Function: mspetersson
Section: modular_symbols
C-Name: mspetersson
Prototype: GDGDG
Help: mspetersson(M,{F},{G=F}): M being a full modular symbol space,
 as given by msinit, calculate the intersection product {F,G} of modular
 symbols F and G on M.
Doc: $M$ being a full modular symbol space for $\Gamma = \Gamma_{0}(N)$,
 as given by \kbd{msinit},
 calculate the intersection product $\{F, G\}$ of modular symbols $F$ and $G$
 on $M=\Hom_{\Gamma}(\Delta_{0}, V_{k})$ extended to an hermitian bilinear
 form on $M \otimes \C$ whose radical is the Eisenstein subspace of $M$.

 Suppose that $f_{1}$ and $f_{2}$ are two parabolic forms. Let $F_{1}$
 and $F_{2}$ be the attached modular symbols
 $$ F_{i}(\delta)= \int_{\delta} f_{i}(z) \cdot (z X + Y)^{k-2} \,dz$$
 and let $F^{\R}_{1}$, $F^{\R}_{2}$ be the attached real modular symbols
 $$ F^{\R}_{i}(\delta)= \int_{\delta}
    \Re\big(f_{i}(z) \cdot (z X + Y)^{k-2} \,dz\big) $$
 Then we have
 $$
 \{ F^{\R}_{1}, F^{\R}_{2} \} = -2 (2i)^{k-2} \cdot
    \Im(<f_{1},f_{2}>_{\var{Petersson}}) $$
 and
 $$\{ F_{1}, \bar{F_{2}} \} = (2i)^{k-2} <f_{1},f_{2}>_{\var{Petersson}}$$
 In weight 2, the intersection product $\{F, G\}$ has integer values on the
 $\Z$-structure on $M$ given by \kbd{mslattice} and defines a Riemann form on
 $H^{1}_{par}(\Gamma,\R)$.

 For user convenience, we allow $F$ and $G$ to be matrices and return the
 attached Gram matrix. If $F$ is omitted: treat it as the full modular space
 attached to $M$; if $G$ is omitted, take it equal to $F$.
 \bprog
 ? M = msinit(37,2);
 ? C = mscuspidal(M)[1];
 ? mspetersson(M, C)
 %3 =
 [ 0 -17 -8 -17]
 [17   0 -8 -25]
 [ 8   8  0 -17]
 [17  25 17   0]
 ? mspetersson(M, mslattice(M,C))
 %4 =
 [0 -1 0 -1]
 [1  0 0 -1]
 [0  0 0 -1]
 [1  1 1  0]
 ? E = ellinit("33a1");
 ? [M,xpm] = msfromell(E); [xp,xm,L] = xpm;
 ? mspetersson(M, mslattice(M,L))
 %7 =
 [0 -3]
 [3  0]
 ? ellmoddegree(E)
 %8 = [3, -126]
 @eprog
 \noindent The coefficient $3$ in the matrix is the degree of the
 modular parametrization.
