The OSCAR Book Companion
The OSCAR Book Companion
The preprint of this chapter is available here.
import Pkg
Pkg.add(name="GenericCharacterTables", version=v"0.2"; io=devnull)
using GenericCharacterTables
# for nicer printing
using GenericCharacterTables: ParameterException
julia> G_perm = @permutation_group(5, (2,5)(3,4), (1,3)(4,5))
Permutation group of degree 5
julia> order(G_perm)
10
julia> describe(G_perm)
"D10"
julia> r = G_perm[1] * G_perm[2]
(1,3,5,2,4)
julia> g = perm([1,2,3,5,4]) # 'tabular' notation
(4,5)
julia> g in G_perm
false
julia> G_perm([2,1,5,4,3]) # 'tabular' notation with explicit parent
(1,2)(3,5)
julia> cperm([1,2],[4,5]) # cycle decomposition
(1,2)(4,5)
julia> @perm (1,2)(4,5) # cycle notation via macro
(1,2)(4,5)
julia> K = algebraic_closure(QQ)
Field of algebraic numbers
julia> e = one(K)
Root 1.00000 of x - 1
julia> s, c = sinpi(2*e/5), cospi(2*e/5)
(Root 0.951057 of 16x^4 - 20x^2 + 5, Root 0.309017 of 4x^2 + 2x - 1)
julia> mat_rot = matrix([ c -s ; s c ]);
julia> mat_sigma1 = matrix(K, [ -1 0 ; 0 1 ]);
julia> G_mat = matrix_group(mat_rot, mat_sigma1)
Matrix group of degree 2
over field of algebraic numbers
julia> is_isomorphic(G_mat, G_perm)
true
julia> p = K.([0,1]); # coordinates of vertex 1, expressed over K
julia> orb = orbit(G_mat, *, p)
G-set of
matrix group of degree 2 over QQBar
with seeds Vector{QQBarFieldElem}[[Root 0 of x, Root 1.00000 of x - 1]]
julia> pts = collect(orb)
5-element Vector{Vector{QQBarFieldElem}}:
[Root 0 of x, Root 1.00000 of x - 1]
[Root 0.951057 of 16x^4 - 20x^2 + 5, Root 0.309017 of 4x^2 + 2x - 1]
[Root 0.587785 of 16x^4 - 20x^2 + 5, Root -0.809017 of 4x^2 + 2x - 1]
[Root -0.951057 of 16x^4 - 20x^2 + 5, Root 0.309017 of 4x^2 + 2x - 1]
[Root -0.587785 of 16x^4 - 20x^2 + 5, Root -0.809017 of 4x^2 + 2x - 1]
julia> visualize(convex_hull(pts))
julia> R2 = free_module(K, 2) # the "euclidean" plane over K
Vector space of dimension 2 over QQBar
julia> A = R2([0,1])
(Root 0 of x, Root 1.00000 of x - 1)
julia> pts = [A*mat_rot^i for i in 0:4];
julia> sigma_1 = hom(R2, R2, [-R2[1], R2[2]])
Module homomorphism
from vector space of dimension 2 over QQBar
to vector space of dimension 2 over QQBar
julia> rot = hom(R2, R2, mat_rot);
julia> G_generic = generic_group(closure([rot, sigma_1], *), *)[1]
Generic group of order 10 with multiplication table
julia> permutation_group(G_generic)
Permutation group of degree 10
julia> is_isomorphic(ans, G_perm)
true
julia> G_fp = fp_group(G_perm)
Finitely presented group of order 10
julia> gens(G_fp)
2-element Vector{FPGroupElem}:
F1
F2
julia> relators(G_fp)
3-element Vector{FPGroupElem}:
F1^2
F1^-1*F2*F1*F2^-4
F2^5
julia> F = free_group(2)
Free group of rank 2
julia> G_cox, _ = quo(F, [F[1]^2, F[2]^2, (F[1]*F[2])^5])
(Finitely presented group, Hom: F -> G_cox)
julia> is_isomorphic(G_cox, G_perm)
true
julia> F = free_group(:a, :b, :c); a,b,c = gens(F);
julia> G, _ = quo(F, [a^2, b^2, c^2, (a*b)^3, (a*c)^2, (b*c)^3])
(Finitely presented group, Hom: F -> G)
julia> H, _ = quo(F, [a^2, b^2, c^2, (a*b)^3, (a*c)^3, (b*c)^3])
(Finitely presented group, Hom: F -> H)
julia> is_finite(G)
true
julia> describe(G)
"S4"
julia> describe(H)
"a finitely presented group"
julia> H1, _ = derived_subgroup(H)
(Sub-finitely presented group, Hom: H1 -> H)
julia> H2, _ = derived_subgroup(H1)
(Sub-finitely presented group, Hom: H2 -> H1)
julia> describe(H2)
"Z x Z"
julia> describe(quo(H, H2)[1])
"(C3 x C3) : C2"
julia> dihedral_group(10)
Pc group of order 10
julia> dihedral_group(PermGroup, 10)
Permutation group of degree 5
julia> gset(alternating_group(4))
G-set of
Alt(4)
with seeds 1:4
julia> G = dihedral_group(6)
Pc group of order 6
julia> U = sub(G, [g for g in gens(G) if order(g) == 2])[1]
Sub-pc group of order 2
julia> r = right_cosets(G, U)
Right cosets of
sub-pc group of order 2 in
pc group of order 6
julia> acting_group(r)
Pc group of order 6
julia> collect(r)
3-element Vector{GroupCoset{PcGroup, PcGroupElem}}:
Right coset of U with representative <identity> of ...
Right coset of U with representative f2
Right coset of U with representative f2^2
julia> action_function(r)
* (generic function with 1736 methods)
julia> permutation(r, G[1])
(2,3)
julia> phi = right_coset_action(G,U)
Group homomorphism
from pc group of order 6
to permutation group of degree 3 and order 6
julia> phi(G[1]), phi(G[2])
((2,3), (1,2,3))
julia> function optimal_perm_rep(G)
is_natural_symmetric_group(G) && return hom(G,G,gens(G))
is_natural_alternating_group(G) && return hom(G,G,gens(G))
cand = [] # pairs (U,h) with U ≤ G and h a map G -> Sym(G/U)
for C in subgroup_classes(G)
U = representative(C)
h = right_coset_action(G, U)
is_injective(h) && push!(cand, (U, h))
end
return argmax(a -> order(a[1]), cand)[2]
end;
julia> U = dihedral_group(8)
Pc group of order 8
julia> optimal_perm_rep(U)
Group homomorphism
from pc group of order 8
to permutation group of degree 4 and order 8
julia> isomorphism(PermGroup, U)
Group homomorphism
from pc group of order 8
to permutation group of degree 8 and order 8
julia> permutation_group(U)
Permutation group of degree 8 and order 8
julia> for g in all_transitive_groups(degree => 3:9, !is_primitive)
h = image(optimal_perm_rep(g))[1]
if degree(h) < degree(g)
id = transitive_group_identification(g)
id_new = transitive_group_identification(h)
println(id => id_new)
end
end
(6, 2) => (3, 2)
(6, 4) => (4, 4)
(6, 7) => (4, 5)
(6, 8) => (4, 5)
(8, 4) => (4, 3)
(8, 13) => (6, 6)
(8, 14) => (4, 5)
(8, 24) => (6, 11)
(9, 4) => (6, 5)
(9, 8) => (6, 9)
The characters of SL(2,5) … and the representation of chapter 8
julia> G = SL(2, 5)
SL(2,5)
julia> T = character_table(G)
Character table of SL(2,5)
2 3 1 1 3 1 1 1 1 2
3 1 . . 1 . . 1 1 .
5 1 1 1 1 1 1 . . .
1a 10a 10b 2a 5a 5b 3a 6a 4a
X_1 1 1 1 1 1 1 1 1 1
X_2 2 z_5^3 + z_5^2 + 1 -z_5^3 - z_5^2 -2 -z_5^3 - z_5^2 - 1 z_5^3 + z_5^2 -1 1 .
X_3 2 -z_5^3 - z_5^2 z_5^3 + z_5^2 + 1 -2 z_5^3 + z_5^2 -z_5^3 - z_5^2 - 1 -1 1 .
X_4 3 -z_5^3 - z_5^2 z_5^3 + z_5^2 + 1 3 -z_5^3 - z_5^2 z_5^3 + z_5^2 + 1 . . -1
X_5 3 z_5^3 + z_5^2 + 1 -z_5^3 - z_5^2 3 z_5^3 + z_5^2 + 1 -z_5^3 - z_5^2 . . -1
X_6 4 -1 -1 4 -1 -1 1 1 .
X_7 4 1 1 -4 -1 -1 1 -1 .
X_8 5 . . 5 . . -1 -1 1
X_9 6 -1 -1 -6 1 1 . . .
julia> R = gmodule(T[end])
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> schur_index(T[end])
2
julia> gmodule_minimal_field(S)
G-module for G acting on vector space of dimension 6 over number field
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
julia> K = grunwald_wang(b)
Class field defined mod (<40, 360>, InfPlc{AbsSimpleNumField, AbsSimpleNumFieldEmbedding}[Infinite place corresponding to (Complex embedding corresponding to 1.00 of number field)]) of structure Z/2
julia> F, _ = absolute_simple_field(number_field(K))
(Number field of degree 2 over QQ, Map: F -> non-simple number field)
julia> L, _, F_to_L = compositum(base_ring(S), F)
(Number field of degree 8 over QQ, Map: cyclotomic field of order 5 -> L, Map: F -> L)
julia> gmodule_over(F_to_L, gmodule(L, S))
G-module for G acting on vector space of dimension 6 over F
julia> t = character_table("J2");
julia> mx = character_table.(maxes(t));
julia> pis = [trivial_character(s)^t for s in mx];
julia> ords = orders_class_representatives(t); println(ords)
[1, 2, 2, 3, 3, 4, 5, 5, 5, 5, 6, 6, 7, 8, 10, 10, 10, 10, 12, 15, 15]
julia> ord7 = findall(is_equal(7), ords);
julia> filt = filter(pi -> pi[ord7[1]] != 0, pis);
julia> degree.(filt)
2-element Vector{QQFieldElem}:
100
1800
julia> ord5 = findall(is_equal(5), ords);
julia> s = sum(filt);
julia> all(i -> s[i] == 0, ord5)
true
julia> (x -> [x["1a"], x["2b"], x["3a"], x["3b"]]).(filt)
2-element Vector{Vector{QQAbElem{AbsSimpleNumFieldElem}}}:
[100, 0, 10, 4]
[1800, 20, 0, 6]
julia> ord3 = findall(is_equal(3), ords);
julia> [class_multiplication_coefficient(t, 3, i, ord7[1]) for i in ord3]
2-element Vector{ZZRingElem}:
0
70
julia> s = mx[findfirst(is_equal(filt[2]), pis)];
julia> identifier(s)
"L3(2).2"
julia> possible_class_fusions(s, t)
1-element Vector{Vector{Int64}}:
[1, 2, 5, 6, 13, 3, 12, 14, 14]
julia> class_multiplication_coefficient(s, 6, 3, 5)
0
julia> Oscar.randseed!(42)
julia> G = dihedral_group(PermGroup, 10);
julia> h = epimorphism_from_free_group(G)
Group homomorphism
from free group of rank 2
to permutation group of degree 5
julia> gens(G)
2-element Vector{PermGroupElem}:
(1,2,3,4,5)
(2,5)(3,4)
julia> g = rand(G)
(1,4)(2,3)
julia> w = preimage(h, g)
x1^-1*x2^-1*x1^-3
julia> map_word(w, gens(G)) # evaluate w at generators to get back g
(1,4)(2,3)
julia> k, mk = kernel(h); # mk is a map from k to domain(h)
julia> ngens(k)
11
julia> mk(k[2]) # pick any kernel generator ...
x1*x2^-2*x1^-1
julia> G[1]*G[-2]^2*G[-1] # ... and it evaluates to the identity
()
julia> U = cyclic_group(5); V = cyclic_group(2); F = free_group(2);
julia> q = quo(F, [F[1]^5, F[2]^2*F[1]])[1];
julia> describe(q)
"C10"
julia> q, mq = quo(F, [F[1]^5, F[2]^2, F[-1]*F[-2]*F[1]*F[2]*F[1]^2])
(Finitely presented group, Hom: F -> q)
julia> describe(q)
"D10"
julia> irreducible_modules(symmetric_group(3))
3-element Vector{GModule}:
G-module for Sym(3) acting on vector space of dimension 1 over abelian closure of Q
G-module for Sym(3) acting on vector space of dimension 1 over abelian closure of Q
G-module for Sym(3) acting on vector space of dimension 2 over abelian closure of Q
julia> G = symmetric_group(3);
julia> A = abelian_group([2,2,2])
(Z/2)^3
julia> M = gmodule(G, [hom(A, A, permuted(gens(A), g)) for g in gens(G)])
G-module for G acting on A
julia> function regular_gmodule(F::Field, G::Oscar.GAPGroup)
FM = free_module(F, order(Int, G));
gs = gset(G, *, [one(G)])
return gmodule(G, [hom(FM, FM, permutation_matrix(F, permutation(gs, g))) for g in gens(G)])
end;
julia> Base.:^(a::T, b::T) where T <: MatElem = inv(b)*a*b
julia> G = dihedral_group(10);
julia> M = regular_gmodule(GF(7), G);
julia> C = composition_factors_with_multiplicity(M)
3-element Vector{Any}:
(G-module for G acting on vector space of dimension 1 over GF(7), 1)
(G-module for G acting on vector space of dimension 1 over GF(7), 1)
(G-module for G acting on vector space of dimension 4 over GF(7), 2)
julia> [is_absolutely_irreducible(x[1]) for x in C]
3-element Vector{Bool}:
1
1
0
julia> phi = embed(GF(7), splitting_field(C[3][1]))
Morphism of finite fields
from prime field of characteristic 7
to finite field of degree 2 and characteristic 7
julia> M = extension_of_scalars(C[3][1], phi)
G-module for G acting on vector space of dimension 4 over GF(7, 2)
julia> composition_factors_with_multiplicity(M)
2-element Vector{Any}:
(G-module for G acting on vector space of dimension 2 over GF(7, 2), 1)
(G-module for G acting on vector space of dimension 2 over GF(7, 2), 1)
julia> G = pc_group(symmetric_group(4));
julia> C = abelian_closure(QQ)[1];
julia> F = free_module(C, 1);
julia> s, ms = sub(G, [G[3], G[4]]); # subgroup generated by c, d
julia> T = trivial_gmodule(s, F);
julia> z = root_of_unity(C, 3);
julia> ss, mss = sub(G, [G[2], G[3], G[4]]);
julia> M = gmodule(ss, [hom(F, F, [z*F[1]]), action(T, s[1]), action(T, s[2])])
G-module for ss acting on F
julia> FF = free_module(C, 2);
julia> zm = 0*matrix(action(M, one(ss)));
julia> im = [hom(FF, FF, [matrix(action(M, x)) zm; zm matrix(action(M, preimage(mss, mss(x)^G[1])))]) for x in gens(ss)];
julia> pushfirst!(im, hom(FF, FF, matrix(C, [0 1; 1 0])));
julia> phi = gmodule(G, im);
julia> character(phi)
class_function(character table of G, QQAbElem{AbsSimpleNumFieldElem}[2, 0, -1, 2, 0])
julia> schur_index(ans)
1
julia> T = matrix(C, [C(1) -z; z+1 z]);
julia> [matrix(x)^T for x in action(phi)]
4-element Vector{AbstractAlgebra.Generic.MatSpaceElem{QQAbElem{AbsSimpleNumFieldElem}}}:
[1 0; -1 -1]
[0 1; -1 -1]
[1 0; 0 1]
[1 0; 0 1]
julia> G = SL(2, 5)
SL(2,5)
julia> T = character_table(G)
Character table of SL(2,5)
2 3 1 1 3 1 1 1 1 2
3 1 . . 1 . . 1 1 .
5 1 1 1 1 1 1 . . .
1a 10a 10b 2a 5a 5b 3a 6a 4a
X_1 1 1 1 1 1 1 1 1 1
X_2 2 z_5^3 + z_5^2 + 1 -z_5^3 - z_5^2 -2 -z_5^3 - z_5^2 - 1 z_5^3 + z_5^2 -1 1 .
X_3 2 -z_5^3 - z_5^2 z_5^3 + z_5^2 + 1 -2 z_5^3 + z_5^2 -z_5^3 - z_5^2 - 1 -1 1 .
X_4 3 -z_5^3 - z_5^2 z_5^3 + z_5^2 + 1 3 -z_5^3 - z_5^2 z_5^3 + z_5^2 + 1 . . -1
X_5 3 z_5^3 + z_5^2 + 1 -z_5^3 - z_5^2 3 z_5^3 + z_5^2 + 1 -z_5^3 - z_5^2 . . -1
X_6 4 -1 -1 4 -1 -1 1 1 .
X_7 4 1 1 -4 -1 -1 1 -1 .
X_8 5 . . 5 . . -1 -1 1
X_9 6 -1 -1 -6 1 1 . . .
julia> R = gmodule(T[end])
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> schur_index(T[end])
2
julia> gmodule_minimal_field(S)
G-module for G acting on vector space of dimension 6 over number field
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
julia> K = grunwald_wang(b)
Class field defined mod (<40, 360>, InfPlc{AbsSimpleNumField, AbsSimpleNumFieldEmbedding}[Infinite place corresponding to (Complex embedding corresponding to 1.00 of number field)]) of structure Z/2
julia> F, _ = absolute_simple_field(number_field(K))
(Number field of degree 2 over QQ, Map: F -> non-simple number field)
julia> L, _, F_to_L = compositum(base_ring(S), F)
(Number field of degree 8 over QQ, Map: cyclotomic field of order 5 -> L, Map: F -> L)
julia> gmodule_over(F_to_L, gmodule(L, S))
G-module for G acting on vector space of dimension 6 over F
This section showcases the
GenericCharacterTables.jl package.
An extended and possibly updated version of this example can be found in
the package manual.
julia> T = genchartab("SL3.n1")
Generic character table
of order q^8 - q^6 - q^5 + q^3
with 8 irreducible character types
with 8 class types
with parameters (a, b, m, n)
julia> printval(T,char=4,class=4)
Value of character type 4 on class type
4: (q + 1) * exp(2π𝑖(1//(q - 1)*a*n)) + (1) * exp(2π𝑖(-2//(q - 1)*a*n))
julia> h = tensor!(T,2,2)
9
julia> scalar(T,4,h)
(0, Set(ParameterException{QQPolyRingElem}[(2*n1)//(q - 1) ∈ ℤ]))
julia> print_decomposition(T, h)
Decomposing character 9:
<1,9> = 1
<2,9> = 2
<3,9> = 2
<4,9> = 0 with possible exceptions:
(2*n1)//(q - 1) ∈ ℤ
<5,9> = 0 with possible exceptions:
(2*n1)//(q - 1) ∈ ℤ
<6,9> = 0 with possible exceptions:
(m1 + n1)//(q - 1) ∈ ℤ
(2*m1 - n1)//(q - 1) ∈ ℤ
(m1)//(q - 1) ∈ ℤ
(n1)//(q - 1) ∈ ℤ
(m1 - n1)//(q - 1) ∈ ℤ
(m1 - 2*n1)//(q - 1) ∈ ℤ
<7,9> = 0 with possible exceptions:
(n1)//(q - 1) ∈ ℤ
<8,9> = 0 with possible exceptions:
((q + 1)*n1)//(q^2 + q + 1) ∈ ℤ
(q*n1)//(q^2 + q + 1) ∈ ℤ
(n1)//(q^2 + q + 1) ∈ ℤ
julia> chardeg(T, lincomb!(T,[1,2,2],[1,2,3]))
2*q^3 + 2*q^2 + 2*q + 1
julia> chardeg(T, h)
q^4 + 2*q^3 + q^2
julia> printcharparam(T,4)
4 n ∈ {1,…, q - 1} except (n)//(q - 1) ∈ ℤ
julia> T2 = setcongruence(T, (0,2));
julia> (q, (a, b, m, n)) = params(T2);
julia> x = param(T2, "x"); # create an additional "free" variable
julia> speccharparam!(T2, 6, m, -n + (q-1)*x) # force m = -n (mod q-1)
julia> s, e = scalar(T2,6,h); s
1
julia> e
Set{ParameterException{QQPolyRingElem}} with 2 elements:
(2*n1)//(q - 1) ∈ ℤ
(3*n1)//(q - 1) ∈ ℤ