha_j) for alpha_i in roots_simple,
+      alpha_j in roots_simple
+    ],
+  )
+  R = root_system(cm; check=true) # TODO: disable check
+
+  # compute a Chevalley basis of L
+  root_vectors = [
+    begin
+      concrete_root = sum(
+        c * root_simple for
+        (c, root_simple) in zip(coefficients(abstract_root), roots_simple)
+      )
+      @assert concrete_root in roots
+      root_vector = only(basis(root_spaces[concrete_root]))
+    end for abstract_root in Oscar.roots(R)
+  ]
+  xs = root_vectors[1:n_positive_roots(R)]
+  ys = [
+    begin
+      x = xs[i]
+      y = root_vectors[n_positive_roots(R) + i]
+      y *= 2//only(solve(_matrix(x), _matrix(bracket(bracket(x, y), x)); side=:left))
+    end for i in 1:n_positive_roots(R)
+  ]
+  hs = [xs[i] * ys[i] for i in 1:n_simple_roots(R)]
+
+  return R, (xs, ys, hs)
+end
+
 ###############################################################################
 #
 #   Root system getters

From bf6c60894b6589f3a30420227f769f4c8d64eb64 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Thu, 12 Sep 2024 14:37:35 +0200
Subject: [PATCH 08/17] Handle root system caching

---
 .../LieAlgebras/src/AbstractLieAlgebra.jl     | 31 ++++++-----
 .../LieAlgebras/src/DirectSumLieAlgebra.jl    | 23 ++++++--
 experimental/LieAlgebras/src/LieAlgebra.jl    | 53 ++++++++++---------
 .../LieAlgebras/src/LinearLieAlgebra.jl       | 25 +++++++++
 experimental/LieAlgebras/src/Types.jl         | 10 ++++
 5 files changed, 102 insertions(+), 40 deletions(-)

diff --git a/experimental/LieAlgebras/src/AbstractLieAlgebra.jl b/experimental/LieAlgebras/src/AbstractLieAlgebra.jl
index afa9bb6660be..4f5f16ffcd98 100644
--- a/experimental/LieAlgebras/src/AbstractLieAlgebra.jl
+++ b/experimental/LieAlgebras/src/AbstractLieAlgebra.jl
@@ -95,22 +95,20 @@ end
 has_root_system(L::AbstractLieAlgebra) = isdefined(L, :root_system)
 
 function root_system(L::AbstractLieAlgebra)
-  @req has_root_system(L) "no root system known."
+  assure_root_system(L)
   return L.root_system
 end
 
 function chevalley_basis(L::AbstractLieAlgebra)
-  @req has_root_system(L) "no root system known."
-  # TODO: once there is root system detection, this function needs to be updated to indeed return the Chevalley basis
-
-  npos = n_positive_roots(root_system(L))
-  b = basis(L)
-  # root vectors
-  r_plus = b[1:npos]
-  r_minus = b[(npos + 1):(2 * npos)]
-  # basis for cartan algebra
-  h = b[(2 * npos + 1):dim(L)]
-  return (r_plus, r_minus, h)
+  assure_root_system(L)
+  return L.chevalley_basis::NTuple{3,Vector{elem_type(L)}}
+end
+
+function set_root_system_and_chevalley_basis!(
+  L::AbstractLieAlgebra{C}, R::RootSystem, chev::NTuple{3,Vector{AbstractLieAlgebraElem{C}}}
+) where {C<:FieldElem}
+  L.root_system = R
+  L.chevalley_basis = chev
 end
 
 ###############################################################################
@@ -264,7 +262,14 @@ function lie_algebra(
   ]
 
   L = lie_algebra(R, struct_consts, s; check=false)
-  L.root_system = rs
+
+  npos = n_positive_roots(rs)
+  # set Chevalley basis
+  r_plus = basis(L)[1:npos]
+  r_minus = basis(L)[(npos + 1):(2 * npos)]
+  h = basis(L)[(2 * npos + 1):end]
+  chev = (r_plus, r_minus, h)
+  set_root_system_and_chevalley_basis!(L, rs, chev)
   return L
 end
 
diff --git a/experimental/LieAlgebras/src/DirectSumLieAlgebra.jl b/experimental/LieAlgebras/src/DirectSumLieAlgebra.jl
index f03c10225694..88ccabac2793 100644
--- a/experimental/LieAlgebras/src/DirectSumLieAlgebra.jl
+++ b/experimental/LieAlgebras/src/DirectSumLieAlgebra.jl
@@ -135,9 +135,26 @@ end
 #
 ###############################################################################
 
-# The following implementation needs direct sums of root systems, which
-# is not yet implemented.
-# has_root_system(D::DirectSumLieAlgebra) = all(has_root_system, D.summands)
+has_root_system(D::DirectSumLieAlgebra) = isdefined(D, :root_system)
+
+function root_system(D::DirectSumLieAlgebra)
+  assure_root_system(D)
+  return D.root_system
+end
+
+function chevalley_basis(D::DirectSumLieAlgebra)
+  assure_root_system(D)
+  return D.chevalley_basis::NTuple{3,Vector{elem_type(D)}}
+end
+
+function set_root_system_and_chevalley_basis!(
+  D::DirectSumLieAlgebra{C},
+  R::RootSystem,
+  chev::NTuple{3,Vector{DirectSumLieAlgebraElem{C}}},
+) where {C<:FieldElem}
+  D.root_system = R
+  D.chevalley_basis = chev
+end
 
 ###############################################################################
 #
diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index 5d7801a5136b..a511270b9f3d 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -8,10 +8,10 @@
 #   symbols(L::MyLieAlgebra) -> Vector{Symbol}
 #   bracket(x::MyLieAlgebraElem{C}, y::MyLieAlgebraElem{C}) -> MyLieAlgebraElem{C}
 #   Base.show(io::IO, x::MyLieAlgebra)
-# If the subtype supports root systems:
 #   has_root_system(::MyLieAlgebra) -> Bool
 #   root_system(::MyLieAlgebra) -> RootSystem
-#   chevalley_basis(L::MyLieAlgebra) -> NTuple{3,Vector{elem_type(L)}}
+#   chevalley_basis(L::MyLieAlgebra{C}) -> NTuple{3,Vector{MyLieAlgebraElem{C}}}
+#   set_root_system_and_chevalley_basis!(L::MyLieAlgebra{C}, R::RootSystem, chev::NTuple{3,Vector{MyLieAlgebraElem{C}}}}})
 
 ###############################################################################
 #
@@ -549,24 +549,6 @@ function engel_subalgebra(x::LieAlgebraElem{C}) where {C<:FieldElem}
   return L0adx
 end
 
-@doc raw"""
-    Oscar.LieAlgebras.cartan_subalgebra(L::LieAlgebra{C}) where {C<:FieldElem} -> LieSubalgebra{C,elem_type(L)}
-
-Return a Cartan subalgebra of `L`.
-
-If `L` knows its root system, this function uses the Chevalley basis to construct a Cartan subalgebra.
-Otherise, it uses the algorithm described in [Gra00; Ch. 3.2](@cite).
-The subalgebra object returned by this function may differ in the two cases, in particular, the bases may be different.
-However, their spans are guaranteed to be equal.
-"""
-function cartan_subalgebra(L::LieAlgebra{C}) where {C<:FieldElem}
-  if has_root_system(L)
-    return sub(L, chevalley_basis(L)[3]; is_basis=true)
-  else
-    return _cartan_subalgebra(L)
-  end
-end
-
 function _cartan_subalgebra(L::LieAlgebra{C}) where {C<:FieldElem}
   F = coefficient_ring(L)
   n = dim(L)
@@ -756,6 +738,14 @@ function _root_system_and_chevalley_basis(
   return R, (xs, ys, hs)
 end
 
+function assure_root_system(L::LieAlgebra{C}) where {C<:FieldElem}
+  if !has_root_system(L)
+    R, chev = _root_system_and_chevalley_basis(L)
+    set_root_system_and_chevalley_basis!(L, R, chev)
+  end
+  @assert has_root_system(L)
+end
+
 ###############################################################################
 #
 #   Root system getters
@@ -774,10 +764,9 @@ has_root_system(L::LieAlgebra) = false # to be implemented by subtypes
 
 Return the root system of `L`.
 
-This function will error if no root system is known (see [`has_root_system(::LieAlgebra)`](@ref)).
+This function will error if no root system is known and none can be computed.
 """
 function root_system(L::LieAlgebra) # to be implemented by subtypes
-  @req has_root_system(L) "root system of `L` not known."
   throw(Hecke.NotImplemented())
 end
 
@@ -788,13 +777,29 @@ Return the Chevalley basis of the Lie algebra `L` in three vectors, stating firs
 then the negative root vectors, and finally the basis of the Cartan subalgebra. The order of root vectors corresponds
 to the order of the roots in [`root_system(::LieAlgebra)`](@ref).
 
-This function will error if no root system is known (see [`has_root_system(::LieAlgebra)`](@ref)).
+This function will error if no root system is known and none can be computed.
 """
 function chevalley_basis(L::LieAlgebra) # to be implemented by subtypes
-  @req has_root_system(L) "root system of `L` not known."
   throw(Hecke.NotImplemented())
 end
 
+@doc raw"""
+    cartan_subalgebra(L::LieAlgebra{C}) where {C<:FieldElem} -> LieSubalgebra{C,elem_type(L)}
+
+Return a Cartan subalgebra of `L`.
+
+If `L` knows its root system, this function uses the Chevalley basis to construct a Cartan subalgebra.
+Otherise, it uses the algorithm described in [Gra00; Ch. 3.2](@cite).
+The return value of this function may change when the root system of `L` is first computed.
+"""
+function cartan_subalgebra(L::LieAlgebra{C}) where {C<:FieldElem}
+  if has_root_system(L)
+    return sub(L, chevalley_basis(L)[3]; is_basis=true)
+  else
+    return _cartan_subalgebra(L)
+  end
+end
+
 ###############################################################################
 #
 #   Universal enveloping algebra
diff --git a/experimental/LieAlgebras/src/LinearLieAlgebra.jl b/experimental/LieAlgebras/src/LinearLieAlgebra.jl
index 8e5dddba355d..61d8183922ac 100644
--- a/experimental/LieAlgebras/src/LinearLieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LinearLieAlgebra.jl
@@ -146,6 +146,31 @@ function bracket(
   return coerce_to_lie_algebra_elem(L, x_mat * y_mat - y_mat * x_mat)
 end
 
+###############################################################################
+#
+#   Root system getters
+#
+###############################################################################
+
+has_root_system(L::LinearLieAlgebra) = isdefined(L, :root_system)
+
+function root_system(L::LinearLieAlgebra)
+  assure_root_system(L)
+  return L.root_system
+end
+
+function chevalley_basis(L::LinearLieAlgebra)
+  assure_root_system(L)
+  return L.chevalley_basis::NTuple{3,Vector{elem_type(L)}}
+end
+
+function set_root_system_and_chevalley_basis!(
+  L::LinearLieAlgebra{C}, R::RootSystem, chev::NTuple{3,Vector{LinearLieAlgebraElem{C}}}
+) where {C<:FieldElem}
+  L.root_system = R
+  L.chevalley_basis = chev
+end
+
 ###############################################################################
 #
 #   Constructor
diff --git a/experimental/LieAlgebras/src/Types.jl b/experimental/LieAlgebras/src/Types.jl
index 7144089ed6f8..073a2c080081 100644
--- a/experimental/LieAlgebras/src/Types.jl
+++ b/experimental/LieAlgebras/src/Types.jl
@@ -157,6 +157,7 @@ abstract type LieAlgebraElem{C<:FieldElem} <: AbstractAlgebra.SetElem end
 
   # only set if known
   root_system::RootSystem
+  chevalley_basis::NTuple{3,Vector{<:LieAlgebraElem{C}}}
 
   function AbstractLieAlgebra{C}(
     R::Field,
@@ -205,6 +206,10 @@ end
   basis::Vector{MatElem{C}}
   s::Vector{Symbol}
 
+  # only set if known
+  root_system::RootSystem
+  chevalley_basis::NTuple{3,Vector{<:LieAlgebraElem{C}}}
+
   function LinearLieAlgebra{C}(
     R::Field,
     n::Int,
@@ -240,6 +245,10 @@ end
   summands::Vector{<:LieAlgebra{C}}
   s::Vector{Symbol}
 
+  # only set if known
+  root_system::RootSystem
+  chevalley_basis::NTuple{3,Vector{<:LieAlgebraElem{C}}}
+
   function DirectSumLieAlgebra{C}(
     R::Field,
     summands::Vector{<:LieAlgebra{C}},
@@ -248,6 +257,7 @@ end
     totaldim = sum(dim, summands; init=0)
     s = [Symbol("$(x)^($(i))") for (i, S) in enumerate(summands) for x in symbols(S)]
     L = new{C}(R, totaldim, summands, s)
+    # TODO: glue root systems if all summands have one
     return L
   end
 end

From b0cf75f3dfb6ae087a42c31a9269ae6c43a9bc99 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Thu, 12 Sep 2024 14:41:39 +0200
Subject: [PATCH 09/17] Misc fixes and some tests

---
 experimental/LieAlgebras/src/LieAlgebra.jl    | 15 +++++-----
 experimental/LieAlgebras/src/serialization.jl | 13 ++++----
 experimental/LieAlgebras/test/setup_tests.jl  | 30 ++++++++++++++-----
 3 files changed, 37 insertions(+), 21 deletions(-)

diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index a511270b9f3d..eaa85f48bdeb 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -587,6 +587,8 @@ function _root_system_and_chevalley_basis(
   @req base_lie_algebra(H) === L "Incompatible Lie algebras."
   # we just assume that H is indeed a Cartan subalgebra
 
+  @req is_invertible(killing_matrix(L)) "The Killing form is degenerate"
+
   F = coefficient_ring(L)
 
   # compute the common eigenspaces of the adjoint action of H.
@@ -637,9 +639,7 @@ function _root_system_and_chevalley_basis(
     end
   end
 
-  function CartanInt(
-    roots::Vector{Vector{QQFieldElem}}, a::Vector{QQFieldElem}, b::Vector{QQFieldElem}
-  )
+  function CartanInt(roots::Vector{RootType}, a::RootType, b::RootType) where {RootType}
     # `a` and `b` are two roots in `roots`.
     a == b && return 2
     a == -b && return -2
@@ -718,7 +718,7 @@ function _root_system_and_chevalley_basis(
   root_vectors = [
     begin
       concrete_root = sum(
-        c * root_simple for
+        c .* root_simple for
         (c, root_simple) in zip(coefficients(abstract_root), roots_simple)
       )
       @assert concrete_root in roots
@@ -726,14 +726,15 @@ function _root_system_and_chevalley_basis(
     end for abstract_root in Oscar.roots(R)
   ]
   xs = root_vectors[1:n_positive_roots(R)]
-  ys = [
+  ys = Vector{elem_type(L)}([
     begin
       x = xs[i]
       y = root_vectors[n_positive_roots(R) + i]
       y *= 2//only(solve(_matrix(x), _matrix(bracket(bracket(x, y), x)); side=:left))
+      y
     end for i in 1:n_positive_roots(R)
-  ]
-  hs = [xs[i] * ys[i] for i in 1:n_simple_roots(R)]
+  ])
+  hs = elem_type(L)[xs[i] * ys[i] for i in 1:n_simple_roots(R)]
 
   return R, (xs, ys, hs)
 end
diff --git a/experimental/LieAlgebras/src/serialization.jl b/experimental/LieAlgebras/src/serialization.jl
index 9d74c7a26357..c5178178c538 100644
--- a/experimental/LieAlgebras/src/serialization.jl
+++ b/experimental/LieAlgebras/src/serialization.jl
@@ -96,12 +96,13 @@ end
 
 function load_root_system_data(s::DeserializerState, L::LieAlgebra)
   if haskey(s, :root_system)
-    @assert L isa AbstractLieAlgebra # TODO: adapt once we have a proper interface for this
-    L.root_system = load_typed_object(s, :root_system)
-    chevalley_basis = load_object(
-      s, Tuple, [(Vector, (AbstractLieAlgebraElem, L)) for _ in 1:3], :chevalley_basis
-    )
-    # chevalley basis will become an attribute in the near future
+    rs = load_typed_object(s, :root_system)
+    chev = NTuple{3,Vector{elem_type(L)}}(
+      load_object(
+        s, Tuple, [(Vector, (AbstractLieAlgebraElem, L)) for _ in 1:3], :chevalley_basis
+      ),
+    ) # coercion needed due to https://github.com/oscar-system/Oscar.jl/issues/3983
+    set_root_system_and_chevalley_basis!(L, rs, chev)
   end
 end
 
diff --git a/experimental/LieAlgebras/test/setup_tests.jl b/experimental/LieAlgebras/test/setup_tests.jl
index 2c55ed3ed13d..baaa7ee27ce9 100644
--- a/experimental/LieAlgebras/test/setup_tests.jl
+++ b/experimental/LieAlgebras/test/setup_tests.jl
@@ -110,14 +110,28 @@ if !isdefined(Main, :lie_algebra_conformance_test) || isinteractive()
     end
 
     @testset "Root systems" begin
-      if has_root_system(L)
-        rs = root_system(L)
-        @test rs isa RootSystem
-        @test dim(L) == n_roots(rs) + n_simple_roots(rs)
-        chev = @inferred chevalley_basis(L)
-        @test length(chev) == 3
-        @test length(chev[1]) == length(chev[2])
-        @test dim(L) == sum(length, chev; init=0)
+      if !(L isa DirectSumLieAlgebra) # TODO: make root_system work for DirectSumLieAlgebra
+        try
+          root_system(L)
+        catch e
+          e isa ArgumentError || rethrow()
+          @test any(
+            msg -> contains(e.msg, msg),
+            ["Killing form is degenerate", "Cartan subalgebra is not split"],
+          )
+        end
+        if has_root_system(L)
+          rs = root_system(L)
+          @test rs isa RootSystem
+          @test dim(L) == n_roots(rs) + n_simple_roots(rs)
+          chev = @inferred chevalley_basis(L)
+          @test length(chev) == 3
+          @test length(chev[1]) == length(chev[2])
+          @test dim(L) == sum(length, chev; init=0)
+          H = cartan_subalgebra(L)
+          @test all(h -> h in H, chev[3])
+          @test all(xs -> bracket(xs...) in H, zip(chev[1], chev[2]))
+        end
       end
     end
 

From 77925d7577926616263eec838c0a97a223380b3c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Thu, 12 Sep 2024 14:44:13 +0200
Subject: [PATCH 10/17] Simplify some code using root system detection

---
 .../LieAlgebras/src/LieAlgebraModule.jl       | 84 +++----------------
 1 file changed, 10 insertions(+), 74 deletions(-)

diff --git a/experimental/LieAlgebras/src/LieAlgebraModule.jl b/experimental/LieAlgebras/src/LieAlgebraModule.jl
index 562c86122eb6..08ddecd9f3b1 100644
--- a/experimental/LieAlgebras/src/LieAlgebraModule.jl
+++ b/experimental/LieAlgebras/src/LieAlgebraModule.jl
@@ -1413,17 +1413,8 @@ julia> dim_of_simple_module(L, [1, 1, 1])
 ```
 """
 function dim_of_simple_module(T::Type, L::LieAlgebra, hw::Vector{<:IntegerUnion})
-  if has_root_system(L)
-    R = root_system(L)
-    return dim_of_simple_module(T, R, hw)
-  else # TODO: remove branch once root system detection is implemented
-    @req is_dominant_weight(hw) "Not a dominant weight."
-    return T(
-      GAPWrap.DimensionOfHighestWeightModule(
-        codomain(Oscar.iso_oscar_gap(L)), GAP.Obj(hw; recursive=true)
-      ),
-    )
-  end
+  R = root_system(L)
+  return dim_of_simple_module(T, R, hw)
 end
 
 function dim_of_simple_module(L::LieAlgebra, hw::Vector{<:IntegerUnion})
@@ -1456,24 +1447,8 @@ julia> dominant_weights(L, [1, 0, 3])
 ```
 """
 function dominant_weights(T::Type, L::LieAlgebra, hw::Vector{<:IntegerUnion})
-  if has_root_system(L)
-    R = root_system(L)
-    return dominant_weights(T, R, hw)
-  else # TODO: remove branch once root system detection is implemented
-    @req is_dominant_weight(hw) "Not a dominant weight."
-    return first.(
-      sort!(
-        collect(
-          T(w) => d for (w, d) in
-          zip(
-            GAP.Globals.DominantWeights(
-              GAP.Globals.RootSystem(codomain(Oscar.iso_oscar_gap(L))),
-              GAP.Obj(hw; recursive=true),
-            )...,
-          )
-        ); by=last)
-    )
-  end
+  R = root_system(L)
+  return dominant_weights(T, R, hw)
 end
 
 function dominant_weights(L::LieAlgebra, hw::Vector{<:IntegerUnion})
@@ -1503,20 +1478,8 @@ Dict{Vector{Int64}, Int64} with 4 entries:
 ```
 """
 function dominant_character(L::LieAlgebra, hw::Vector{<:IntegerUnion})
-  if has_root_system(L)
-    R = root_system(L)
-    return dominant_character(R, hw)
-  else # TODO: remove branch once root system detection is implemented
-    @req is_dominant_weight(hw) "Not a dominant weight."
-    return Dict{Vector{Int},Int}(
-      Vector{Int}(w) => d for (w, d) in
-      zip(
-        GAPWrap.DominantCharacter(
-          codomain(Oscar.iso_oscar_gap(L)), GAP.Obj(hw; recursive=true)
-        )...,
-      )
-    )
-  end
+  R = root_system(L)
+  return dominant_character(R, hw)
 end
 
 @doc raw"""
@@ -1547,22 +1510,8 @@ Dict{Vector{Int64}, Int64} with 10 entries:
 ```
 """
 function character(L::LieAlgebra, hw::Vector{<:IntegerUnion})
-  if has_root_system(L)
-    R = root_system(L)
-    return character(R, hw)
-  else # TODO: remove branch once root system detection is implemented
-    @req is_dominant_weight(hw) "Not a dominant weight."
-    dc = dominant_character(L, hw)
-    c = Dict{Vector{Int},Int}()
-    W = GAPWrap.WeylGroup(GAPWrap.RootSystem(codomain(Oscar.iso_oscar_gap(L))))
-    for (w, d) in dc
-      it = GAPWrap.WeylOrbitIterator(W, GAP.Obj(w))
-      while !GAPWrap.IsDoneIterator(it)
-        push!(c, Vector{Int}(GAPWrap.NextIterator(it)) => d)
-      end
-    end
-    return c
-  end
+  R = root_system(L)
+  return character(R, hw)
 end
 
 @doc raw"""
@@ -1595,19 +1544,6 @@ MSet{Vector{Int64}} with 6 elements:
 function tensor_product_decomposition(
   L::LieAlgebra, hw1::Vector{<:IntegerUnion}, hw2::Vector{<:IntegerUnion}
 )
-  if has_root_system(L)
-    R = root_system(L)
-    return tensor_product_decomposition(R, hw1, hw2)
-  else # TODO: remove branch once root system detection is implemented
-    @req is_dominant_weight(hw1) && is_dominant_weight(hw2) "Both weights must be dominant."
-    return multiset(
-      Tuple{Vector{Vector{Int}},Vector{Int}}(
-        GAPWrap.DecomposeTensorProduct(
-          codomain(Oscar.iso_oscar_gap(L)),
-          GAP.Obj(hw1; recursive=true),
-          GAP.Obj(hw2; recursive=true),
-        ),
-      )...,
-    )
-  end
+  R = root_system(L)
+  return tensor_product_decomposition(R, hw1, hw2)
 end

From 11ceb45bb588cc201c57afd49ac3e0e5e41abb92 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Thu, 12 Sep 2024 15:18:22 +0200
Subject: [PATCH 11/17] Update some flaky B2/C2 test

---
 .../LieAlgebras/test/LieAlgebraModule-test.jl | 52 ++++++++++++-------
 1 file changed, 33 insertions(+), 19 deletions(-)

diff --git a/experimental/LieAlgebras/test/LieAlgebraModule-test.jl b/experimental/LieAlgebras/test/LieAlgebraModule-test.jl
index 28abe42dee2f..0f7a031634ad 100644
--- a/experimental/LieAlgebras/test/LieAlgebraModule-test.jl
+++ b/experimental/LieAlgebras/test/LieAlgebraModule-test.jl
@@ -772,10 +772,10 @@
       @test dim == 16
     end
 
-    let L = special_orthogonal_lie_algebra(QQ, 5) # type B_2 but without known root system
-      dim = @inferred dim_of_simple_module(L, ZZ.([1, 2]))
+    let L = special_orthogonal_lie_algebra(QQ, 7) # type B_3 but without known root system
+      dim = @inferred dim_of_simple_module(L, ZZ.([1, 2, 1]))
       @test dim isa Int
-      @test dim == 35
+      @test dim == 2800
     end
   end
 
@@ -896,14 +896,22 @@
       )
     end
 
-    let L = special_orthogonal_lie_algebra(QQ, 5), hw = ZZ.([1, 2]) # type B_2 but without known root system
+    let L = special_orthogonal_lie_algebra(QQ, 7), hw = ZZ.([1, 2, 0]) # type B_3 but without known root system
       domchar = check_dominant_character(L, hw)
       @test domchar == Dict(
-        [1, 2] => 1,
-        [2, 0] => 1,
-        [0, 2] => 2,
-        [1, 0] => 3,
-        [0, 0] => 3,
+        [1, 2, 0] => 1,
+        [2, 0, 2] => 1,
+        [2, 1, 0] => 1,
+        [0, 1, 2] => 2,
+        [0, 2, 0] => 2,
+        [3, 0, 0] => 2,
+        [1, 0, 2] => 3,
+        [1, 1, 0] => 6,
+        [2, 0, 0] => 6,
+        [0, 0, 2] => 9,
+        [0, 1, 0] => 9,
+        [1, 0, 0] => 15,
+        [0, 0, 0] => 15,
       )
     end
   end
@@ -984,7 +992,7 @@
       )
     end
 
-    let L = special_orthogonal_lie_algebra(QQ, 5), hw = ZZ.([1, 2]) # type B_2 but without known root system
+    let L = special_orthogonal_lie_algebra(QQ, 7), hw = ZZ.([1, 2, 1]) # type B_3 but without known root system
       char = check_character(L, hw)
     end
   end
@@ -1090,18 +1098,24 @@
       ))
     end
 
-    let L = special_orthogonal_lie_algebra(QQ, 5), hw1 = ZZ.([1, 2]), hw2 = ZZ.([1, 1]) # type B_2 but without known root system
+    let L = special_orthogonal_lie_algebra(QQ, 7), hw1 = ZZ.([1, 1, 0]),
+      hw2 = ZZ.([0, 1, 1]) # type B_3 but without known root system
       dec = test_tensor_product_decomposition(L, hw1, hw2)
       @test dec == multiset(
         Dict(
-          [2, 3] => 1,
-          [3, 1] => 1,
-          [0, 5] => 1,
-          [1, 3] => 2,
-          [2, 1] => 2,
-          [0, 3] => 2,
-          [1, 1] => 2,
-          [0, 1] => 1,
+          [1, 2, 1] => 1,
+          [2, 0, 3] => 1,
+          [2, 1, 1] => 1,
+          [0, 1, 3] => 1,
+          [0, 2, 1] => 1,
+          [3, 0, 1] => 1,
+          [1, 0, 3] => 2,
+          [1, 1, 1] => 3,
+          [2, 0, 1] => 2,
+          [0, 0, 3] => 2,
+          [0, 1, 1] => 2,
+          [1, 0, 1] => 2,
+          [0, 0, 1] => 1,
         ),
       )
     end

From 2626dad1a2bbf5aa5a62f6c4f573b8848d89ca9a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Tue, 17 Sep 2024 09:52:44 +0200
Subject: [PATCH 12/17] Make the formatter happy

---
 experimental/LieAlgebras/test/LieAlgebraModule-test.jl | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/experimental/LieAlgebras/test/LieAlgebraModule-test.jl b/experimental/LieAlgebras/test/LieAlgebraModule-test.jl
index 0f7a031634ad..c077a1862e11 100644
--- a/experimental/LieAlgebras/test/LieAlgebraModule-test.jl
+++ b/experimental/LieAlgebras/test/LieAlgebraModule-test.jl
@@ -1098,8 +1098,10 @@
       ))
     end
 
-    let L = special_orthogonal_lie_algebra(QQ, 7), hw1 = ZZ.([1, 1, 0]),
-      hw2 = ZZ.([0, 1, 1]) # type B_3 but without known root system
+    let L = special_orthogonal_lie_algebra(QQ, 7) # type B_3 but without known root system
+      hw1 = ZZ.([1, 1, 0])
+      hw2 = ZZ.([0, 1, 1])
+
       dec = test_tensor_product_decomposition(L, hw1, hw2)
       @test dec == multiset(
         Dict(

From 8118bbeb50281e6fc86b604b7685ccdadbe736b8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Tue, 17 Sep 2024 09:54:47 +0200
Subject: [PATCH 13/17] Try to fix docs

---
 experimental/LieAlgebras/docs/src/ideals_and_subalgebras.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/experimental/LieAlgebras/docs/src/ideals_and_subalgebras.md b/experimental/LieAlgebras/docs/src/ideals_and_subalgebras.md
index e0c1c2bcad63..be741d34084d 100644
--- a/experimental/LieAlgebras/docs/src/ideals_and_subalgebras.md
+++ b/experimental/LieAlgebras/docs/src/ideals_and_subalgebras.md
@@ -17,7 +17,7 @@ They are used similarly in most cases.
 dim(::LieAlgebraIdeal)
 basis(::LieAlgebraIdeal)
 basis(::LieAlgebraIdeal, ::Int)
-Base.in(::LieAlgebraElem, ::LieAlgebraIdeal)
+Base.in(::LieAlgebraElem{C}, ::LieAlgebraIdeal{C}) where {C<:FieldElem}
 bracket(::LieAlgebraIdeal{C,LieT}, ::LieAlgebraIdeal{C,LieT}) where {C<:FieldElem,LieT<:LieAlgebraElem{C}}
 normalizer(::LieAlgebra, ::LieAlgebraIdeal)
 centralizer(::LieAlgebra, ::LieAlgebraIdeal)
@@ -29,7 +29,7 @@ centralizer(::LieAlgebra, ::LieAlgebraIdeal)
 dim(::LieSubalgebra)
 basis(::LieSubalgebra)
 basis(::LieSubalgebra, ::Int)
-Base.in(::LieAlgebraElem, ::LieSubalgebra)
+Base.in(::LieAlgebraElem{C}, ::LieSubalgebra{C}) where {C<:FieldElem}
 bracket(::LieSubalgebra{C,LieT}, ::LieSubalgebra{C,LieT}) where {C<:FieldElem,LieT<:LieAlgebraElem{C}}
 normalizer(::LieAlgebra, ::LieSubalgebra)
 centralizer(::LieAlgebra, ::LieSubalgebra)

From 8e0f354a9876a4c1d24984e8e4c830564f007456 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Wed, 18 Sep 2024 11:47:22 +0200
Subject: [PATCH 14/17] Update experimental/LieAlgebras/src/LieAlgebra.jl
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Co-authored-by: Felix Röhrich <47457568+felix-roehrich@users.noreply.github.com>
---
 experimental/LieAlgebras/src/LieAlgebra.jl | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index eaa85f48bdeb..e7bdda177b31 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -699,11 +699,7 @@ function _root_system_and_chevalley_basis(
       alpha_j in roots_simple
     ],
   )
-  R = root_system(cm; check=true) # TODO: disable check
-
-  # reorder the simple roots according to the Dynkin diagram.
-  # users usually expect this ordering, but it is not necessary for the root system.
-  _, ordering = root_system_type_with_ordering(R)
+  _, ordering = cartan_type_with_ordering(cm)
   permute!(roots_simple, ordering)
   cm = matrix(
     ZZ,

From b46fb11860ae28df8cc31d82e7a1510aaac1e5af Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Wed, 18 Sep 2024 18:23:39 +0200
Subject: [PATCH 15/17] Skip second cartan matrix computation

---
 experimental/LieAlgebras/src/LieAlgebra.jl | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index e7bdda177b31..a7a08882788c 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -701,14 +701,10 @@ function _root_system_and_chevalley_basis(
   )
   _, ordering = cartan_type_with_ordering(cm)
   permute!(roots_simple, ordering)
-  cm = matrix(
-    ZZ,
-    [
-      CartanInt(roots, alpha_i, alpha_j) for alpha_i in roots_simple,
-      alpha_j in roots_simple
-    ],
-  )
-  R = root_system(cm; check=true) # TODO: disable check
+  p = inv(Perm(ordering))
+  cm = p * cm * p
+  R = root_system(cm; check=false)
+  @assert root_system_type_with_ordering(R)[2] == 1:rank(R)
 
   # compute a Chevalley basis of L
   root_vectors = [

From 5491bc32fc93ef751a898af242167b0e1d84fabd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Wed, 18 Sep 2024 18:50:13 +0200
Subject: [PATCH 16/17] Update experimental/LieAlgebras/src/LieAlgebra.jl
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Co-authored-by: Felix Röhrich <47457568+felix-roehrich@users.noreply.github.com>
---
 experimental/LieAlgebras/src/LieAlgebra.jl | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index a7a08882788c..c38b4430ac0b 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -699,11 +699,9 @@ function _root_system_and_chevalley_basis(
       alpha_j in roots_simple
     ],
   )
-  _, ordering = cartan_type_with_ordering(cm)
+  type, ordering = cartan_type_with_ordering(cm; check=false)
   permute!(roots_simple, ordering)
-  p = inv(Perm(ordering))
-  cm = p * cm * p
-  R = root_system(cm; check=false)
+  R = root_system(type)
   @assert root_system_type_with_ordering(R)[2] == 1:rank(R)
 
   # compute a Chevalley basis of L

From 5c289adc1cb9dd99722ed7c9175067c878da1f86 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Wed, 18 Sep 2024 18:58:47 +0200
Subject: [PATCH 17/17] Update experimental/LieAlgebras/src/LieAlgebra.jl
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Co-authored-by: Felix Röhrich <47457568+felix-roehrich@users.noreply.github.com>
---
 experimental/LieAlgebras/src/LieAlgebra.jl | 1 -
 1 file changed, 1 deletion(-)

diff --git a/experimental/LieAlgebras/src/LieAlgebra.jl b/experimental/LieAlgebras/src/LieAlgebra.jl
index c38b4430ac0b..04a487f5d124 100644
--- a/experimental/LieAlgebras/src/LieAlgebra.jl
+++ b/experimental/LieAlgebras/src/LieAlgebra.jl
@@ -702,7 +702,6 @@ function _root_system_and_chevalley_basis(
   type, ordering = cartan_type_with_ordering(cm; check=false)
   permute!(roots_simple, ordering)
   R = root_system(type)
-  @assert root_system_type_with_ordering(R)[2] == 1:rank(R)
 
   # compute a Chevalley basis of L
   root_vectors = [