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\chapter{The Birch and Swinnerton-Dyer conjecture}
\label{chap:bsd}\index{Birch and Swinnerton-Dyer conjecture|see{BSD conjecture}}%
\index{Conjecture!Birch and Swinnerton-Dyer|see{BSD conjecture}}%
Now that the Shimura-Taniyama%
\index{Shimura-Taniyama conjecture}\index{Conjecture!Shimura and Taniyama} 
conjecture has been proved, many experts consider the Birch and
Swinnerton-Dyer conjecture (BSD conjecture)
to be one of the main outstanding problems in the field 
(see~\cite[pg.~549]{darmon-bsd} and \cite[Intro.]{cime-1997}).  
This conjecture ties together many of the arithmetic
and analytic invariants of an elliptic curve.  At present, there is no
general class of elliptic curves for which the full BSD 
conjecture\index{BSD conjecture!is still unknown} is
known, though a slightly weakened form is known for a fairly broad
class of complex multiplication elliptic
curves of analytic rank~$0$ (see~\cite{rubin:main-conjectures}), and
several deep partial results have been obtained during
the last twenty years (see, e.g.,~\cite{gross-zagier} and 
\cite{kolyvagin:mordellweil}).

Approaches to the BSD conjecture\index{BSD conjecture} 
that rely on congruences between\index{Congruences!and BSD conjecture}
modular forms\index{Modular forms!and BSD} 
are likely to require a deeper
understanding of the analogue of the BSD conjecture\index{BSD conjecture!in higher dimensions} 
for higher-dimensional abelian varieties.  As a first step, this chapter
presents theorems and explicit computations of some of the arithmetic
invariants of modular abelian varieties.

The reader is urged to also read A.~Agashe's 2000
Berkeley Ph.D.\ thesis which cover similar themes.\index{Agashe}  The paper of
Cremona and Mazur's~\cite{cremona-mazur}\index{Mazur} paints a detailed
experimental picture of the way in which congruences link
Mordell-Weil and Shafarevich-Tate groups of elliptic curves.
\index{Congruences!and BSD conjecture}

\section{The BSD conjecture}\index{BSD conjecture|textit}
By~\cite{breuil-conrad-diamond-taylor} we now know 
that every elliptic curve over~$\Q$ is
a quotient of the curve~$X_0(N)$, whose complex points 
are the isomorphism classes of pairs consisting of a 
(generalized) elliptic curve and a cyclic subgroup of order~$N$.
Let~$J_0(N)$ denote the Jacobian\index{Jacobian} of $X_0(N)$; this is an abelian
variety of dimension equal to the genus of~$X_0(N)$ whose points
correspond to the degree~$0$ divisor classes on~$X_0(N)$.
The survey article~\cite{diamond-im} is a good 
guide to the facts and literature
about the family of abelian varieties $J_0(N)$.

Following Mazur~\cite{mazur:rational}\index{Mazur}, we make the following definition.
\begin{definition}[Optimal quotient]\index{Optimal quotient|textit}
An {\em optimal quotient} of $J_0(N)$ is a quotient~$A$ of
$J_0(N)$ by an abelian subvariety.
\end{definition}
Consider an optimal quotient~$A$ such that $L(A,1)\neq 0$.
By~\cite{kolyvagin-logachev:totallyreal},~$A(\Q)$ and~$\Sha(A)$ 
are both finite.
The BSD conjecture\index{BSD conjecture!statement of}%
asserts that 
$$\frac{L(A,1)}{\Omega_A} =
\frac{\#\Sha(A)\cdot\prod_{p\mid N} c_p} 
{\# A(\Q)\cdot\#\Adual(\Q)}.$$
Here the Shafarevich-Tate group\index{Shafarevich-Tate group} 
$$\Sha(A) := \ker\left(H^1(\Q,A) \ra \prod_{v} H^1(\Q_v,A)\right)$$
is a measure of the failure
of the local-to-global principle\index{Local-to-global principle};
the Tamagawa numbers~$c_p$\index{Tamagawa numbers} are the 
orders of the groups of rational points of the
component groups of~$A$ (see Chapter~\ref{chap:compgroups}); 
the real number~$\Omega_A$ is 
the measure of~$A(\R)$ with respect to a basis of differentials having 
everywhere nonzero good reduction (see Section~\ref{sec:realmeasure}); 
and~$\Adual$ is the abelian variety dual to~$A$ (see \cite[\S9]{milne:abvars}).
This chapter makes a small contribution to the long-term goal 
of verifying the above conjecture for many specific abelian varieties 
on a case-by-case basis.  In a large list of examples, we compute
the conjectured order of $\Sha(A)$, up to a power of $2$, and then
show that $\Sha(A)$ is at least as big as conjectured. 
We also discuss methods to obtain upper bounds on $\#\Sha(A)$, but do
not carry out any computations in this direction.
This is the first step in a program to verify the above 
conjecture for an infinite family of quotients of~$J_0(N)$.

\subsection{The ratio $L(A,1)/\Omega_A$}
Extending classical work on elliptic curves, 
A.~Agashe\index{Agashe} and the author proved the following 
theorem.   
\begin{theorem}\label{thm:ratpart}
Let~$m$ be the largest square dividing~$N$.
The ratio $L(A,1)/\Omega_A$ is a rational number that can be
explicitly computed, up to a unit (conjecturally $1$) in $\Z[1/(2m)]$.
\end{theorem}
\begin{proof}
The proof uses modular symbols\index{Modular symbols} 
combined with an extension of the argument
used by Mazur\index{Mazur} in~\cite{mazur:rational} to bound 
the Manin constant\index{Manin constant}.
The modular symbols part of the proof for $L$-functions attached
to newforms of weight $k\geq 2$ is given in Section~\ref{sec:rationalvals};
it involves expressing the
ratio $L(A,1)/\Omega_A$ as the lattice 
index\index{Lattice index} of 
two modules over the Hecke algebra\index{Hecke algebra}. 
The bound on the Manin constant\index{Manin constant} is given in 
Section~\ref{sec:maninconstant}.
\end{proof}

The author has computed $L(A,1)/\Omega_A$ for all simple optimal
quotients of level $N\leq 1500$; this table can be
obtained from the author's web page.

\begin{remark}
The method of proof should also  give similar results for special 
values of twists of $L(A,s)$, just as it does in the case $\dim A=1$
(see~\cite[Prop.~2.11.2]{cremona:algs}).
\end{remark}


\subsection{Torsion subgroup\index{Torsion subgroup}}
We can compute upper and lower bounds on $\#A(\Q)_{\tor}$,
see Section~\ref{sec:torsionsubgroup}; 
these frequently determine $\#A(\Q)_{\tor}$.

These methods, combined with the method used
to obtain Theorem~\ref{thm:ratpart}, 
yield the following corollary, which supports the expected
cancellation between torsion and~$c_p$ coming from the reduction
map sending rational points to their image in the component 
group of~$A$. The corollary also generalizes to higher weight forms, 
thus suggesting a geometric way to think about reducibility
of modular Galois representations.
\begin{corollary}
Let~$n$ be the order of the image of 
$(0)-(\infty)$ in $A(\Q)$, and
let~$m$ be the largest square dividing~$N$.
Then 
 $n\cdot L(A,1)/\Omega_A \in \Z[1/(2m)].$
\end{corollary}
For the proof, see Corollary~\ref{cor:denominator}
in Chapter~\ref{chap:computing}.

\subsection{Tamagawa numbers\index{Tamagawa numbers}}
We prove the following theorem in Chapter~\ref{chap:compgroups}. 
\begin{theorem}\label{thm:tamagawa}
When $p^2\nmid N$, the number~$c_p$ can be explicitly computed
(up to a power of~$2$).
\end{theorem}

We can compute the order~$c_p$ of the group of rational
points of the component group, but not
its structure as a group.
When $p^2 \mid N$ it may be possible 
to compute~$c_p$ using the 
Drinfeld-Katz-Mazur model
of~$X_0(N)$, but we have not yet done this.
There are also good bounds on the primes that can divide $c_p$ when
$p^2\mid N$. 

Systematic computations (see Section~\ref{sec:compgroupconjectures})
using this formula suggest the 
following conjectural refinement of a result 
of Mazur~\cite{mazur:eisenstein}\index{Mazur}.
\begin{conjecture}
Suppose~$N$ is prime and~$A$ 
is an optimal quotient of $J_0(N)$ corresponding
to a newform~$f$.  Then $A(\Q)_{\tor}$ is 
generated by the image of $(0)-(\infty)$
and $c_p = \#A(\Q)_{\tor}$.  Furthermore,
the product of the~$c_p$ over all simple optimal quotients
corresponding to newforms equals the numerator of $(N-1)/12$.
\end{conjecture}
I have checked this conjecture for all $N\leq 997$ and, 
up to a power of~$2$, for all $N\leq 2113$.
The first part is known when~$A$ is an elliptic 
curve (see~\cite{mestre-oesterle:crelle}).
Upon hearing of this conjecture, Mazur\index{Mazur} reportedly
proved it  when all ``$q$-Eisenstein quotients'' are simple.
There are three promising approaches to finding 
a complete proof. One involves the explicit 
formula of Theorem~\ref{thm:tamagawa};
another is based on Ribet's\index{Ribet} level lowering theorem
(see~\cite{ribet:modreps}),
and a third makes use of a simplicity result of Merel\index{Merel}
(see~\cite{merel:weil}).


The formula that lies behind Theorem~\ref{thm:tamagawa} probably
has a natural analogue in weight greater than~$2$.  
One could then guess that it produces Tamagawa numbers\index{Tamagawa numbers} 
of motifs\index{Motifs} attached to eigenforms of higher weight; however,
we have no idea if this is really the case.  These numbers appear
in the conjectures of Bloch and Kato,
\index{Conjecture!Bloch and Kato}%
\index{Bloch and Kato conjecture}%
which generalize the BSD conjecture\index{BSD conjecture!generalization of} to 
motifs (see~\cite{bloch-kato}).  
Anyone wishing to
try to compute them should be aware of Neil Dummigan's
paper~\cite{dummigan:cp}, which gives some information
about the Tamagawa numbers\index{Tamagawa numbers} 
of motifs\index{Motifs} attached by
Scholl in~\cite{scholl:motivesinvent}
to modular eigenforms.

\subsection{Upper bounds on $\#\Sha(A)$}
V.~Kolyvagin (see \cite{kolyvagin:structureofsha}) 
and K.~Kato (see, e.g., \cite{scholl:kato})
constructed Euler systems\index{Euler system} that
were used to prove that $\Sha(A)$ is {\em finite} 
when $L(A,1)\neq 0$. 
To verify the full BSD conjecture\index{BSD conjecture!verification of} 
for certain abelian varieties, we must make the Kolyvagin-Kato 
finiteness bound explicit. 
Kolyvagin's bounds involve computations with Heegner 
points\index{Heegner points},
and Kato's involve a study of the Galois representations 
associated to~$A$.

\subsubsection{Kolyvagin's bounds}%
\index{Bound of!Kolyvagin}
In~\cite{kolyvagin:mordellweil}, Kolyvagin obtains explicit upper 
bounds for $\#\Sha(A)$ for a certain (finite) list of elliptic curves~$A$ 
by computing the index in $A(K)$ of the subgroup 
generated by the Heegner point, where~$K$ 
is a suitable imaginary quadratic extension.
In~\cite{kolyvagin-logachev:totallyreal}, Kolyvagin and Logachev
generalize Kolyvagin's earlier results; in Section~1.6, ``Unsolved
problems'', they say that: ``If one were to compute the
height of a Heegner point~$y$ [...]
considered in the present paper, then one would have succeeded in 
obtaining an upper bound for $\#\Sha$ for this curve.''
(By ``curve'' they mean abelian variety.)
This suggests that explicit computations should yield upper
bounds on the order of $\Sha(A)$, but that they had not yet 
figured out how to carry out such computations.

\subsubsection{Kato's bounds}%
\index{Bound of!Kato}
Kato has constructed Euler systems\index{Euler system} coming 
from $K_2$-groups of modular
curves.  These can be used to prove the following theorem (see, e.g.,
\cite[Cor.~3.5.19]{rubin:book}).
\begin{theorem}[Kato]
Suppose~$E$ is an elliptic curve over~$\Q$ without complex 
multiplication that~$E$ has conductor~$N$,
that~$E$ has good reduction at~$p$, that~$p$ does not divide
$2r_E\prod_{q\mid N} L_q(q^{-1})\#E(\Q_q)_{\tor}$, and 
the  Galois representation $\rho_{E,p}:\GQ\ra\Aut(E[p])$
is surjective.  Then
$$\#\Sha(E)_{p^{\infty}}\text{ divides }
   \frac{L(E,1)}{\Omega_E}.$$
\end{theorem}
Here $L_q(x)$ is the local Euler factor at~$q$ and the constant
$r_E$ arises in the construction of Kato's Euler system.  
Rubin suggests that computing $r_E$ is not very 
difficult (private communication).
Appropriate variants of Kato's arguments 
give similar results for quotients of $J_0(N)$ of arbitrary
dimension, though these have not been written down.

\comment{
   >How mysterious is the constant r_E in, for example, Corollary 3.5.19 of
   >your Euler Systems book?

   I think it's not too bad.  Certainly nothing like Heegner points are
   involved.  When I wrote that part of my book, and the similar paper in
   the Durham proceedings, I did not really know what it was because Kato
   hadn't written anything.  I don't have Scholl's paper here so I'm not
   certain, but I suspect that the only contribution to $r_E$ comes from
   passing from the modular curve to $E$, and perhaps some extra 2's and
   3's.

   Karl
}


\subsection{Lower bounds on $\#\Sha(A)$}
One approach to showing that~$\Sha(A)$ is as {\em at least} as
large as predicted
by the BSD conjecture\index{BSD conjecture!and $\Sha$}
is suggested by Mazur's\index{Mazur} notion of
the visible part $\Sha(A)^{\circ}$ 
of~$\Sha(A)$ (see~\cite{cremona-mazur,  mazur:visthree}).
Let~$\Adual\subset J_0(N)$ be the dual to~$A$.
The \defn{visible part}\index{Visibility!of $\Sha$|textit}%
\index{Shafarevich-Tate group!visible part of}
of $\Sha(\Adual)$ is the 
kernel of the natural map
$\Sha(\Adual)\ra \Sha(J_0(N))$.
Mazur\index{Mazur} observed that if an element of order~$p$ 
in~$\Sha(\Adual)$ is visible, 
then it is explained by a ``jump in the rank of Mordell-Weil''
in the sense that there is another abelian subvariety $B\subset J_0(N)$
such that $p \mid \#(\Adual\intersect B)$ and the rank of~$B$ is positive.

Mazur's\index{Mazur} observation can be turned around: if there is another abelian
variety~$B$ of positive rank such that $p\mid \#(\Adual\intersect B)$,
then, under mild hypotheses (see Theorem~\ref{thm:shaexists}),  there 
is an element of~$\Sha(\Adual)$ of order~$p$.  From a computational
point of view it is easy to understand the intersections 
$\Adual\intersect B$; see Section~\ref{sec:intersection}.
From a theoretical point of view, nontrivial
intersections ``correspond'' to congruences between modular forms.
Thus the well-developed
theory  of congruences between modular forms%
\index{Modular forms!congruences between}%
\index{Congruences!and lower bounds on $\Sha$}
can be used  to obtain a lower bound on~$\#\Sha(\Adual)$. 

\subsubsection{Invisible elements of $\#\Sha(\Adual)$}
\index{Shafarevich-Tate group!invisible elements of}
\index{Invisible elements of $\Sha$}
Numerical experiments suggest 
 that as $\Adual$ varies, $\Sha(\Adual)$ is
often {\em not} visible inside of~$J_0(N)$.
For example (see Table~\ref{table:primesha}), the 
BSD conjecture\index{BSD conjecture!predicts invisible elements} 
predicts the existence of invisible elements of odd 
order in~$\Sha(\Adual)$  
for almost  half of the~$37$ optimal quotients
of prime level $\leq 2113$.

\subsubsection{Visibility at higher level}
\index{Shafarevich-Tate group!visibility at higher level}
\index{Visibility!at higher level}
For every integer~$M$ (Ribet~\cite{ribet:raising}\index{Ribet} 
tells us which~$M$
to choose), we can ask whether $\Sha(\Adual)$ maps to~$0$ 
under one of the natural maps $\Adual\ra J_0(NM)$; that is, we
can ask whether $\Sha(\Adual)$ ``becomes visible at 
level $NM$.''
We have been unable to prove in any particular case that $\Sha(\Adual)$ is
not visible at level~$N$, but becomes visible at some level $NM$.
See Section~\ref{sec:higherlevel} for some computations which strongly
indicate that such examples exist. 

\subsubsection{Visibility in some Jacobian}%
\index{Visibility!in some Jacobian}%
\index{Jacobian!visibility in}%
Johan de Jong proved that if~$E$ is an elliptic curve
over a number field~$K$ and $c\in H^1(K,E)$ then there is a
Jacobian~$J$ and an imbedding $E\hookrightarrow J$ such that~$c$ maps
to~$0$ under the natural map $H^1(K,E)\ra H^1(K,J)$ (see Remark~3
in~\cite{cremona-mazur}); de Jong's proof appears to generalize
when~$E$ is replaced by an abelian variety, but time does not permit
going into the details here.

\subsection{Motivation for considering abelian varieties}
If~$A$ is an elliptic curve, then explaining~$\Sha(A)$ using 
only congruences between elliptic curves will probably fail.
\index{Congruences!between elliptic curves}
This is because pairs of non-isogenous elliptic curves with isomorphic 
$p$-torsion for large~$p$ are, according to E.~Kani's\index{Kani} 
conjecture,  extremely rare.%
\index{Conjecture!Kani}
It is crucial to understand what happens in all dimensions.

Within the range accessible by computer, abelian varieties exhibit
more richly textured structure than elliptic curves.  For example, there
is a visible element of prime order $83341$ in the
Shafarevich-Tate group\index{Shafarevich-Tate group} of an abelian
variety of prime conductor~$2333$; in contrast, over all optimal
elliptic curves of conductor up to $5500$, it appears that the largest
order of an element of a Shafarevich-Tate group is~$7$ (see the
discussion in~\cite{cremona-mazur}).

\section{Existence of nontrivial visible elements of $\Sha(A)$}%
The reader who wants to see tables of Shafarevich-Tate groups can
safely skip to the next section.  

Without relying on any unverified conjectures,
we will use the following theorem to exhibit abelian varieties~$A$
such that the visible part of $\Sha(A)$ is nonzero.  
In the following theorem we do {\em not} assume that~$J$ is the
Jacobian\index{Jacobian} of a curve.
\begin{theorem}\label{thm:shaexists}\index{Visibility!existence theorem}
Let~$A$ and~$B$ be abelian subvarieties of an abelian variety~$J$ such 
that $A\intersect B$ is finite and $A(\Q)$ is finite. 
Assume that~$B$ has purely toric reduction
at each prime at which~$J$ has
bad reduction.
Let~$p$ be an odd prime at which~$J$ has good reduction, and
assume that~$p$ does not divide the orders of any of
the (geometric) component groups\index{Component group!geometric} 
of~$A$ and~$B$,
or the orders of the torsion subgroups of $(J/B)(\Q)$ and $B(\Q)$.  
Suppose further that  $B[p] \subset A\intersect B$.
Then there exists an injection 
         $$B(\Q)/pB(\Q)\hookrightarrow \Sha(A)^{\circ}$$
of $B(\Q)/p B(\Q)$ into the visible part of $\Sha(A)$.
\end{theorem}

\begin{proof}
Let $C=J/A$.
The long exact sequence of Galois cohomology
associated to the short exact sequence
$$0 \ra A \ra J \ra C \ra 0$$ 
begins
$$0\ra A(\Q) \ra J(\Q) \ra C(\Q) \xrightarrow{\,\delta\,}
          H^1(\Q,A) \ra \cdots.$$
Because $B[p]\subset A$, the map $B\ra C$, obtained by composing
the inclusion
$B\hookrightarrow J$ with $J\ra C$, factors through multiplication-by-$p$,
giving the following commutative diagram:
$$\xymatrix{
& B\ar[d] \ar[r]^{p}& B\ar[d]\\
A\ar[r]&J\ar[r]&C.}$$
Because $B(\Q)[p]=0$ and $B(\Q)\intersect A(\Q)=0$, we 
deduce the following commutative diagram with exact
rows:
$$\xymatrix{
         & 0\ar[d] & K_1\ar[d]& K_2\ar[d]\\
0 \ar[r] & B(\Q) \ar[r]^{p}\ar[d] & B(\Q)\ar[dr]^{\pi} \ar[r]\ar[d]
         & B(\Q)/pB(\Q)\ar[r]\ar[d] & 0\\
0 \ar[r] & J(\Q)/A(\Q)\ar[r]\ar[d] & C(\Q) \ar[r] & \delta(C(\Q)) \ar[r] & 0\\
         & K_3,
}$$
where $K_1$ and $K_2$ are the indicated kernels and $K_3$ is the cokernel.
We have the snake lemma exact sequence
$$0\ra K_1 \ra K_2 \ra K_3.$$
Because $B(\Q)[p]=0$ and $K_2$ is a $p$-torsion group, 
we have $K_1=0$.  
The quotient $J(\Q)/B(\Q)$ has no $p$-torsion because
it is a subgroup  of $(J/B)(\Q)$; also, $A(\Q)$ is a finite group 
of order coprime to~$p$,
so $K_3 = J(\Q)/(A(\Q)+B(\Q))$ has no $p$-torsion.  Thus $K_2=0$.

The above argument shows that $B(\Q)/p B(\Q)$ is a subgroup of
$H^1(\Q,A)$.  However, $H^1(\Q,A)$ contains infinitely many elements of
order~$p$ (see~\cite{shafarevich:exp}),
whereas $\Sha(A)[p]$ is a finite group, so we must work
harder to deduce that $B(\Q)/p B(\Q)$ lies in
$\Sha(A)[p]$.   Fix $x\in B(\Q)$.  We must show 
that $\pi(x)$ lies in $\Sha(A)[p]$; equivalently, that
$\res_v(\pi(x))=0$ for all places~$v$ of~$\Q$. 

At the archimedean place $v=\infty$, the restriction
$\res_v(\pi(x))$ is killed by~$2$ and the odd prime~$p$, 
hence $\res_v(\pi(x))=0$.

Suppose that~$v$ is a place at which~$J$ has bad reduction.
By hypothesis, $B$ has purely toric reduction\index{Purely toric reduction}, 
so over the maximal unramified extension $\Q_v^{\ur}$
of $\Q_v$ there is an isomorphism $B\isom\Gm^d/\Gamma$ 
of $\Gal(\Qbar_v/\Q_v^{\ur})$-modules,
for some ``lattice'' $\Gamma$.  
For example, when 
$\dim B=1$, this is the Tate curve representation of~$B$. 
Let~$n$ be the order of the component group of~$B$ at~$v$; thus~$n$
equals the order of the cokernel of the valuation 
map $\Gamma\ra \Z^d$.  Choose a representative $P=(x_1,\ldots,x_d)\in\Gm^d$
for the point~$x$.  Then $nP$ can be adjusted by elements of~$\Gamma$ 
so that each of its components $x_i\in\Gm$ has valuation~$0$.
By assumption,~$p$ is a prime at which~$J$ has good reduction, so 
$p\neq v$;
it follows that there is a point $Q\in\Gm^d(\Q_v^{\ur})$ such that
$pQ = nP$.
Thus the cohomology class $\res_v(\pi(nx))$ is unramified
at~$v$.  By \cite[Prop.~I.3.8]{milne:duality}, 
$$H^1(\Q_v^{\ur}/\Q_v,A(\Q_v^{\ur}))
  =H^1(\Q_v^{\ur}/\Q_v,\Phi_{A,v}(\Fbar_v)),$$
where $\Phi_{A,v}$ is the component group of~$A$ at~$v$.
Since the component group $\Phi_{A,v}(\Fbar_v)$ has
order~$n$, it follows that $$\res_v(\pi(nx))=n\res_v(\pi(x))=0.$$
Since the order~$p$ of $\res_v(\pi(x))$ is coprime to~$n$,
we conclude that $\res_v(\pi(x))=0$.

Next suppose that~$J$ has good reduction at~$v$ 
and that~$v$ is {\em odd}, in the sense that the
residue characteristic of~$v$ is odd.  To simplify notation in 
this paragraph, since~$v$ is a non-archimedean place
of $\Q$, we will also let~$v$ denote the odd prime number
which is the residue characteristic of~$v$.
Let $\cA$, $\cJ$, $\cC$, be the N\'eron models
of~$A$,~$J$, and~$C$, respectively (for more on N\'eron 
models, see Chapter~\ref{chap:compgroups}).  
Let $A$, $J$, $C$, also denote the sheaves on
the \'etale-site over $\Spec(\Z_v)$ determined
by the group schemes $\cA$, $\cJ$, and $\cC$, respectively.
Since~$v$ is odd, $1=e<v-1$, so we may apply
\cite[Thm.~7.5.4]{neronmodels} to conclude that
the sequence of group schemes
$$0\ra \cA \ra \cJ\ra \cC \ra 0$$ 
is exact; in particular, it
is exact as a sequence of sheaves on the
\'etale site (see the proof of~\cite[Thm.~7.5.4]{neronmodels}).
Thus it is exact on the stalks, so by~\cite[2.9(d)]{milne:etale}
the sequence
$$0\ra \cA(\Z_v^{\ur})\ra \cJ(\Z_v^{\ur}) \ra \cC(\Z_v^{\ur})\ra 0$$
is exact; by the N\'eron mapping property the sequence
$$0\ra A(\Q_v^{\ur})\ra J(\Q_v^{\ur}) \ra C(\Q_v^{\ur})\ra 0$$
is also exact. 
Thus $\res_v(\pi(x))$ in unramified, 
so it arises by inflation from
an element of $H^1(\Q_v^{\ur}/\Q_v,A)$.
By \cite[Prop.~I.3.8]{milne:duality},
 $$H^1(\Q_v^{\ur}/\Q_v,A) \isom H^1(\Q_v^{\ur}/\Q_v,\Phi_{A,v}),$$
where $\Phi_{A,v}$ is the component group of~$A$ at~$v$.
Since~$A$ has good reduction, $\Phi_{A,v}=0$, hence
$\res_v(\pi(x))=0$. 

If~$J$ has bad reduction at~$v=2$, then we already dealt with~$2$ above.  
Consider the case when~$J$ has good reduction at~$2$.   Because the
absolute ramification index~$e$ of $\Z_2$ is~$1$, which is
{\em not} less than $v-1=1$, we can not apply \cite[Thm.~7.5.4]{neronmodels}.
However, we can modify our situation by an isogeny of degree a power
of~$2$, then apply a different theorem as follows.
The $2$-primary subgroup~$\Psi$ of $A\intersect B$ is 
rational as a subgroup over~$\Q$, in the sense that the
conjugates of any point in $\Psi$ are
also contained in $\Psi$.
The abelian varieties
$\tilde{J}=J/\Psi$, $\tilde{A}=A/\Psi$, and 
$\tilde{B}=B/\Psi$ also satisfy the hypothesis of 
the theorem we are proving.
By \cite[Prop.~7.5.3(a)]{neronmodels}, the corresponding sequence of
N\'eron models
$$0\ra\tilde{\cA}\ra\tilde{\cJ}\ra\tilde{\cC}\ra 0$$
is exact, so the sequence
$$0\ra \tilde{A}(\Q_v^{\ur})\ra\tilde{J}(\Q_v^{\ur})
\ra\tilde{C}(\Q_v^{\ur})\ra 0$$
is exact. Thus the image of 
$\res_v(\pi(x))$  in $H^1(\Q_v,\tilde{A})$ is unramified.
It equals~$0$, again by \cite[Prop.~3.8]{milne:duality},
since the component group of $\tilde{A}$ at~$v$ has order a power
of~$2$ (in fact it is trivial, since $\tilde{A}$ has
good reduction at~$2$), whereas $\pi(x)$ has odd prime order~$p$.
Thus $\res_v(\pi(x))=0$, since 
the kernel of $H^1(\Q_v,A)\ra H^1(\Q_v,\tilde{A})$ is a 
finite group of $2$-power order.
\end{proof}




\section{Description of tables}
In this section we describe our tables of optimal quotients of
$J_0(N)$, which have nontrivial Shafarevich-Tate group. 
The tables, which can be found on 
pages~\pageref{table:primesha}--\pageref{table:shacompgps}, 
were computed using a combination of
\hecke{}~\cite{stein:hecke}, {\sc LiDIA}, {\sc NTL}, {\sc Pari}, and
most successfully \magma{}~\cite{magma}.  The component group
computations at non-prime level rely on Kohel's quaternion
algebra package, which was also written in \magma{}.

We compute the conjectural order of the Shafarevich-Tate group of an
abelian variety~$A$, and then make assertions about the
Shafarevich-Tate group of~$\Adual$.  This is justified because the
order of $\Sha(\Adual)$ equal the order of~$\Sha(A)$, since both are
finite and the Cassells-Tate pairing sets up a nondegenerate duality
between them.

\subsection{Notation}\label{sec:optquo-notation}
Each optimal quotient~$A$ of $J_0(N)$ is denoted by a label, such as
{\bf 389E}, which consists of a level~$N$ and a letter indicating the
isogeny class.  In the labeling,~$N$ is a positive integer and the
isogeny class is given by a letter: the first isogeny class is labeled
{\bf A}, the second is labeled {\bf B}, the third labeled {\bf C}, and
so on.  Thus {\bf 389E} is the fifth isogeny class of optimal quotient
of $J_0(389)$, corresponding to a Galois-conjugacy class of newforms.
The isogeny classes that we consider are in bijection with the
Galois-conjugacy classes of newforms in $S_2(\Gamma_0(N))$.  The
classes of newforms are ordered as described in Section~\ref{sec:sorting}.

{\bf WARNING:} The {\em odd part} of a rational number $x$ is $x/2^v$,
where $v=\ord_2(x)$.  In the tables, only the {\bf odd parts} of the
arithmetic invariants of~$A$ are given.

\subsection{Table~\ref{table:primesha}: Shafarevich-Tate 
groups at prime level} 

Table~\ref{table:primesha} was constructed as follows.  Using the
results of Section~\ref{sec:ratpartformula}, we computed the odd part
of the conjectural order $\Shaan(A)$ of the Shafarevich-Tate group of
every optimal quotient of $J_0(p)$ that corresponds to a single Galois
conjugacy-class of eigenforms and has analytic rank~$0$, for~$p$ a
prime with $p\leq 2161$.  We also computed a few sporadic examples of
prime level~$p$ with $p>2161$.  The sporadic examples appear at the
bottom of the table below a horizontal line.

\subsubsection{Notation}
The columns of the table contain the following information.  The
abelian varieties~$A$ for which $\Shaan(A)$ is greater than~$1$ are
laid out in the first column of Table~\ref{table:primesha}.  The
second column contains the dimensions of the abelian varieties in the
first column.  The third column contains the {\em odd part} (i.e.,
largest odd divisor) of the order of the Shafarevich-Tate group, as
predicted by the BSD conjecture.\index{BSD conjecture!predicted order of $\Sha$}  
Column four contains the odd parts
of the modular degrees of the abelian varieties in the first column.

The fifth column contains an optimal quotient~$B$ of $J_0(p)$ of
positive analytic rank, such that the $\ell$-torsion of $\Bdual$ is
contained in~$\Adual$, when one exists, where $\ell$ is a divisor
of $\Shaan(A)$.  We computed this
intersection using the algorithm described in
Section~\ref{sec:intersection}.  Such a~$B$ is called an
\defn{explanatory factor}.\index{Explanatory factor} 
When no such abelian varieties exists, we write ``NONE'' in
the fifth column.  The sixth column contains the dimensions of the
abelian varieties in the fifth column, and the seventh column contains
the odd parts of the modular degrees of the abelian varieties in the
fifth column.

\subsubsection{Ranks of the explanatory factors}
That the explanatory factors have positive analytic rank follows from
our modular symbols computation of $L(B,1)/\Omega_B$.  
This is supported by the table in~\cite{brumer:rank}, except
in the case {\bf 2333A}, where there is a mistake in \cite{brumer:rank}
(see below).

The explanatory factor {\bf 389A} is the first elliptic curve of
rank~$2$.  The table in \cite{brumer:rank} suggests that the
explanatory factor {\bf 1061B} is the first $2$-dimensional abelian
variety (attached to a newform) whose Mordell-Weil group when tensored
with the field of fractions~$F$ of the corresponding ring of Fourier
coefficients, is of dimension~$2$ over~$F$.  Similarly 
{\bf 1567B} appears to be the first $3$-dimensional one of rank~$2$, and
{\bf 2333A} is the first $4$-dimensional one of rank~$2$. 
It thus seems very likely that the ranks of each explanatory factor
is exactly~$2$, though we have not proved this.

\subsubsection{Discussion of the data}
There are~$23$ examples in which~$\Sha(A)$ is 
visible and~$18$ in which~$\Sha(A)$ 
is invisible.  The largest visible
$\Sha(A)$ found occurs at level $2333$ and has order at least $83341^2$ 
($83341$ is prime).  
The largest invisible\index{Invisible elements of $\Sha$} $\Sha(A)$ 
occurs in a $112$-dimensional abelian variety at level 
$2111$ and has order at least $211^2$. 

The example {\bf 1283C} demonstrates that $\Shaan(A)$ can divide the 
modular degree, yet be {\em invisible}.  In this case~$5$ divides
$\Shaan(A)$. Since~$5$ divides the 
modular degree, it follows that there must be
other non-isogenous subvarieties of $J_0(1283)$ that 
intersect {\bf 1283C} in a subgroup of order divisible
by~$5$. In this case, the only such subvariety is
{\bf 1283A}, which has dimension~$2$ and whose $5$-torsion is contained
in {\bf 1283C}.  However {\bf 1283A} has analytic (hence algebraic) rank~$0$,
so $\Shaan(A)$ cannot be visible.

The cases {\bf 1483D}, {\bf 1567D}, {\bf 2029C}, and {\bf 2593B} are
interesting because {\em all} of~$\Sha$, even though it has two
nontrivial $p$-primary components in each of these cases, is made
visible in a single~$B$.  In the case {\bf 1913E} only
the $5$-primary component of $\Sha$ is visible in {\bf 1913A}, but
still {\em both} the $5$-primary and 
$61$-primary components of $\Sha$ are visible in {\bf 1913C}.

Examples {\bf 1091C} and {\bf 1429B} were first found in
\cite{agashe} and {\bf 1913B} in \cite{cremona-mazur}.

\subsubsection{Errata to Brumer's paper}
Contrary to our computations, \cite{brumer:rank} suggests that 
{\bf 2333A} has rank~$0$.  However, the author pointed the discrepancy out
to Brumer who replied:
\begin{quote}
I looked in vain for information about $\theta$-relations on~$2333$
and have concluded that I never ran the interval from~$2300$ to~$2500$
or else had lost all info by the time I wrote up the paper. The
punchline:~$4$ relations for~$2333$ and~$2$ relations for~$2381$ (also
missing from the table).
\end{quote}

\comment{
Date: Wed, 08 Sep 1999 18:24:10 -0400
From: armand brumer <brumer@murray.fordham.edu>
To: William Arthur Stein <was@math.berkeley.edu>
CC: ab <brumer@murray.fordham.edu>
Subject: Re: The rank of J_0(2333)

Dear William,
         I just found your 3 emails (including one from the end of June)
sitting on a mail server I did not know existed until a few days ago (the
university did not tell us that the two addresses were on different
servers!!)

I then looked in vain for information about theta relations on 2333 and have
concluded that I never ran the interval from 2300 to 2500 or else had lost
all info by the time I wrote up the paper. The punchline:4 relations for 2333
and 2 relations for 2381 (also missing from the table). I may try to check as
much as possible in the background and would be grateful if you mention this
errata when you write up your stuff (I don't know any other way of
publicizing the correction).

Best regards and hope you did not think I was ``blowing you off" as my son
would say!

Armand
}                            


\subsection{Tables~\ref{table:newvis}--\ref{table:shacompgps}:
New visible Shafarevich-Tate groups} 

Let~$n$ denote the largest odd square dividing the numerator of
$L(A,1)/\Omega_A$.  Table~\ref{table:newvis} lists those~$A$ such that
for some $p\mid n$ there exists a quotient~$B$ of $J_0(N)$,
corresponding to a {\em newform} and having positive analytic rank,
such that $(\Adual\intersect B^{\vee})[p]\neq 0$.  Our search was
systematic up to level $1001$, but there might be omitted examples
between levels $1001$ and $1028$.
Table~\ref{table:explain} contains
further arithmetic information about each explanatory factor.  
Table~\ref{table:shacompgps} gives the quantities involved in the
formula of Chapter~\ref{chap:compgroups} for Tamagawa numbers, for
each of the abelian varieties~$A$ in Table~\ref{table:newvis}. 


\subsubsection{Notation}
Most of the notation is the same as in Table~\ref{table:primesha}.
However the additional columns $w_q$ and $c_p$ contain the signs
of the Atkin-Lehner involutions and the Tamagawa numbers, respectively.
These are given in order, from smallest to largest prime divisor
of~$N$.

In each case~$B$ has dimension~$1$.  When $4\mid N$, we write ``$a$''
for $c_2$ to remind us that we did not compute $c_2$ because the
reduction at~$2$ is additive.  Again only
{\em odd parts} of the invariants are given.
Section~\ref{sec:compgrptables} contains a discussion of 
the notation used in the 
headings of Table~\ref{table:shacompgps}.

\subsubsection{Remarks on the data}
The explanatory factors~$B$ of level $\leq 1028$ are {\em exactly} the 
set of rank~$2$ elliptic curves of level $\leq 1028$. 


\section{Further visibility computations}



\subsection{Does $\Sha$ become visible at higher level?}
\label{sec:higherlevel}

This section is concerned with whether or not the examples of invisible
elements of Shafarevich-Tate groups of elliptic curves of level~$N$
that are given in \cite{cremona-mazur} become visible in abelian
surfaces inside appropriate $J_0(Np)$.  We analyze each of the cases
in Table~1 of \cite{cremona-mazur}.  For the reader's convenience, the
part of this table which concerns us is reproduced as
Table~\ref{table:cm}.
The most interesting examples we give
are {\bf 2849A} and {\bf 5389A}.  As in
\cite{cremona-mazur}, the assertions below assume the 
truth of the BSD conjecture.\index{BSD conjecture}


\begin{table}\index{Table of!odd invisible $\Sha_E$}
\ssp
\caption{Odd invisible $|\Sha_E|>1$, all $N\leq 5500$ (from Table~1 of~\cite{cremona-mazur})}
\label{table:cm}
$$
\begin{array}{lcclcl}
\mbox{\rm\bf E}&\sqrt{|\Sha_E|}& m_E & \mbox{\rm\bf F}  
      & m_F & \text{Remarks}\\ 
& & & & & \vspace{-3ex} \\
\mbox{\rm\bf 2849A}& 3 &2^5\cdot 5\cdot 61&\mbox{\rm\bf NONE}& - &\\
\mbox{\rm\bf 3364C}& 7 &2^6\cdot3^2\cdot5^2\cdot7  &\mbox{\rm\bf none}& - &\\
\mbox{\rm\bf 4229A}& 3 &2^3\cdot3\cdot7\cdot13  &\mbox{\rm\bf none}& - &\\
\mbox{\rm\bf 4343B}& 3 &2^4\cdot1583  &\mbox{\rm\bf NONE}& -&\\
\mbox{\rm\bf 4914N}& 3 &2^4\cdot 3^5  &\mbox{\rm\bf none}& - &\text{{\bf E} has rational $3$-torsion}\\
\mbox{\rm\bf 5054C}& 3 &2^3\cdot 3^3\cdot 11&\mbox{\rm\bf none}& - &\\
\mbox{\rm\bf 5073D}& 3 &2^5\cdot 3\cdot 5\cdot7\cdot23 
                           &\mbox{\rm\bf none}& - & \\
\mbox{\rm\bf 5389A}& 3 &2^2\cdot 2333  &\mbox{\rm\bf NONE}& - &\\
\end{array}
$$
\end{table}


\subsubsection{How we found the explanatory curves}
We use a naive heuristic observation to find possible explanatory
curves of higher level, even though their conductors are out of the
range of Cremona's tables.  Note that we have not proved that
these factors are actually explanatory in any cases, and expect
that in some cases they are not.

First we recall some of the notation from Table~1
of~\cite{cremona-mazur}, which is partially reproduced below.  
The ``NONE'' label in the row corresponding
to an elliptic curve~$E$ indicates that there are elements in
$\Sha(E)$ whose order does not divide the modular degree of~$E$, and
hence they must be invisible.  The label ``none'' indicates only that
no suitable explanatory elliptic curves could be found, so $\Sha(E)$ is
not visible in an {\em abelian surface} inside $J_0(N)$; it could, 
however, be visible in the full abelian variety $J_0(N)$.


Studying the Weierstrass equations corresponding to the curves in 
\cite{cremona-mazur} reveals that the elliptic curves labeled
``NONE'' have unusually large height, as compared to their conductors.
However, the explanatory factors often have unusually small height.
Motivated by this purely heuristic observation, we make a table of 
all elliptic curves of the form
    $$y^2 + a_1 xy + a_3 y = x^3 + a_2x^2 + a_4 x+ a_6,$$
with $a_1, a_2, a_3 \in \{-1,0,1\}$, $|a_4|, |a_6| < 1000$,
and conductor bounded by $50000$.  The bound on the conductor 
is required only so that the table will fit within 
computer storage.  This table took a few days to generate.

\subsubsection{2849A} Barry Mazur\index{Mazur} and Adam Logan\index{Logan} 
found the first known example of an 
{\em invisible} Shafarevich-Tate group\index{Shafarevich-Tate group!first invisible example}.  This 
was $\Sha(E)$, where~$E$ is the elliptic curve {\bf 2849A}, 
which has minimal Weierstrass equation
       $$E:\quad y^2 + xy + y = x^3 + x^2 - 53484x - 4843180.$$ 
Consulting our table of curves of small height, 
we find an elliptic curve~$F$ of conductor
$8547=2849\cdot 3$ such that $f_E \con f_F \pmod{3}$, where $f_E$
and $f_F$ are the newforms attached to~$E$ and~$F$, respectively.
This is a congruence for {\em all} eigenvalues $a_p$ attached to~$E$ and~$F$.
The elliptic curve~$F$ is defined by the equation
       $$F:\quad y^2 + xy + y = x^3 + x^2 - 154x - 478.$$
 Cremona's program {\tt mwrank} reveals that
the Mordell-Weil group of~$F$ has rank~$2$.  
Thus maybe $\Sha(E)$ becomes visible at level~$8547$.  
Unfortunately, visibility of $\Sha(E)$ is not 
implied by Theorem~\ref{thm:shaexists} because
the geometric component group of~$F$ at~$3$ has order 
divisible by~$3$.  

\subsubsection{4343B} Consider the elliptic curve $E$ labeled
{\bf 4343B}.  According to Table~1 of \cite{cremona-mazur}, 
$\Sha(E)$ has order~$9$, but the modular degree prevents~$\Sha(E)$ 
from being visible in $J_0(4343)$.  
At level $21715 = 5\cdot 4343$ 
there is an elliptic curve~$F$ of rank~$1$ that is
congruent to~$E$.  Its equation is
       $$F:\quad y^2 - xy - y = x^3 - x^2 + 78x - 256.$$

\subsubsection{5389A} The last curve labeled ``NONE'' in the table is curve
{\bf 5389A}, which has minimal Weierstrass equation
       $$y^2+xy+y =x^3 - 35590x-2587197.$$

The main theorem of~\cite{ribet:raising} implies that there exists a
newform that is congruent modulo~$3$ to the newform corresponding to
{\bf 5389A} and of level $3\cdot 5389$.  This is because $(-2)^2 =
(3+1)^2 \pmod{3}$.  However, our table of curves of small height does
not contain any curve of conductor $3\cdot 5389$.  Next we observe that
$(-2)^2 \con (7+1)^2 \pmod{3}$, so using Ribet's\index{Ribet} theorem we can
instead augment the level by~$7$.  Our table of small-height curves
contains just one (up to isogeny) elliptic curve of 
conductor~$37723$, and {\em luckily} the
corresponding newform is congruent modulo~$3$ to the newform
corresponding to {\bf 5389A} (away from primes dividing the level)!  
The Weierstrass equation of this curve is
            $$F:\quad y^2 - y = x^3 + x^2 + 34x - 248.$$
According to Cremona's program {\tt mwrank}, 
the rank of~$F$ is~$2$.


\subsubsection{3364C, 4229A, 5073D} 
Perhaps $\Sha(E)$ is already visible in some of the cases in which the 
curve is labeled ``none'', because the method fails in most
of these cases.  Each of the curves {\bf 3364C}, {\bf
4229A}, and {\bf 5073D} is labeled ``none''.
In none of these 3 cases are we able to find
an explanatory factor at higher level, within the range of our table
of elliptic curves of small height.  

\subsubsection{4194N, 5054C} The curve {\bf 4914N} is labeled ``none'' 
and we find the remark ``$E$ has rational $3$-torsion''.
There is a congruent curve~$F$ of conductor $24570$ given
by the equation
         $$F: \quad y^2 - xy = x^3 - x^2 - 15x - 75,$$  
and $F(\Q) = \{0\}$. The curve {\bf 5054C} is labeled ``none'' 
and its Shafarevich-Tate group contains invisible elements of
order~$3$.  We find a congruent curve of level~$25270$ and rank~$1$.
The equation of the congruent curve is 
         $$F: y^2 - xy = x^3 + x^2 - 178x + 882.$$


\subsection{Positive rank example}
The abelian varieties with nontrivial $\Sha(A)$ that one
finds in both ours and Cremona's
tables all have rank~$0$.  In this section we outline a computation
which sugggests, but does not prove, that there is a positive-rank abelian
subvariety $A$ of $J_0(5077)$ such that $\Sha(A)$
possesses a nontrivial visible element of order~$31$.

According to \cite{cremona:algs}, 
the smallest conductor elliptic curve~$E$ of rank~$3$ is found in
$J=J_0(5077)$.  The number $5077$ is prime, and the isogeny
decomposition of~$J$ is\footnote{
This decomposition was found in about one minute
using the Mestre-Oesterl\'e\index{Mestre} 
method of graphs (see~\cite{mestre:graphs}).}
             $$J \sim A \cross B \cross E,$$
where each of~$A$, $B$, and~$E$ are abelian subvarieties of~$J$ 
associated to newforms, which have 
dimensions $205$, $216$, and~$1$, respectively.
Using Remark~\ref{rem:moddegmestre} or 
\cite{zagier:parametrizations}, 
we find that the modular degree of~$E$ is $1984=2^6\cdot 31$.
The sign of the Atkin-Lehner involution on~$E$ is the same
as its sign on~$A$, so $E[31]\subset A$.
We have $E(\Q)\isom \Z\cross\Z\cross\Z$, and the 
component group of~$E$ is trivial.
The numerator of $(5077-1)/12$ is $3^2\cdot 47$, so \cite{mazur:eisenstein}
implies that none of the abelian varieties above have $31$-torsion.
It might be possible to find an analogue of Theorem~\ref{thm:shaexists}
that applies when~$A$ has positive rank, and deduce in this case
that $\Sha(A)$ contains visible elements of order~$31$.

%\section{Tables}
\begin{table}\index{Table of!$\Sha$ at prime level}
\ssp
\caption{Shafarevich-Tate groups at prime level.
(The entries in the columns 
``mod deg'' and ``$\Shaan$'' are only really
the odd parts of ``mod deg'' and ``$\Shaan$''.)\label{table:primesha}}
\vspace{-.25in}$$
\begin{array}{lccclcc}
\mbox{\rm\bf A}& \mbox{\rm dim}& \Shaan(A) & \mbox{\rm mod deg}(A) & \mbox{\rm\bf B}  & \mbox{\rm dim} & \mbox{\rm mod deg} (B)\\ 
& & & & & & \vspace{-3ex} \\
\mbox{\rm\bf 389E}& 20 &5^{2}&5&\mbox{\rm\bf 389A}& 1 &5\\
\mbox{\rm\bf 433D}& 16 &7^{2}&3\cdot7\cdot37&\mbox{\rm\bf 433A}& 1 &7\\
\mbox{\rm\bf 563E}& 31 &13^{2}&13&\mbox{\rm\bf 563A}& 1 &13\\
\mbox{\rm\bf 571D}& 2 &3^{2}&3^{2}\cdot127&\mbox{\rm\bf 571B}& 1 &3\\
\mbox{\rm\bf 709C}& 30 &11^{2}&11&\mbox{\rm\bf 709A}& 1 &11\\
\mbox{\rm\bf 997H}& 42 &3^{4}&3^{2}&\mbox{\rm\bf 997B}& 1 &3\\
\mbox{\rm\bf 1061D}& 46 &151^{2}&61\cdot151\cdot179&\mbox{\rm\bf 1061B}& 2 &151\\
\mbox{\rm\bf 1091C}& 62 &7^{2}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 1171D}& 53 &11^{2}&3^{4}\cdot11&\mbox{\rm\bf 1171A}& 1 &11\\
\mbox{\rm\bf 1283C}& 62 &5^{2}&5\cdot41\cdot59&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 1429B}& 64 &5^{2}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 1481C}& 71 &13^{2}&5^{2}\cdot2833&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 1483D}& 67 &3^{2}\cdot5^{2}&3\cdot5&\mbox{\rm\bf 1483A}& 1 &3\cdot5\\
\mbox{\rm\bf 1531D}& 73 &3^{2}&3&\mbox{\rm\bf 1531A}& 1 &3\\
\mbox{\rm\bf 1559B}& 90 &11^{2}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 1567D}& 69 &7^{2}\cdot41^{2}&7\cdot41&\mbox{\rm\bf 1567B}& 3 &7\cdot41\\
\mbox{\rm\bf 1613D}& 75 &5^{2}&5\cdot19&\mbox{\rm\bf 1613A}& 1 &5\\
\mbox{\rm\bf 1621C}& 70 &17^{2}&17&\mbox{\rm\bf 1621A}& 1 &17\\
\mbox{\rm\bf 1627C}& 73 &3^{4}&3^{2}&\mbox{\rm\bf 1627A}& 1 &3^{2}\\
\mbox{\rm\bf 1693C}& 72 &1301^{2}&1301&\mbox{\rm\bf 1693A}& 3 &1301\\
\mbox{\rm\bf 1811D}& 98 &31^{2}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 1847B}& 98 &3^{6}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 1871C}& 98 &19^{2}&14699&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 1877B}& 86 &7^{2}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 1907D}& 90 &7^{2}&3\cdot5\cdot7\cdot11&\mbox{\rm\bf 1907A}& 1 &7\\
\mbox{\rm\bf 1913B}& 1 &3^{2}&3\cdot103&\mbox{\rm\bf 1913A}& 1 &3\cdot5^{2}\\
\mbox{\rm\bf 1913E}& 84 &5^{4}\cdot61^{2}&5^{2}\cdot61\cdot103&\mbox{\rm\bf 1913A,C}& 1,2 &3\cdot5^{2}, 5^2\cdot 61\\
\mbox{\rm\bf 1933C}& 83 &3^{2}\cdot7^{2}&3\cdot7&\mbox{\rm\bf 1933A}& 1 &3\cdot7\\
\mbox{\rm\bf 1997C}& 93 &17^{2}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 2027C}& 94 &29^{2}&29&\mbox{\rm\bf 2027A}& 1 &29\\
\mbox{\rm\bf 2029C}& 90 &5^{2}\cdot269^{2}&5\cdot269&\mbox{\rm\bf 2029A}& 2 &5\cdot269\\
\mbox{\rm\bf 2039F}& 99 &3^{4}\cdot5^{2}&19\cdot29\cdot7759\cdot3214201&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 2063C}& 106 &13^{2}&61\cdot139&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 2089J}& 91 &11^{2}&3\cdot5\cdot11\cdot19\cdot73\cdot139&\mbox{\rm\bf 2089B}& 1 &11\\
\mbox{\rm\bf 2099B}& 106 &3^{2}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 2111B}& 112 &211^{2}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 2113B}& 91 &7^{2}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 2161C}& 98 &23^{2}&1&\mbox{\rm NONE} & & \\\hline
\mbox{\rm\bf 2333C}& 101 &83341^{2}&83341&\mbox{\rm\bf 2333A}& 4 &83341\\
\mbox{\rm\bf 2339C}& 114 &3^{8}&6791&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 2411B}& 123 &11^{2}&1&\mbox{\rm NONE} & & \\
\mbox{\rm\bf 2593B}& 109 &67^2\cdot 2213^2 & 67 \cdot 2213
       &\mbox {\bf 2593A}& 4 
        & 67 \cdot 2213\\
\end{array}
$$
\end{table}

\begin{table}\index{Table of!new visible $\Sha$}
\ssp
\caption{New visible Shafarevich-Tate groups\label{table:newvis}}\vspace{-2ex}
$$\begin{array}{lcccccccl}
\mathbf{A}    &\text{dim} &\Shaan   &w_q& c_p &\#A(\Q)
                                &\frac{\#A(\Q)\cdot L(A,1)}{\Omega_A} 
                                &\mbox{\rm mod deg(A)} & \quad\mathbf{B}\\
\vspace{-2ex} & &  &&& & &  \\
\mathbf{389E} & 20&5^2&-&97&97&5^2&5&\mathbf{389A}\\
\mathbf{433D} & 16&7^2&-&3^2&3^2&7^2&3\cdot 7\cdot 37&\mathbf{433A}\\
\mathbf{446F}&8&11^2&+- &1,3&3&11^2&11\cdot359353&\mathbf{446B}\\
\mathbf{563E}&31  & 13^2 & - & 281   &281 &13^2  &13           &\mathbf{563A}\\
\mathbf{571D}&2  & 3^2  & - & 1   &1   & 3^2  & 3^2\cdot 127&\mathbf{571B}\\
\mathbf{655D}&13  & 3^4  &+- & 1,1   & 1  & 3^4  & 3^2\cdot 19\cdot 515741&\mathbf{655A}\\

\mathbf{664F} & 8&5^2 &-+ &a,1&1&  5^2 &5  & \mathbf{664A}\\
%             Sha    dim   Wq   c_p    T    TL/O        delta           B
\mathbf{681B}&1 & 3^2  &+- & 1,1   & 1  &  3^2  &   3\cdot 5^3    & \mathbf{681C}\\
\mathbf{707G}& 15& 13^2 &+- & 1,1   & 1  &  13^2 &   13\cdot 800077& \mathbf{707A}\\
\mathbf{709C}&30& 11^2 &-  & 59   &59  &  11^2 &   11                  & \mathbf{709A}\\
\mathbf{718F}&7& 7^2  &+- & 1,1   & 1 &   7^2   &7\cdot 151\cdot 35573  & \mathbf{718B}\\ 
\mathbf{794G}&14& 11^2 &+- & 3,1   & 3 &  11^2   &3\cdot7\cdot11\cdot47\cdot35447& \mathbf{794A}\\
\mathbf{817E}& 15& 7^2  &+- & 1,5   & 5 &   7^2   &  7\cdot 79            & \mathbf{817A}\\
\mathbf{916G}&9& 11^2  &-+ & a,1   & 1 &  11^2   &  3^9\cdot 11\cdot 17\cdot 239 & \mathbf{916C}\\
\mathbf{944O} &6& 7^2   &+- & a,1   & 1 &  7^2    &  7                    & \mathbf{944E}\\ 
\mathbf{997H}&42& 3^4  &-  & 83    & 83 &  3^4   &     3^2        & \mathbf{997BC}\\
\mathbf{1001L}&7& 7^2  &+-+& 1,1,1  & 1 & 7^2     & 7\cdot19\cdot47\cdot2273&\mathbf{1001C}\\  
\mathbf{1028E}&14& 11^2&-+ & a,1  & 3 &  3^4\cdot 11^2 & 3^{13}\cdot 11 & \mathbf{1028A}\\ 
\end{array}$$
\end{table}

\begin{table}\index{Table of!explanatory factors}
\ssp
\caption{Explanatory factors\label{table:explain}}\vspace{-2ex}
$$\begin{array}{lcccccc}
\mathbf{B}&\text{rank}&w_q&c_p&\#A(\Q)&\mbox{\rm mod deg(A)}&\text{Comments}\\
\vspace{-2ex} & &  && & &  \\
\mathbf{389A}& 2 &-&1 &1&5&\text{first curve of rank $2$}\\
\mathbf{433A}&2 &-&1&1&7&\\
\mathbf{446B}&2 &+-&1,1& 1 &11&\text{called $\mathbf{446D}$ in \cite{cremona:algs}}\\
\mathbf{563A}&2 &- & 1   & 1  & 13    & \\
\mathbf{571B}&2 &- & 1   & 1  &  3    & \\
\mathbf{655A}&2 &+-& 1,1 & 1  &  3^2  & \\
\mathbf{664A}&2 &-+& 1,1 & 1  &  5    & \\
%              RANK      wq   g_p  Tor   delta   comments
\mathbf{681C} &  2  & +- & 1,1  & 1  &  3    &  \\
\mathbf{707A} &  2  & +- & 1,1  & 1  &  13    &  \\
\mathbf{709A} &  2  & -  & 1   &  1  &  11    &  \\
\mathbf{718B} &  2  & +- & 1,1  & 1  &  7   &  \\ 
\mathbf{794A} &  2  & +- & 1,1  & 1  &  11   &  \\
\mathbf{817A} &  2  & +- & 1,1  & 1  &  7   &  \\
\mathbf{916C} &  2  & -+ & 3,1  & 1  & 3\cdot 11 & \\
\mathbf{944E} &  2  & +- & 1,1  & 1  &  7    &  \\
\mathbf{997B} &  2  & -  & 1    & 1  &  3    &  \\
\mathbf{997C} &  2  & -  & 1    & 1  &  3    &  \\
\mathbf{1001C} & 2  & +-+& 1,3,1& 1  &  3^2\cdot 7  & \\  
\mathbf{1028A} & 2  & -+ & 3,1  & 1 &  3\cdot 11& \text{intersects $\mathbf{1028E}$ mod $11$}\\
\end{array}$$
\end{table}


\begin{table}\index{Table of!factorizations}
\ssp
\caption{Factorizations\label{table:factor}}\vspace{-1ex}
$$\begin{array}{llll}
\mathbf{446}=2\cdot 223&
\mathbf{655}=5\cdot 131&
\mathbf{664}=2^3\cdot 83&
\mathbf{681}=3\cdot 227\\
\mathbf{707}=7\cdot 101&
\mathbf{718}=2\cdot 359&
\mathbf{794}=2\cdot 397&
\mathbf{817}=19\cdot 43\\
\mathbf{916}=2^2\cdot 229&
\mathbf{944}=2^4\cdot 59&
\mathbf{1001}=7\cdot 11\cdot 13&
\mathbf{1028}=2^2\cdot 257\\
\end{array}$$
\end{table}

\begin{table}\index{Table of!component groups of explanatory factors}
\ssp
\caption{Component groups\label{table:shacompgps}}\vspace{-2ex}
$$\begin{array}{lcccccc}
\vspace{-2ex}&&&&&&\\
\mathbf{A} &\text{dim}           &  p & w_q &\#\Phi_{X,p} &m_{X,p}
          &\#\Phi_{A,p}(\Fpbar) \\
\vspace{-2ex}&    &  &       &                 &                  \\
\mathbf{389E}&20& 389&-&  97 & 5\cdot 97 &  97      \\
\mathbf{433D}&16& 433&-& 3^2& 3^3\cdot 7\cdot 37   &  3^2   \\
\mathbf{446F}&8 & 223&-& 3 & 3\cdot 11\cdot 359353   &      3        \\
               && 2 &+ & 3 & 3\cdot 11&                                     3\cdot 359353\\
\mathbf{563E}&31& 563&-& 281& 13\cdot 281&   281 \\
\mathbf{571D}&2 & 571&-& 1  & 3^2\cdot 127  &1 \\
\mathbf{655D}&13& 131&-& 1 & 3^{2}\cdot19\cdot515741 &  1\\
               &&   5&+& 1  & 3^2  &          19\cdot 515741  \\
\mathbf{664F}&8 &  83&+& 1 & 5     & 1     \\
\mathbf{681B}&1 & 227&-&  1 &  3\cdot 5^3 &   1    \\
              &      &   3&+&  1      &  3\cdot 5^2&             5    \\
\mathbf{707G}&15& 101&-& 1 &  13\cdot800077 &  1 \\
               &&   7&+& 1 &   13&   800077         \\
\mathbf{709C}&30& 709&-&  59&  11\cdot 59 &   59   \\
\mathbf{718F}&7 & 359&-&  1   &  7\cdot 151\cdot 35573  &1 \\
               && 2  &+&  1   &  7    &       151\cdot 35573 \\
\mathbf{794G}&14& 397&-& 3   & 3^2\cdot7\cdot11\cdot47\cdot35447 & 3    \\
             &&   2&+& 3 &   3\cdot11&    3^2\cdot 7\cdot 47\cdot 35447  \\
\mathbf{817E}&15&  43&-  & 5  &  5\cdot7\cdot 79 &     5        \\
               &&  19&+ &1        &   7 &                                  79  \\
\mathbf{916G}&9 & 229&+ &1  & 3^9\cdot 11\cdot 17\cdot 239 &  1 \\

\mathbf{944O}&6 &  59&-& 1 & 7          &   1\\
\mathbf{997H}&42& 997&-& 83&  3^2\cdot 83 &          83  \\
\mathbf{1001L}&7&  13&+& 1&  7\cdot 19\cdot 47\cdot 2273& 1\\
               &&  11&-& 1&  7\cdot19\cdot47\cdot2273 &      1 \\
               &&   7&+& 1&  7\cdot 19\cdot 47 &     2273   \\
\mathbf{1028E}&14&257&+&  1  &   3^{13}\cdot 11 &   1  \\
\end{array}$$
\end{table}



\comment{

\begin{table}\index{Table of!odd square numerators}
\caption{Square roots of odd square divisors of $L(A,1)/\Omega_A$\label{table:oddnumer}}\vspace{2ex}
$$\begin{array}{lc}
\mathbf{305D7}&3\\
\mathbf{309D8}&5\\ 
\mathbf{335E11}&3^2\\
\mathbf{389E20}&5\\
\mathbf{394A2}&5\\
\mathbf{399G5}&3^4\\
\mathbf{433D16}&7\\
\mathbf{435G2}&3\\
\mathbf{436C4}&3\\
\mathbf{446E7}&3\\
\mathbf{446F8}&11\\
\mathbf{455D4}&3\\
\mathbf{473F9}&3\\
\mathbf{500C4}&3\\
\mathbf{502E6}&11\\
\mathbf{506I4}&5\\
\mathbf{524D4}&3\\
\mathbf{530G4}&7\\
\mathbf{538E7}&3\\
\mathbf{551H18}&3\\
\mathbf{553D13}&3\\
\mathbf{555E2}&3\\
\mathbf{556C7}&3\\
\mathbf{563E31}&13\\
\mathbf{564C3}&3\\
\mathbf{571D2}&3\\
\end{array}\qquad
\begin{array}{lc}
\mathbf{579G13}&15\\
\mathbf{597E14}&19\\
\mathbf{602G3}&3\\
\mathbf{604C6}&3 \\
\mathbf{615F6}&5 \\
\mathbf{615G8}&7 \\
\mathbf{620D3}&3\\ 
\mathbf{620E4}&3\\
\mathbf{626F12}&5\\
\mathbf{629G15}&3\\
\mathbf{642D2}&3\\
\mathbf{644C5}&3\\
\mathbf{644D5}&3\\
\mathbf{655D13}&3^2\\
\mathbf{660F2}&3\\
\mathbf{662E10}\!&\!\!43\\
\mathbf{664F8}&5\\
\mathbf{668B5}&3\\
\mathbf{678I2}&3\\
\mathbf{681B1}&3\\
\mathbf{681I10}&3\\
\mathbf{682I6}&11\\
\mathbf{707G15}&13\\
\mathbf{709C30}&11\\
\mathbf{718F7}&7\\
\mathbf{721F14}&3^2\\ 
\end{array}\qquad
\begin{array}{lc}
\mathbf{724C8}&3\\
\mathbf{756G2}&3\\
\mathbf{764A8}&3\\
\mathbf{765M4}&3\\
\mathbf{766B4}&3\\
\mathbf{772C9}&3\\
\mathbf{790H6}&3\\
\mathbf{794G12}\!&\!\!11\\
\mathbf{794H14}&5^2\\
\mathbf{796C8}&3\\
\mathbf{817E15}&7\\
\mathbf{820C4}&3\\
\mathbf{825E2}&3\\
\mathbf{844C10}\!&\!\!3^2\\
\mathbf{855M4}&3\\
\mathbf{860D4}&3\\
\mathbf{868E5}&3\\
\mathbf{876E5}&3\\
\mathbf{878C2}&3\\
\mathbf{884D6}&3\\
\mathbf{885L9}&3^2\\ 
\mathbf{894H2}&3\\
\mathbf{902I5}&3\\
\mathbf{913G17}&3\\
\mathbf{916G9}&11\\
\mathbf{918O2}&5\\
\end{array}\qquad
\begin{array}{lc}
\mathbf{918P2}&3\\
\mathbf{925K7}&3\\
\mathbf{932B13}&3^2\\
\mathbf{933E14}&19\\
\mathbf{934I12}&7\\   %-+
\mathbf{944O6}&7\\
\mathbf{946K7}&3\\
\mathbf{949B2}&3\\
\mathbf{951D19}&3\\
\mathbf{959D24}&3\\
\mathbf{964C12}&3^2\\   % -+, same as EC 964A but that has rank=0.
\mathbf{966J1}&3\\
\mathbf{970I5}&3\\   
\mathbf{980F1}&3\\   
\mathbf{980J2}&3\\
\mathbf{986J7}&5\\ 
\mathbf{989E22}&5\\
\mathbf{993B3}&3^2\\
\mathbf{996E4}&3\\ 
\mathbf{997H42}&3^2\\ 
\mathbf{998A2}&3\\   % ++
\mathbf{998H9}&3\\
\mathbf{999J10}&3\\  
&\\
&\\
&\\
\end{array}$$
\end{table}


}



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%




\comment{

\begin{lemma}
Let~$A$ and~$B$ be abelian subvarieties of an abelian variety~$J$ such 
that $\Phi=A\intersect B$ is finite and and $A(\Q)$ is finite. 
Suppose that~$p$ 
is a prime such that neither $(J/B)(\Q)$ nor $B(\Q)$ have any 
$p$-torsion and such that $B[p]\subset \Phi$.  
Then $B(\Q)/p B(\Q)$ is a subgroup of 
$(J/B)(\Q)/J(\Q)$.
\end{lemma}
\begin{proof}

\end{proof}



\section{Explanatory factors at higher level}
Consider one of the items in Table~\ref{table:primesha} for which
$\Sha$ is invisible.  It is natural to ask whether these
elements of~$\Sha$ ``become visible somewhere.''
For example, Mazur~\cite{mazur:visthree}\index{Mazur} proved that if
$E$ is an elliptic curve and $c\in\Sha(E)$ has order $3$ then
there is some abelian surface $A$ and an 
injection $\iota: E\hookrightarrow A$ such that 
$\iota_*(c)=0\in H^1(\Q,A)$.  T. Klenke has proved
a partial statement in this direction for elements of 
order $2$ as part of his Harvard Ph.D. thesis. 
J.~de Jong (see \cite[Remark 3]{cremona-mazur})
showed that every element of the Shafarevich-Tate
group of an elliptic curve is visible in some Jacobian.\index{Jacobian}

Consider an abelian variety $A$, taken 
from Table~\ref{table:primesha},  for which
$\Sha$ has an invisible element $c$.   Thus 
$A$ sits inside $J_0(p)$ for some prime $p$, 
and we ask ``is there a prime $q$ such that $\delta(c)=0$ 
for one of the degeneracy maps 
$\delta : J_0(p)\ra J_0(pq)$?''

The author has no idea\footnote{Lo\"\i{}c Merel\index{Merel} suspects
the answer might be yes whereas Richard Taylor is more skeptical.}.
To get a feeling for what might happen we consider in detail abelian
variety $A=A_f$ at level $p=1091$ in which $\Shaan$ is divisible by
$7$.

There is a prime $\lambda$ of the ring 
$\Z[f] = \Z[\ldots a_n\ldots]$ attached to $A$. 
The Fourier coefficients of $f$ modulo $\lambda$ are
$$\begin{array}{rcccccccccccccccc}
p=   &2  &3  &5  &7  &11  &13  &17  &19  &23  &29  &31  &37  &41  &43  &47  &53\\
a_p= &3  &0  &1  &5  &0   &2   &0   &5   &4   &6   &3   &3   &5   &5   &6   &5
\end{array}$$
These were computed by finding an eigenvector in  $H_1(X_0(N);\F_7)$. 
[[SAY MORE ABOUT THE TRICK FOR FINDING ALL RIBET $q$'s.]]

According to Ribet's\index{Ribet} level raising theorem \cite{ribet:raising} 
there is a newform $g$ of level $1091\ell$ such that 
$f\con g$ modulo [[something]] if $a_\ell = \pm (\ell+1)\pmod{\lambda}$. 
Fortunately this criterion is already satisfied for $\ell=2$.
Looking closely at level $2\cdot 1091$ (for example, 
in Cremona's online tables \cite{cremona:onlinetables})
we find an elliptic curve $E$ whose corresponding newform
$g=\sum b_n q^n$ has Fourier coefficients
$$\begin{array}{rcccccccccccccccc}
p = &2 & 3 & 5 & 7& 11& 13& 17& 19& 23&  29&  31&  37&41&43&47&53\\
b_p=&1 & 0 & 1 &-2& 0 & -5& 0 & -2&  4&   6&  -4&  -4&-9&-9&-8&-2\\
\end{array}$$

This is convincing evidence that one of the 
two images of~$A$ in $J_0(2\cdot 1091)$ shares some
$7$-torsion with the elliptic curve \abvar{2182B}. 
This can be [[WILL BE!!, EASILY]] established by
a direct computation with the period lattices. 
This is at first disconcerting because the rank of
this elliptic curve is {\em not} $2$.  However, the 
rank is still positive; it is $1$ with 
Mordell-Weil group $\E(\Q)=Z$.

I would not be at all surprised if your 
$7$-torsion in Sha does become visible in $J_0(2\cdot1091)$.
The curve \abvar{2182B}, which shares 7-torsion with $A$ is

\begin{verbatim}
   e=ellinit([1,-1,1,-67,67]);
The Tamagawa number c_2 is 14   (!!)
  ? elllocalred(e,2)
  %2 = [1, 18, [1, 0, 0, 0], 14]
The Tamagawa number c_1091 is 1.
  ? elllocalred(e,1091)
  %3 = [1, 5, [1, 0, 0, 0], 1]

I have this feeling that the right statement about congruence
and mordell-Weil is really something like
    congruence ==> "Selmer + Comp group"'s are identified.
Anyway, the extra component group of order 7 may perhaps
account for the other nontrivial element of Sha.  This might
just be wild speculation.

Good luck.

william
            Barry,

Amod asked me to investigate whether his element of order 7
in the winding quotient J_e at level 1091 becomes visible at
higher level.  Luckily, Ribet's\index{Ribet} level raising theorem predicts the
existence of a form at level 2*1091 congruent mod a prime over
7 to the form corresponding to J_e. Even more luckily, one of the
two rational newforms does the trick.  Thus an image of J_e in
J_0(2*1091) shares 7-torsion with an elliptic curve E (2182B
in Cremona's tables).  This elliptic curve has:

     E(Q) = Z
     Sha(E/Q) = 0
     c_2 = 14,   c_1091 = 1
     L^(1)(f,1)/1! = 4.27332686791516


So there is reasonable hope that the elements of order 7 in
Sha(J_e) are visible at this higher level, even though they
are invisible even in J_1(1091).

Best,
William     


  Dear William,
    This is terrific. I assume that you will be showing that for J_e the
winding
    (not quite quotient, but more conveniently sub-thing) in J_0(1091),
the image of

                 Sha(J_e)  --->  Sha(J_0(2*1091))

     just dies?  Since our working hope, I think, is that for any N there
is an
     M so that
                      Sha(J_0(N)) ---. Sha(J_0(N.M))


     dies, this suggests returning to the (mod 3) N=2849 example, where I
"know" that there must exist such an M  (because all three-torsion in Sha
on elliptic curves is visible in some appropriate abelian surface which is
isogenous to a product of two elliptic curves, and therefore, is abelian
surface is probably "modular").  But I do not know a specific M.

Barry
            
\end{verbatim}


\comment{
\begin{remark}
One reason we must assume~$p$ is odd, is because
when~$B$ has good reduction at~$2$,
in the proof we change~$J$ by an isogeny of $2$-power
degree in order to apply~\cite[\S7.5, Prop.~3]{neronmodels} at $p=2$.
When~$B$ has purely toric reduction,\index{Toric reduction}
at~$2$ we use Tate
uniformization to directly verify that points of $B(\Q)$ map into
$\Sha(A)$, thereby avoiding exactness properties of 
N\'eron models\index{N\'eron model}.
\end{remark}}

\subsubsection{Table~\ref{table:oddnumer}: 
Odd square divisors of $L(A,1)/\Omega_A$}
In order to find candidate~$A$ with nontrivial visible
$\Sha(A)$, we first enumerated those~$A$ for which the numerator 
of  $L(A,1)/\Omega_A$ is divisible by an odd square~$n>1$.  
For $N<1000$, these are given in Table~\ref{table:oddnumer}.  
Any odd visible $\Sha(A)$ coprime to 
primes dividing torsion and $c_p$ must show up as a divisor
of the numerator; it should show up as a square
divisor because the Mordell-Weil rank of the explanatory factor
should be even.  It would be interesting to compute the conjectural
order of $\Sha(A)$ for each abelian variety in this table, but
not in table 1, and show (when possible) that the visible
$\Sha(A)$ is old. 
}






