This file contains functions that are used by the various ambient modular symbols classes to compute presentations of spaces in terms of generators and relations, using the standard methods based on Manin symbols.
Compute a matrix whose echelon form gives the quotient by 3-term T relations. Despite the name, this is used for all modular symbols spaces (including those with character and those for \(\Gamma_1\) and \(\Gamma_H\) groups), not just \(\Gamma_0\).
INPUT:
OUTPUT: A sparse matrix whose rows correspond to the reduction of the T relations modulo the S and I relations.
EXAMPLE:
sage: from sage.modular.modsym.relation_matrix import *
sage: L = sage.modular.modsym.manin_symbols.ManinSymbolList_gamma_h(GammaH(36, [17,19]), 2)
sage: modS = sparse_2term_quotient(modS_relations(L), 216, QQ)
sage: T_relation_matrix_wtk_g0(L, modS, QQ, False)
72 x 216 dense matrix over Rational Field (use the '.str()' method to see the entries)
sage: T_relation_matrix_wtk_g0(L, modS, GF(17), True)
72 x 216 sparse matrix over Finite Field of size 17 (use the '.str()' method to see the entries)
Compute the presentation for self, as a quotient of Manin symbols modulo relations.
INPUT:
OUTPUT:
ALGORITHM:
Let \(S = [0,-1; 1,0], T = [0,-1; 1,-1]\), and \(I = [-1,0; 0,1]\).
Let \(x_0,\ldots, x_{n-1}\) by a list of all non-equivalent Manin symbols.
Form quotient by 2-term S and (possibly) I relations.
Create a sparse matrix \(A\) with \(m\) columns, whose rows encode the relations
There are about n such rows. The number of nonzero entries per row is at most 3*(k-1). Note that we must include rows for all i, since even if \([x_i] = [x_j]\), it need not be the case that \([x_i T] = [x_j T]\), since \(S\) and \(T\) do not commute. However, in many cases we have an a priori formula for the dimension of the quotient by all these relations, so we can omit many relations and just check that there are enough at the end–if there aren’t, we add in more.
Compute the reduced row echelon form of \(A\) using sparse Gaussian elimination.
Use what we’ve done above to read off a sparse matrix R that uniquely expresses each of the n Manin symbols in terms of a subset of Manin symbols, modulo the relations. This subset of Manin symbols is a basis for the quotient by the relations.
EXAMPLE:
sage: L = sage.modular.modsym.manin_symbols.ManinSymbolList_gamma0(8,2)
sage: sage.modular.modsym.relation_matrix.compute_presentation(L, 1, GF(9,'a'), True)
(
[2 0 0]
[1 0 0]
[0 0 0]
[0 2 0]
[0 0 0]
[0 0 2]
[0 0 0]
[0 2 0]
[0 0 0]
[0 1 0]
[0 1 0]
[0 0 1], [1, 9, 11], [(1, 2), (1, 1), (0, 0), (9, 2), (0, 0), (11, 2), (0, 0), (9, 2), (0, 0), (9, 1), (9, 1), (11, 1)]
)
Compute echelon form of 3-term relation matrix, and read off each generator in terms of basis.
INPUT:
OUTPUT:
EXAMPLE:
sage: from sage.modular.modsym.relation_matrix import *
sage: L = sage.modular.modsym.manin_symbols.ManinSymbolList_gamma1(4, 3)
sage: modS = sparse_2term_quotient(modS_relations(L), 24, GF(3))
sage: gens_to_basis_matrix(L, T_relation_matrix_wtk_g0(L, modS, GF(3), 24), modS, GF(3), True)
(24 x 2 sparse matrix over Finite Field of size 3, [13, 23])
Compute quotient of Manin symbols by the I relations.
INPUT:
OUTPUT:
EXAMPLE:
sage: L = sage.modular.modsym.manin_symbols.ManinSymbolList_gamma1(4, 3)
sage: sage.modular.modsym.relation_matrix.modI_relations(L, 1)
set([((14, 1), (20, 1)), ((0, 1), (0, -1)), ((7, 1), (7, -1)), ((9, 1), (3, -1)), ((3, 1), (9, -1)), ((16, 1), (22, 1)), ((10, 1), (4, -1)), ((1, 1), (1, -1)), ((19, 1), (19, 1)), ((8, 1), (2, -1)), ((12, 1), (12, 1)), ((20, 1), (14, 1)), ((21, 1), (15, 1)), ((5, 1), (11, -1)), ((15, 1), (21, 1)), ((22, 1), (16, 1)), ((6, 1), (6, -1)), ((2, 1), (8, -1)), ((17, 1), (23, 1)), ((4, 1), (10, -1)), ((18, 1), (18, 1)), ((11, 1), (5, -1)), ((23, 1), (17, 1)), ((13, 1), (13, 1))])
Warning
We quotient by the involution eta((u,v)) = (-u,v), which has the opposite sign as the involution in Merel’s Springer LNM 1585 paper! Thus our +1 eigenspace is his -1 eigenspace, etc. We do this for consistency with MAGMA.
Compute quotient of Manin symbols by the S relations.
Here S is the 2x2 matrix [0, -1; 1, 0].
INPUT:
OUTPUT:
EXAMPLES:
sage: from sage.modular.modsym.manin_symbols import ManinSymbolList_gamma0
sage: from sage.modular.modsym.relation_matrix import modS_relations
sage: syms = ManinSymbolList_gamma0(2, 4); syms
Manin Symbol List of weight 4 for Gamma0(2)
sage: modS_relations(syms)
set([((3, -1), (4, 1)), ((5, -1), (5, 1)), ((1, 1), (6, 1)), ((0, 1), (7, 1)), ((3, 1), (4, -1)), ((2, 1), (8, 1))])
sage: syms = ManinSymbolList_gamma0(7, 2); syms
Manin Symbol List of weight 2 for Gamma0(7)
sage: modS_relations(syms)
set([((3, 1), (4, 1)), ((2, 1), (7, 1)), ((5, 1), (6, 1)), ((0, 1), (1, 1))])
Next we do an example with Gamma1:
sage: from sage.modular.modsym.manin_symbols import ManinSymbolList_gamma1
sage: syms = ManinSymbolList_gamma1(3,2); syms
Manin Symbol List of weight 2 for Gamma1(3)
sage: modS_relations(syms)
set([((3, 1), (6, 1)), ((0, 1), (5, 1)), ((0, 1), (2, 1)), ((3, 1), (4, 1)), ((6, 1), (7, 1)), ((1, 1), (2, 1)), ((1, 1), (5, 1)), ((4, 1), (7, 1))])
Compute the matrix of relations. Despite the name, this is used for all spaces (not just for Gamma0). For a description of the algorithm, see the docstring for compute_presentation.
INPUT:
Note that ManinSymbolList objects already have a specific weight, so there is no need for an extra weight parameter.
OUTPUT: a pair (R, mod) where
EXAMPLE:
sage: L = sage.modular.modsym.manin_symbols.ManinSymbolList_gamma0(8,2)
sage: A = sage.modular.modsym.relation_matrix.relation_matrix_wtk_g0(L, 0, GF(2), True); A
(
[0 0 0 0 0 0 0 0 1 0 0 0]
[0 0 0 0 0 0 0 0 1 1 1 0]
[0 0 0 0 0 0 1 0 0 1 1 0]
[0 0 0 0 0 0 1 0 0 0 0 0], [(1, 1), (1, 1), (8, 1), (10, 1), (6, 1), (11, 1), (6, 1), (9, 1), (8, 1), (9, 1), (10, 1), (11, 1)]
)
sage: A[0].is_sparse()
True
Performs Sparse Gauss elimination on a matrix all of whose columns have at most 2 nonzero entries. We use an obvious algorithm, which runs fast enough. (Typically making the list of relations takes more time than computing this quotient.) This algorithm is more subtle than just “identify symbols in pairs”, since complicated relations can cause generators to surprisingly equal 0.
INPUT:
OUTPUT:
EXAMPLE: We quotient out by the relations
to get
sage: v = [((int(0),3), (int(1),-1)), ((int(1),1), (int(3),1)), ((int(2),1),(int(3),1)), ((int(4),1),(int(5),-1))]
sage: rels = set(v)
sage: n = 6
sage: from sage.modular.modsym.relation_matrix import sparse_2term_quotient
sage: sparse_2term_quotient(rels, n, QQ)
[(3, -1/3), (3, -1), (3, -1), (3, 1), (5, 1), (5, 1)]