GitHub

Cartan matrices

Cartan matrices can be constructed from a Cartan type, and are represented as a square ZZMatrix.

Many functions taking a Cartan matrix as input (like root_system and weyl_group) will also accept a Cartan type as input. Both cases are semantically equivalent, but the latter may be more efficient.

Note

The convention for Cartan matrices in OSCAR is $(a_{ij}) = (\langle \alpha_i^\vee, \alpha_j \rangle)$ for simple roots $\alpha_i$.

Note

See Cartan types for our conventions on Cartan types and ordering of simple roots.

Table of contents

Constructors

cartan_matrix — Method
cartan_matrix(fam::Symbol, rk::Int) -> ZZMatrix

Return the Cartan matrix of finite type, where fam is the family ($A$, $B$, $C$, $D$, $E$, $F$ $G$) and rk is the rank of the associated the root system.

The input must be a valid Cartan type, see is_cartan_type(::Symbol, ::Int).

Examples

julia> cartan_matrix(:B, 2)
[ 2   -1]
[-2    2]

julia> cartan_matrix(:C, 2)
[ 2   -2]
[-1    2]
source
cartan_matrix — Method
cartan_matrix(type::Vector{Tuple{Symbol,Int}}) -> ZZMatrix
cartan_matrix(type::Tuple{Symbol,Int}...) -> ZZMatrix

Construct a block diagonal matrix of indecomposable Cartan matrices as defined by type that are constructed by cartan_matrix(::Symbol, ::Int).

Each element of type must be a valid Cartan type, see is_cartan_type(::Symbol, ::Int). The vararg version needs at least one element.

Examples

julia> cartan_matrix([(:A, 2), (:B, 2)])
[ 2   -1    0    0]
[-1    2    0    0]
[ 0    0    2   -1]
[ 0    0   -2    2]

julia> cartan_matrix((:C, 2), (:A, 2))
[ 2   -2    0    0]
[-1    2    0    0]
[ 0    0    2   -1]
[ 0    0   -1    2]
source

Properties

is_cartan_matrix — Method
is_cartan_matrix(mat::ZZMatrix; generalized::Bool=true) -> Bool

Check if mat is a generalized Cartan matrix. If generalized=false this check is restricted to Cartan matrices of finite type.

Examples

julia> is_cartan_matrix(ZZ[2 -2; -2 2])
true

julia> is_cartan_matrix(ZZ[2 -2; -2 2]; generalized=false)
false
source
cartan_symmetrizer — Method
cartan_symmetrizer(gcm::ZZMatrix; check::Bool=true) -> Vector{ZZRingElem}

Return a vector $d$ of coprime integers such that $(d_i a_{ij})_{ij}$ is a symmetric matrix, where $a_{ij}$ are the entries of the Cartan matrix gcm.

If check=true the function will verify that gcm is indeed a generalized Cartan matrix.

Warning

Currently only Cartan matrices of finite type are supported.

Examples

julia> cartan_symmetrizer(cartan_matrix(:B, 2))
2-element Vector{ZZRingElem}:
 2
 1
source
cartan_bilinear_form — Method
cartan_bilinear_form(gcm::ZZMatrix; check::Bool=true) -> ZZMatrix

Return the matrix of the symmetric bilinear form associated to the Cartan matrix from cartan_symmetrizer.

If check=true the function will verify that gcm is indeed a generalized Cartan matrix.

Examples

julia> bil = cartan_bilinear_form(cartan_matrix(:B, 2))
[ 4   -2]
[-2    2]

julia> is_symmetric(bil)
true
source

Cartan types

Cartan types and, in particular, the ordering of simple roots are used differently in the literature. OSCAR follows the conventions of [Bou02, Plates I-IX] and [Hum72, Thm 11.4], i.e. we consider the following Dynkin diagrams for the Cartan types (where the ordering of simple roots is given by the numbering of the vertices):

  • $A_n$ (for $n \geq 1$):

    1 - 2 - 3 - ... - n-1 - n

  • $B_n$ (for $n \geq 2$):

    1 - 2 - 3 - ... - n-1 >=> n

  • $C_n$ (for $n \geq 2$):

    1 - 2 - 3 - ... - n-1 <=< n

  • $D_n$ (for $n \geq 4$):

                          n-1
                    /
1 - 2 - 3 - ... - n-2
                    \
                      n

  • $E_6$:

    1 - 3 - 4 - 5 - 6
        |
        2

  • $E_7$:

    1 - 3 - 4 - 5 - 6 - 7
        |
        2

  • $E_8$:

    1 - 3 - 4 - 5 - 6 - 7 - 8
        |
        2

  • $F_4$:

    1 - 2 >=> 3 - 4

  • $G_2$:

    1 <<< 2

The following function is used to verify the validity of a Cartan type, and is thus used in input sanitization.

is_cartan_type — Method
is_cartan_type(fam::Symbol, rk::Int) -> Bool

Check if the pair (fam, rk) is a valid Cartan type, i.e., one of

  • A_l ($l >= 1$),
  • B_l ($l >= 2$),
  • C_l ($l >= 2$),
  • D_l ($l >= 4$),
  • E_6, E_7, E_8,
  • F_4,
  • G_2.
source

Given a Cartan matrix, the following functions can be used to determine its Cartan type.

cartan_type — Method
cartan_type(gcm::ZZMatrix; check::Bool=true) -> Vector{Tuple{Symbol, Int}}

Return the Cartan type of a Cartan matrix gcm.

This function is left inverse to cartan_matrix, i.e., in the case of isomorphic types (e.g. $B_2$ and $C_2$) the ordering of the roots does matter (see the example below).

If check=true the function will verify that gcm is indeed a Cartan matrix.

Note

The order of returned components is, in general, not unique and might change between versions. But we guarantee that if this function is called with the output of cartan_matrix(::Vector{Tuple{Symbol,Int}}), it will keep the order of type.

Warning

Currently only Cartan matrices of finite type are supported.

Examples

julia> cartan_type(ZZ[2 -1; -2 2])
1-element Vector{Tuple{Symbol, Int64}}:
 (:B, 2)

julia> cartan_type(ZZ[2 -2; -1 2])
1-element Vector{Tuple{Symbol, Int64}}:
 (:C, 2)
source
cartan_type_with_ordering — Method
cartan_type_with_ordering(gcm::ZZMatrix; check::Bool=true) -> Vector{Tuple{Symbol, Int}}, Vector{Int}

Return the Cartan type of a Cartan matrix gcm together with a vector indicating a canonical ordering of the roots in the Dynkin diagram.

If check=true the function will verify that gcm is indeed a Cartan matrix.

Note

The order of returned components is, in general, not unique and might change between versions. But we guarantee that if this function is called with the output of cartan_matrix(::Vector{Tuple{Symbol,Int}}), it will keep the order of type and the ordering will be the identity.

Warning

Currently only Cartan matrices of finite type are supported.

Examples

julia> cartan_type_with_ordering(cartan_matrix(:E, 6))
([(:E, 6)], [1, 2, 3, 4, 5, 6])

julia> cartan_type_with_ordering(ZZ[2 0 -1 0; 0 2 0 -2; -2 0 2 0; 0 -1 0 2])
([(:B, 2), (:C, 2)], [1, 3, 2, 4])
source