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\documentclass[11pt]{article}
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\title{The Kernel of the Modular Polarization of a Quotient of $J_0(N)$}
\begin{document}
\maketitle
\tableofcontents
\section{The Modular Polarization}
Suppose $A$ is a quotient of $J_0(N)$.
Because $J_0(N)$ is a Jacobian it possesses a canonical
principal polarization.  This induces a polarization on $A$.

\begin{definition}
The {\bf modular polarization} is the map $\delta:A^{\vee}\ra A$
arising from autoduality of $J_0(N)$.
\begin{center}
\begin{picture}(80,140)
\put(0,110){$A^{\vee}$}
\put(65,110){$J_0(N)$}
\put(70,40){$A$}
\put(75,100){\vector(0,-1){45}}
\put(75,63){\vector(0,-1){3}}
\put(13,105){\vector(1,-1){55}}
%\qbezier(65,45)(10,60)(5,103)\put(5,103){\vector(0,1){1}}
\put(18,113){\vector(1,0){43}}
%\qbezier(18,113)(14,115)(18,117)

\put(40,117){$\pi^{\vee}$}
\put(40,80){$\delta$}
%\put(20,50){$\hat{\delta}$}
\put(80,80){$\pi$}
\end{picture}
\end{center}
\end{definition}

Let $f\in S_2(\Gamma_0(N))$ be a newform, $A=A_f$ the corresponding
optimal quotient of $J_0(N)$, and $\p_f=\Ann_\T(f)\subset\T$ the
annihilator of $f$ in the Hecke algebra.
Let $H_1=H_1(X_0(N),\Z)$ be the first integral homology of
the modular curve $X_0(N)$.

\begin{theorem}
Let $\Phi:H_1\ra A(\C)$ be the period map. Then
there is an exact sequence
$$0 \ra \Phi(H_1[\p_f]) \ra \Phi (H_1) \ra \Ker(\delta) \ra 0.$$
\end{theorem}
\begin{proof}
\mbox{}\\
{\noindent\bf Step 1: Pass to lattices.}
Over the complex number we may write each of $A$ and $A^{\vee}$ as complex tori
$T/\Lambda$ where $T\isom\C^d$ and $\Lambda$ is a lattice.  The isogeny $\delta:A^{\vee}\ra A$
induces maps $T(A^{\vee})\ra T(A)$ and $\Lambda(A^{\vee})\ra \Lambda(A)$.  We
thus obtain the following commuting diagram with exact rows and columns.
$$\begin{matrix} & & & 0 & & 0 & & & \\ & & & \da & & \da & & & \\ 0 \lra & 0 &\lra & \Lambda(A^{\vee})& \lra &\Lambda(A) & \lra & L & \lra 0 \\ & \da & & \da & & \da & & \da & \\ 0 \lra & 0 & \lra&T(A^{\vee})& \lra & T(A) & \lra & 0 & \lra 0 \\ & \da & & \da & & \da & & \da & \\ 0 \lra & \Ker(\delta) & \lra& A^{\vee}& \xrightarrow{\,\,\,\delta\,\,\,} & A & \lra & 0 & \lra 0 \\ & & & \da & & \da & & & \\ & & & 0 & & 0 & & & \\ \end{matrix}$$
Applying the snake lemma we see that
$$\Ker(\delta) \isom L = \coker(\Lambda(A^{\vee})\ra \Lambda(A)).$$

{\noindent\bf Step 2: Identify lattices.}
Proposition 6 of \cite{shimura} allows us to identify $\Lambda(A)$ and $\Lambda(A^{\vee})$
in terms of the integral homology $H_1=H_1(X_0(N),\Z)$.

First for $J_0(N)$ we have
$$T(J)=\Hom(S_2(\Gamma_0(N),\C))$$
and an exact sequence
$$0\ra H_1 \ra T(J) \ra J(\C) \ra 0.$$
As for $A$, we have
$$H_1\xrightarrow{\Phi} T(A) \ra A(\C) \ra 0$$
where
$$\Phi(\gamma) = (\int_\gamma f_1, \ldots, \int_\gamma f_d).$$
(See below for the basis for $T(A)$.)
We thus have
\begin{eqnarray*}
T(A)&=&\Hom(S[\p_f],\C)\\
\Lambda(A) &=& \Phi(H_1)
\end{eqnarray*}
In defining $\Phi$ we have chosen the basis $f_1,\ldots, f_d$ for $S[\p_f]$, in
order to obtain a basis for $T(A)$.
For $A^{\vee}$ we have
\begin{eqnarray*}
T(A^{\vee}) &=& T(J)[\p_f] \\
\Lambda(A^{\vee}) &=& H_1[\p_f]
\end{eqnarray*}

{\noindent\bf Step 3: Compute $L$.}

The map $\Lambda(A^{\vee})\ra\Lambda(A)$ induced by $\delta$ is the restriction of $\Phi$
to $H_1[\p_f]$. Thus
$$L = \frac{\Phi(H_1)}{\Phi(H_1[\p_f])},$$
which, combined with step 1, completes the proof.
\end{proof}

\section{The $\infty$-Component Group}
Fix a newform $f\in S_2(\Gamma_0(N))$ as before and let
$A_f$ be the corresponding optimal quotient of $J_0(N)$.
Define groups
\begin{eqnarray*}
Y_A &=& H_1[\p_f]\\
Y_{A^{\vee}} &=& \Hom(H_1,\Z)[\p_f]
\end{eqnarray*}

\begin{conjecture}
There is an exact sequence of abelian groups
$$0\ra Y_{A^{\vee}} \ra \Hom(Y_A,\Z) \ra \Ker(\delta) \ra 0.$$
\end{conjecture}

{\bf Evidence.} It's only a conjecture because I have not worked out
all the details yet.  The basic idea is that in computing
$\Phi(H_1)/\Phi(H_1[\p_f])$ we can replace $\Phi$ by any
homomorphism $\Psi$ eminating from $H_1$ and satisfying
$$\Ker(\Psi) = \Ker(\Phi).$$

Now let's make a few assumptions related to the structure of the $\T$-module
$H_1=H_1(X_0(N),\C)$: Let
$$V_f = \Hom(H_1,\Z)[\p_f].$$
{\bf\noindent Assumption 1.} $\dim_{\Z} V_f = 2d,$
where $d$ is the number of conjugates of $f$.  Fix a basis
$\vphi_1,\ldots,\vphi_{2d}$ for $V_f$ and define
$$\Psi : H_1 \ra \Z,$$
$$\Psi(x) = (\vphi_1(x),\ldots, \vphi_{2d}(x)).$$

{\bf\noindent Assumption 2.} $\Ker(\Psi) = \Ker(\Phi)$ \\

Given these two assumptions, computing $\Psi(H_1)/\Psi(H_1[\p_f])$
and computing the cokernel $Y_{A^{\vee}} \ra \Hom(Y_A,\Z)$ are
the same thing.

I think both of these assumption can be shown by looking
at characteristic polynomials of Hecke operators and using
the Atkin-Lehner multiplicity one theory.

\vspace{.8in}

In analogy with the Grothendieck-Raynaud-Ribet description of the local
component groups of Neron models we make the following definition.
\begin{definition}
The {\bf $\infty$-component group} is $$\Phi_{A,\infty} := \Ker(\delta).$$
\end{definition}

\begin{thebibliography}{HHHHHHH}
\bibitem[S]{shimura} G. Shimura, {\em On the factors of the jacobian
variety of a modular function field}, J. Math. Soc. Japan,
{\bf 25}, No. 3, 523--544 (1973).
\end{thebibliography} \normalsize\vspace*{1 cm}

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