mspathgens

Function: mspathgens
Section: modular_symbols
C-Name: mspathgens
Prototype: G
Help: mspathgens(M): M being a full modular symbol space, as given by
 msinit, return a set of Z[G]-generators for Div^0(P^1 Q). The output
 is [g,R], where g is a minimal system of generators and R the vector of
 Z[G]-relations between the given generators.
Doc: Let $\Delta_0:=\text{Div}^0(\P^1(\Q))$.
 Let $M$ being a full modular symbol space, as given by \kbd{msinit},
 return a set of $\Z[G]$-generators for $\Delta_0$. The output
 is $[g,R]$, where $g$ is a minimal system of generators and $R$
 the vector of $\Z[G]$-relations between the given generators. A
 relation is coded by a vector of pairs $[a_i,i]$ with $a_i\in \Z[G]$
 and $i$ the index of a generator, so that $\sum_i a_i g[i] = 0$.

 An element $[v]-[u]$ in $\Delta_0$ is coded by the ``path'' $[u,v]$,
 where \kbd{oo} denotes the point at infinity $(1:0)$ on the projective
 line.
 An element of $\Z[G]$ is either an integer $n$ ($= n [\text{id}_2]$) or a
 ``factorization matrix'': the first column contains distinct elements $g_i$
 of $G$ and the second integers $n_i$ and the matrix codes $\sum n_i [g_i]$:
 \bprog
 ? M = msinit(11,8); \\ M_8(Gamma_0(11))
 ? [g,R] = mspathgens(M);
 ? g
 %3 = [[+oo, 0], [0, 1/3], [1/3, 1/2]] \\ 3 paths
 ? #R  \\ a single relation
 %4 = 1
 ? r = R[1]; #r \\ ...involving all 3 generators
 %5 = 3
 ? r[1]
 %6 = [[1, 1; [1, 1; 0, 1], -1], 1]
 ? r[2]
 %7 = [[1, 1; [7, -2; 11, -3], -1], 2]
 ? r[3]
 %8 = [[1, 1; [8, -3; 11, -4], -1], 3]
 @eprog\noindent
 The given relation is of the form $\sum_i (1-\gamma_i) g_i = 0$, with
 $\gamma_i\in \Gamma_0(11)$. There will always be a single relation involving
 all generators (corresponding to a round trip along all cusps), then
 relations involving a single generator (corresponding to $2$ and $3$-torsion
 elements in the group:
 \bprog
 ? M = msinit(2,8); \\ M_8(Gamma_0(2))
 ? [g,R] = mspathgens(M);
 ? g
 %3 = [[+oo, 0], [0, 1]]
 @eprog\noindent
 Note that the output depends only on the group $G$, not on the
 representation $V$.