Simple Lie algebras and root systems (#2572)

experimental/LieAlgebras/docs/src/introduction.md

@@ -18,6 +18,7 @@ This part of OSCAR is in an experimental state; please see [Adding new projects
 
 Please direct questions about this part of OSCAR to the following people:
 * [Lars Göttgens](https://lgoe.li/)
+* [Laura Voggesberger](https://www.ruhr-uni-bochum.de/ffm/Lehrstuehle/Lehrstuhl-VI/voggesberger.html)
 
 You can ask questions in the [OSCAR Slack](https://www.oscar-system.org/community/#slack).
 

experimental/LieAlgebras/docs/src/lie_algebras.md

@@ -24,6 +24,7 @@ coefficients(::LieAlgebraElem)
 coeff(::LieAlgebraElem, ::Int)
 getindex(::LieAlgebraElem, ::Int)
 symbols(::LieAlgebra)
+characteristic(L::LieAlgebra)
 ```
 
 ## Special functions for `LinearLieAlgebra`s

experimental/LieAlgebras/src/LieAlgebra.jl

@@ -57,6 +57,13 @@ function basis(L::LieAlgebra, i::Int)
   return L([(j == i ? one(R) : zero(R)) for j in 1:dim(L)])
 end
 
+@doc raw"""
+    characteristic(L::LieAlgebra) -> Int
+
+Return the characteristic of the coefficient ring of the Lie algebra `L`.
+"""
+characteristic(L::LieAlgebra) = characteristic(coefficient_ring(L))
+
 @doc raw"""
     zero(L::LieAlgebra{C}) -> LieAlgebraElem{C}
 

experimental/LieAlgebras/src/LieAlgebras.jl

@@ -21,6 +21,7 @@ import ..Oscar:
   canonical_projections,
   center,
   centralizer,
+  characteristic,
   coeff,
   coefficient_ring,
   coefficients,
@@ -72,17 +73,23 @@ export LieSubalgebra
 export LieAlgebraModule, LieAlgebraModuleElem
 export LieAlgebraModuleHom
 export LinearLieAlgebra, LinearLieAlgebraElem
+export RootSystem
+export SimpleLieAlgebra, SimpleLieAlgebraElem
 
 export abelian_lie_algebra
 export abstract_module
 export base_lie_algebra
 export base_module
 export base_modules
 export bracket
+export cartan_matrix
+export chevalley_basis
 export coefficient_vector
 export coerce_to_lie_algebra_elem
 export combinations
 export derived_algebra
+export dynkin_diagram
+export exterior_power
 export general_linear_lie_algebra
 export highest_weight_module
 export hom_direct_sum
@@ -97,8 +104,11 @@ export is_tensor_product
 export lie_algebra
 export matrix_repr_basis
 export multicombinations
+export number_of_roots
 export permutations
 export permutations_with_sign
+export root_system
+export root_system_type
 export special_linear_lie_algebra
 export special_orthogonal_lie_algebra
 export standard_module
@@ -120,6 +130,8 @@ include("LieAlgebraModuleHom.jl")
 include("iso_oscar_gap.jl")
 include("iso_gap_oscar.jl")
 include("GapWrapper.jl")
+include("root_systems.jl")
+include("simple_lie_algebra.jl")
 
 end
 
@@ -133,15 +145,21 @@ export LieAlgebraModule, LieAlgebraModuleElem
 export LieAlgebraModuleHom
 export LieSubalgebra
 export LinearLieAlgebra, LinearLieAlgebraElem
+export RootSystem
+export SimpleLieAlgebra, SimpleLieAlgebraElem
 
 export abelian_lie_algebra
 export abstract_module
 export base_lie_algebra
 export base_module
 export base_modules
 export bracket
+export cartan_matrix
+export chevalley_basis
 export coerce_to_lie_algebra_elem
 export derived_algebra
+export dynkin_diagram
+export exterior_power
 export general_linear_lie_algebra
 export highest_weight_module
 export hom_direct_sum
@@ -155,6 +173,9 @@ export is_tensor_power
 export is_tensor_product
 export lie_algebra
 export matrix_repr_basis
+export number_of_roots
+export root_system
+export root_system_type
 export special_linear_lie_algebra
 export special_orthogonal_lie_algebra
 export standard_module

experimental/LieAlgebras/src/root_systems.jl

@@ -0,0 +1,237 @@
+mutable struct RootSystem
+  roots::Vector{Vector{Int}}
+  simple_roots::Vector{Vector{Int}}
+  positive_roots::Vector{Vector{Int}}
+  root_system_type::Tuple{Symbol,Int}
+  GAP_root_system::GAP.GapObj
+  function RootSystem(S::Symbol, n::Int)
+    # S is a symbol detailing the type of the indecomposable root system 
+    # e.g. "A", "B", "C",... and n is an integer for the number of simple roots
+    S1 = GAP.Obj(S)
+    RS = GAP.Globals.RootSystem(S1, n)
+    sR = Vector{Vector{Int}}(GAP.Globals.SimpleSystem(RS))
+    Ro1 = Vector{Vector{Int}}(GAP.Globals.PositiveRoots(RS))
+    Ro2 = Vector{Vector{Int}}(GAP.Globals.NegativeRoots(RS))
+    Ro = vcat(Ro1, Ro2)
+    t = (S, n)
+    return new(Ro, sR, Ro1, t, RS)
+  end
+end
+
+###############################################################################
+#
+#   Basic manipulation
+#
+###############################################################################
+@doc raw"""
+    number_of_roots(R::RootSystem)
+
+Return the numbers of roots in the root system `R`.
+"""
+number_of_roots(R::RootSystem) = size(R.roots)[1]
+
+@doc raw"""
+    number_of_roots(S::Symbol, n::Int)
+
+Return the numbers of roots in the root system of type `S`
+"""
+number_of_roots(S::Symbol, n::Int) = number_of_roots(root_system(S, n))
+
+@doc raw"""
+    getindex(R::RootSystem, r::Int)
+
+Return the `r`-th root of the root system `R`.
+"""
+getindex(R::RootSystem, r::Int) = getindex(R.roots, r)
+
+@doc raw"""
+    root_system_type(R::RootSystem)
+
+Return the Dynkin type of the root system `R`.
+"""
+root_system_type(R::RootSystem) = R.root_system_type
+
+root_system_type_string(R::RootSystem) =
+  string(R.root_system_type[1]) * string(R.root_system_type[2])
+
+###############################################################################
+#
+#   String I/O
+#
+###############################################################################
+
+function Base.show(io::IO, R::RootSystem)
+  print(io, "Root system of type $(root_system_type_string(R))")
+end
+
+###############################################################################
+#
+#   Comparison
+#
+###############################################################################
+
+function Base.:(==)(R1::RootSystem, R2::RootSystem)
+  return R1.root_system_type == R2.root_system_type && R1.roots == R2.roots
+end
+
+function Base.hash(R::RootSystem, h::UInt)
+  b = 0x9d96557cb5f07773 % UInt
+  h = hash(R.root_system_type, h)
+  h = hash(R.roots, h)
+  return xor(h, b)
+end
+###############################################################################
+#
+#   Constructor
+#
+###############################################################################
+
+@doc raw"""
+    root_system(S::Symbol, n::Int) -> RootSystem
+    
+Return the root system of type `Sn` where `S` is a symbol consisting out of
+a single letter `A`, `B`, `C`, `D`, `E`, `F`, `G`.
+The allowed values for `n` depend on the choice of `S`.
+"""
+function root_system(S::Symbol, n::Int)
+  @req _root_system_type_supported_by_GAP(S, n) "Unknown Dynkin type or not supported by GAP"
+  return RootSystem(S, n)
+end
+
+###############################################################################
+#
+#   further functions
+#
+###############################################################################
+
+function _root_system_type_supported_by_GAP(S, n)
+  S in [:A, :B, :C, :D, :E, :F, :G] || return false
+  n >= 1 || return false
+  S == :D && n < 4 && return false
+  S == :E && !(n in [6, 7, 8]) && return false
+  S == :F && n != 4 && return false
+  S == :G && n != 2 && return false
+  return true
+end
+
+@doc raw"""
+    cartan_matrix(S::Symbol, n::Int) -> Matrix{QQFieldElem}
+
+Return the Cartan matrix of the root system of type `Sn`.
+For the semantics of the arguments, refer to `root_system(S::Symbol, n::Int)` ref.
+"""
+function cartan_matrix(S::Symbol, n::Int)
+  return cartan_matrix(root_system(S, n))
+end
+
+@doc raw"""
+    cartan_matrix(R::RootSystem) -> Matrix{QQFieldElem}
+
+Return the Cartan matrix of the root system `R`.
+"""
+function cartan_matrix(R::RootSystem)
+  RS = R.GAP_root_system
+  CG = GAP.Globals.CartanMatrix(RS)
+  C = matrix(QQ, CG)
+  return C
+end
+
+@doc raw"""
+    dynkin_diagram(S::Symbol, n::Int)
+
+Return the Dynkin diagram of the root system of type `Sn`.
+For the semantics of the arguments, refer to [root_system(S::Symbol, n::Int)` ref.
+"""
+function dynkin_diagram(S::Symbol, n::Int)
+  @req _root_system_type_supported_by_GAP(S, n) "Unknown Dynkin type or not supported by GAP"
+  D = ""
+
+  if S == :A
+    for i in 1:(n - 1)
+      D = D * string(i) * " - "
+    end
+    D = D * string(n)
+
+  elseif S == :B
+    if n == 1
+      D = string(n)
+    else
+      for i in 1:(n - 2)
+        D = D * string(i) * " - "
+      end
+      D = D * string(n - 1) * " >=> " * string(n)
+    end
+
+  elseif S == :C
+    if n == 1
+      D = string(n)
+    else
+      for i in 1:(n - 2)
+        D = D * string(i) * " - "
+      end
+      D = D * string(n - 1) * " <=< " * string(n)
+    end
+
+  elseif S == :D
+    if n >= 4
+      for i in 1:(4 * n - 10)
+        D = D * " "
+      end
+      D = D * string(n - 1) * "\n"
+      for i in 1:(4 * n - 11)
+        D = D * " "
+      end
+      D = D * "/\n"
+      for i in 1:(n - 3)
+        D = D * string(i) * " - "
+      end
+      D = D * string(n - 2) * "\n"
+      for i in 1:(4 * n - 12)
+        D = D * " "
+      end
+      D = D * " \\ \n"
+      for i in 1:(4 * n - 10)
+        D = D * " "
+      end
+      D = D * string(n)
+    else
+      error("This root system doesn't exist.")
+    end
+
+  elseif S == :E
+    if n == 6
+      D = "1 - 3 - 4 - 5 - 6\n        |\n        2"
+    elseif n == 7
+      D = "1 - 3 - 4 - 5 - 6 - 7\n        |\n        2"
+    elseif n == 8
+      D = "1 - 3 - 4 - 5 - 6 - 7 - 8\n        |\n        2"
+    else
+      error("This root system doesn't exist.")
+    end
+
+  elseif S == :F
+    if n == 4
+      D = "1 - 2 >=> 3 - 4"
+    else
+      error("This root system doesn't exist.")
+    end
+  elseif S == :G
+    if n == 2
+      D = "1 >>> 2"
+    else
+      error("This root system doesn't exist.")
+    end
+  else
+    error("This root system doesn't exist.")
+  end
+  print(D)
+end
+
+@doc raw"""
+    dynkin_diagram(R::RootSystem)
+
+Return the Dynkin diagram of the root system `R`
+"""
+function dynkin_diagram(R::RootSystem)
+  return dynkin_diagram(R.root_system_type...)
+end