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\title{Fermat's Last Theorem\brk;After 356 Years}
\author{William F. Hammond}
\subtitle{A Lecture at the Everyone Seminar\brk;
University at Albany, October 22, 1993}
\date{GELLMU Edition with Retrospective Comments\\
April 21, 2001 \\
Minor revisions: July 15, 2004}
\begin{document}
\tableofcontents
\foreword{Comments for the \gellmu Edition}
Among the challenges that I have been facing with my \gellmu project
are \iseq{(1)} convincing mathematicians that it is possible to use
comfortable \latex;-like markup in a fully rigorous way to prepare our
articles so that they can have formal inclusion in the markup category
known as \anch[href="http://www.w3.org/XML/"]{\abbr{XML}} and
\iseq{(2)} then convincing them that high quality typesetting may be
obtained from the ensuing \abbr{XML} document instance.
Toward this end I have revisited the \latex; markup for the official
notes on my October 1993 Albany seminar presentation and edited what
was \latex source to convert it to \latex;-like source markup for
the \emph{article} document type that is part of the \gellmu didactic
markup production system. Information about this system and my
reasons for developing it may be found at
\urlanch{http://www.albany.edu/\tld;hammond/gellmu/}.
After the time of the original talk and the subsequent preparation of my
\anch[href="http://math.albany.edu:8000/math/pers/hammond/oct93.html"
]{original write-up}, there was a time --- fortunately not long and also not
to have been unexpected in the aftermath of so large a new development ---
when Andrew Wiles's argument underwent some revision in collaboration
with Richard Taylor. Questions about its soundness appeared to
have ceased by the fall of 1994, and the work announced by Wiles in
June 1993, as revised, was published in the May 1995 issue of the
\slnt{Annals of Mathematics}.
There has also been discussion, at times appearing to approach
controversy, about the name of the conjecture arising from the 1955
meeting in Japan. What I termed the ``Shimura-Taniyama-Weil''
conjecture became known as the ``modular curve conjecture'' and then,
from the summer of 1999, as the ``modular curve theorem'' after the work
of Breuil, Conrad, Diamond, and Taylor in the same vein
as the work of Wiles and Taylor for the ``semi-stable'' case.
I list a few references on these matters for the period since my
original talk:
\begin{Menu}
\item A. Wiles, ``Modular elliptic curves and Fermat's Last Theorem'',
\slnt{Annals of Mathematics}, (second series) vol. 141 (1995),
pp. 443--551.
\item R. Taylor \& A. Wiles, ``Ring-theoretic properties of certain
Hecke algebras'',
\slnt{Annals of Mathematics}, (second series) vol. 141 (1995),
pp. 553--572.
\item H. Darmon, F. Diamond, \& R. Taylor, ``Fermat's Last Theorem'',
\slnt{Current Developments in Mathematics, 1995}, International
Press, Cambridge, Massachusetts, 1995.
\item G. Cornell, J. H. Silverman, \& G. Stevens,
\slnt{Modular Forms and Fermat's Last Theorem}, Springer-Verlag, 1997.
\ This volume is the record of an instructional conference on number theory
and arithmetic geometry held August 9-18, 1995 at Boston University.
\item J. Coates \& S.T. Yau,
\slnt{Elliptic curves, modular forms, \& Fermat's last theorem}, 2nd
edition, International Press, Cambridge, MA, 1997. Proceedings of the
Conference on Elliptic Curves and Modular Forms held at the Chinese
University of Hong Kong, Dec. 1993.
\item B. Conrad, F. Diamond, \& R. Taylor, ``Modularity of certain
potentially Barsotti-Tate Galois representations'', \slnt{J. Amer. Math.
Soc. 12 (1999), no. 2, 521-567}. \ In this article the modular curve
conjecture is proved for any elliptic curve defined over $\Q$ with
conductor not divisible by $27$\aos
\item C. Breuil, B. Conrad, F. Diamond, \& R. Taylor, ``On the modularity
of elliptic curves over $\Q$: wild $3$-adic exercises'', \slnt{J. Amer.
Math. Soc.}, to appear.\footnote{Based on a citation found at
\urlanch{http://www.math.harvard.edu/\tld;rtaylor/} on 21 April 2001.}
\end{Menu}
What follows has the same content as the original write-up except
that the title of the Appendix has been changed from ``Current Status''
to ``Late 1993/Early 1994 Status''.
\section{Introduction.}
The purpose of this expository lecture is to explain the basic ideas
underlying the final resolution of ``Fermat's Last Theorem'' after 356
years as a consequence of the reported establishment by Andrew Wiles
of a sufficient portion of the ``Shimura-Taniyama-Weil'' conjecture.
As these notes are being written, the work of Wiles is not available,
and the sources of information available to the author are (1) reports
by electronic mail, (2) the \slnt{AMS Notices} article
\ct{ribetnotices} of K. Ribet, and (3) a preprint \ct{rubinsilvbg} by
K. Rubin and A. Silverberg based on the June, 1993 lectures of Wiles
at the Newton Institute in Cambridge, England. It should be noted
that the fact that ``Fermat's Last Theorem'' is a consequence of
sufficient knowledge of the theory of ``elliptic curves'' has been
fully documented in the publications (\ct{ribinv}, \ct{ribtoul}) of K.
Ribet.
``Fermat's Last Theorem'' is the statement, having origin with Pierre
de Fermat in 1637, that there are no positive integers $x, y, z$ such
that
\(x^n + y^n = z^n\)
for any integer exponent $n > 2$\@. Obviously, if there are no
positive integer solutions $x, y, z$ for a particular $n$, then there
are certainly none for exponents that are multiples of $n$. Since
every integer $n > 2$ is divisible either by $4$ or by some odd prime
$p$, it follows that ``Fermat's Last Theorem'' is true if there are no
solutions in positive integers of the equation
\(x^n + y^n = z^n\)
when $n = 4$ and when $n = p$ for each prime $p > 2$\@. The cases $n
= 3, \ 4$ are standard fare for textbooks (e.g., see Hardy \& Wright
\ct{hardywright}) in elementary number theory. Therefore, this
discussion will focus on the case $n = p$ where $p > 3$ is prime.
Very briefly, the idea is that we now know enough about the
classification of non-degenerate plane cubic curves
\(F(x, y) = 0\)
in two variables, also known as ``elliptic curves'', with \emph{rational}
coefficients to know how to enumerate them in a logical way
so that we may conclude that if there were positive integers $a, b, c$
with
\(a^p + b^p = c^p\),
\ then the curve
\[ y^2 = x (x - a^p) (x + b^p), \]
which is an elliptic curve known as the ``Frey curve'', would fall
inside of the enumeration. Because the classification is
enumerative, when one is presented with a particular elliptic curve
with rational coefficients, one knows where to look for the curve in
the classification. The curve just written is not to be found within
the classification. As a consequence there cannot be positive
integers $a, b, c$ with $a^p + b^p = c^p$\@.
The enumerative classification of non-degenerate plane cubic curves
defined by polynomials with rational coefficients has been entirely
conjectural (variously known as the ``Taniyama Conjecture'', the
``Weil Conjecture'', the ``Taniyama-Shimura Conjecture'', $\ldots$)
until June, 1993. This conjecture, even as a conjecture, has served
as an important motivating example for the idea of the ``Langlands
Program'', or perhaps of an extension of that program, that certain
kinds of objects in geometry should give rise to certain group
representations.
What seems to be believed today\footnote{As of the time of this
write-up Wiles has stated that a portion of what he announced in June
needs further justification and that he expects to be able to complete
it. See the appendix.} is that the portion of the enumerative
classification pertaining to ``semi-stable'' elliptic curves has been
proved by Andrew Wiles. That the existence of positive integers $a,
b, c$ with
\(a^p + b^p = c^p\)
would violate the enumerative classification of semi-stable elliptic
curves was established by 1987 through the work of G. Frey, J.-P.
Serre, and K. Ribet.
The primary purpose of this lecture is to explain the enumerative
classification of elliptic curves and to give a brief indication of
the mathematics involved in showing that the Frey curve violates that
classification.
\section{Elliptic curves}
A polynomial $f(X,Y)$ of degree $d$ in two variables with coefficients
in a field $k$ gives rise to what is called an \emph{affine plane
curve} of degree $d$: for each field $K$ containing $k$ (more
generally, for each commutative ring that is a $k$-algebra) one has
the set
\[ C_0(K) = \{ (x,y) \in K^2 | f(x,y) = 0 \}, \]
and for each $k$-linear homomorphism
\(K \longrightarrow K'\)
one has the induced map
\(C_0(K) \rightarrow C_0(K')\).
From the polynomial $f$ one obtains a homogeneous polynomial of degree
d in three variables with coefficients in $k$:
\[ F(X,Y,Z) = Z^d f(X/Z, Y/Z), \]
and the \emph{projective plane curve} of degree $d$:
\[ C(K) = \{ ((x,y,z)) \in \proj{2}{K} | F(x,y,z) = 0 \}, \]
where $\proj{N}{K}$ denotes $N$-dimensional projective space, which is
the quotient set of \(K^{N+1} - \{ 0 \}\)
obtained by identifying points lying on the same line through the
origin of $K^{N+1}$\@. Since the projective plane $\proj{2}{K}$ is the
disjoint union of the affine plane
\(\{ ((x,y,1)) | (x,y) \in K^2 \}\)
with the ``\emph{(projective) line at infinity}''
\[ \balbr{((x,y,0)) | ((x,y)) \in \proj{1}{K}}\ ,\]
it follows that $C(K)$ is the disjoint union of $C_0(K)$ with the
finite set of its points lying on the projective line at infinity.
An \emph{elliptic curve defined over} $k$ is the (projective) plane
curve $E$ given by a homogeneous polynomial $F$ of degree $3$ in three
variables with coefficients in $k$ such that (i) $F$ is irreducible
over the algebraic closure $\clos{k}$ of $k$, (ii) the gradient vector
$\nabla F$ is a non-vanishing vector at points of
\(\clos{k}^{3} - \{0\}\)
where $F$ vanishes, and (iii) the set $E(k)$ is non-empty.
% \renewcommand{\thefootnote}{\fnsymbol{footnote}}
If $k$ is any field, then after an isomorphism (see Silverman
\ct{silverman}) one may obtain a given elliptic curve $E$ with an
affine equation of the form
\beq
\label{eq.wnormform}
y^2 + a_1 x y + a_3 y = x^3 + a_2 x^2 + a_4 x + a_6 \ .
\eeq
Then the homogeneous equation for the
intersection of $E(K)$ with the line at infinity is
\beq
\label{eq.wnormforminf}
x^3 = 0 \ .
\eeq
Thus, in this case, $E$ has a unique point on the line at infinity.
If the characteristic of $k$ is different from $2$ and
$3$\footnote{Thus, one sees that the primes $2$ and $3$ play a special
role in the theory of elliptic curves.} then one may obtain an
equation in ``Weierstrass normal form'':
\beq
\label{eq.swnormform}
y^2 = 4 x^3 - g_2 x - g_3 \ ,
\eeq
which is non-singular if and only if the cubic polynomial in the
variable $x$ has distinct roots in $\clos{k}$\@.
% \renewcommand{\thefootnote}{\arabic{footnote}}
Elliptic curves are the ``group objects'' in the category of algebraic
curves that reside in projective space: for each extension field $K$
of $k$ the set $E(K)$ of ``$K$-valued points'' of $E$ is an abelian
group. The group law on $E(K)$ is characterized by two conditions:
\be
\item The origin is a given point of $E(k)$\@.
\item The points obtained by intersecting $E(K)$ with any line in
$\proj{2}{K}$, counted with multiplicities, add up to zero.
\ee
When $E$ is given by an equation in the form (\ref{eq.wnormform}), the
origin is usually taken to be the unique point on the line at
infinity. If two distinct points of $E(K)$ are given, they determine
a line in $\proj{2}{K}$; the intersection of that line with $E(K)$ is
given by a cubic polynomial in a parameter for the line which has two
roots in $K$ corresponding to the two given points; hence, there is a
third root of that cubic polynomial in $K$; this root gives rise to a
point of $E(K)$, which is the negative of the sum of the two given
points. The negative of a given point of $E(K)$ is obtained as the
third point in the intersection with $E(K)$ of the line through the
given point and the origin.
For a given field $k$ the set of homogeneous cubic polynomials in
three variables is a vector space over $k$ having the set of
``monomials'' of degree three in three variables as basis. Thus, the
dimension of the space of homogeneous cubics is 10. The linear group
$\Gl{3}{k}$ acts on the space of cubics, and two cubic curves in
$\Proj{2}$ that are related by this action are isomorphic. Since
$\Gl{3}{k}$ is 9-dimensional, one is led to think of the \emph{family}
of isomorphism classes of elliptic curves as 1-dimensional since
``non-singularity'' is an ``open'' condition.
\section{\label{sec.ellipC}Elliptic curves over $\C$}
When $k$ is the field $\C$ of complex numbers, one knows (see, e.g.,
Ahlfors \ct{ahlfors}) that for each lattice $\Lambda$ in $\C$ the set
of $\Lambda$-periodic meromorphic functions on the complex line $\C$
is the field $\C(\wp, \wp')$, which is a quadratic extension of the
rational function field $\C(\wp)$, where $\wp$ is the $\wp$-function
of Weierstrass. Moreover, $\wp$ satisfies the famous Weierstrass
differential equation
\beq
\label{eq.wdiffeq}
\wp'(z)^2 = 4\wp(z)^3 - g_2(\Lambda)\wp(z) - g_3(\Lambda) \ ;
\eeq
thus, the formula
\(z \mapsto (\wp(z), \wp'(z))\)
defines a holomorphic map from the punctured complex torus
\(\C/\Lambda - \{0\}\)
to the affine cubic curve
\beq
\label{eq.cwnormform}
y^2 = 4x^3 - g_2(\Lambda)x - g_3(\Lambda) \ ;
\eeq
it should hardly be necessary to point out that this map extends to a
holomorphic map from the torus $\C/\Lambda$ to the corresponding
(projective) elliptic curve by sending the origin of the torus to the
unique point of the elliptic curve on the line at infinity. The
classical theory of theta functions (see, e.g., Igusa \ct{igusa} or
Siegel \ct{siegel}) leads to a direct demonstration that this map is a
homomorphism from the group law on the complex torus to the group law
previously described for an elliptic curve. It is not difficult to
see that the analytic manifold given by any elliptic curve defined
over $\C$ arises from some complex torus. Indeed each non-singular
cubic curve $E$ in $\proj{2}{\C}$ determines a compact connected
complex-analytic group. Its universal cover is given by a holomorphic
homomorphism
\(\C \rightarrow E\) which has some lattice as kernel.
Any two lattices in $\C$ are related by a change of real basis for
$\C$, i.e., by a matrix in $\Gl{2}{\R}$. Consequently, there is only
one real-analytic isomorphism class for the complex torus $\C/\Lambda$
as $\Lambda$ varies. The tori corresponding to two lattices are
complex-analytically isomorphic if and only if the corresponding
real-linear isomorphism of $\R^2$ satisfies the Cauchy-Riemann partial
differential equations, i.e., if and only if the $\R$-linear
isomorphism is $\C$-linear.
A lattice $\Lambda$ may be represented concretely by an ordered basis
\(\{\omega_1, \omega_2\}\).
If $\tau = \omega_2/\omega_1$, then $\tau$ is not real, and after
permuting the basis members, if necessary, one may assume that $\tau$
is in the ``upper-half plane''\footnote{The fact that the half-plane
is a model of non-Euclidean geometry led a popular columnist in
November, 1993 to question the validity of the work being discussed
here.} $H$ of $\C$\@. Observing that $\Lambda$ is the image under the
$\C$-linear map
\(z \mapsto \omega_1 z\)
of the lattice with ordered basis
\(\{1, \tau\}\),
one may assume that $\Lambda$ is this latter lattice. Let $E(\tau)$
be the complex torus $\C/\Lambda$\@. Allowing for change of basis
subject to these assumptions on the basis, one sees that there is an
isomorphism of complex-analytic groups
\(E(\tau') \cong E(\tau)\)
if
\beq
\label{eq.linfrac}
\tau' = \frac{a\tau + b}{c\tau + d} ,
\eeq
for some matrix
\[
\gamma = \mxtwo{a}{b}{c}{d}
\in \Sl{2}{\Z} .
\]
Conversely, the monodromy principle may be used to show that every
complex-analytic isomorphism among the complex tori $E(\tau)$ arises
in this way.
The coefficients $g_2$ and $g_3$ in the Weierstrass normal form
(\ref{eq.cwnormform}) have very explicit constructions as infinite
series (see, e.g., Ahlfors \ct{ahlfors} or Serre \ct{serrecourse})
determined by the given lattice; from this it is straightforward to
see that $g_w$ is a \emph{modular form of weight} $2w$: if $\tau$ and
$\tau'$ are related by (\ref{eq.linfrac}), then
\[ g_w(\tau') = (\lambda)^{2w}g_w(\tau),\ \ \lambda = c \tau + d \ . \]
Consequently, the map
\[ (x,y) \mapsto (\lambda^2 x, \lambda^3 y) \]
carries the curve given by (\ref{eq.cwnormform}) for $\tau$
isomorphically to the curve given by (\ref{eq.cwnormform}) for
$\tau'$\@. The discriminant of the cubic polynomial in the
Weierstrass normal form (\ref{eq.cwnormform}) is a modular form of
weight $12$, which up to a multiplicative constant, is:
\[ \Delta(\tau) = g_2^3 - 27 g_3^2 \ . \]
$\Delta$ is a non-vanishing holomorphic function in $H$\@. The
\emph{modular invariant} $\jf$ (\ct{serrecourse},\ct{shimiaf}) is defined by:
\[ \jf{\tau} = \frac{(12 g_2)^3}{\Delta} \ ; \]
it is a holomorphic function in the upper-half plane $H$ with the
property that
\[ \jf{\tau} = \jf{\tau'} \]
if and only if $\tau$ and $\tau'$ are related by (\ref{eq.linfrac}).
Furthermore, $\J$ assumes every value in $\C$ at some point of $H$\@.
Consequently, the complex-analytic isomorphism classes of complex tori
or, equivalently, the isomorphism classes of elliptic curves defined
over $\C$, are parameterized via $\J$ in a one-to-one manner by the
complex numbers.
Since this is an expository discourse, it is hoped that the reader
will not feel patronized by having noted the fact that the
coincidence of (1) the category of elliptic curves over $\C$ and
(2) the category of complex tori is the ``genus one'' case of the
coincidence (see Weyl \ct{weyl}) of (i) the category of ``complete''
non-singular algebraic curves over $\C$ and (ii) the category of
compact Riemann surfaces (one-dimensional connected complex-analytic
manifolds).
Although the classification of elliptic curves over $\C$ via the
$\J$-function is a result that is both beautiful and useful, and
although two elliptic curves defined over $\Q$ that are isomorphic as
curves defined over $\Q$ give rise to elliptic curves defined over
$\C$ that have the same $\J$-invariant, it is \emph{not} true that any
two elliptic curves defined over $\Q$ having the same $\J$-invariant
are isomorphic over $\Q$\@. Thus, the classification of elliptic
curves over $\C$ does not lead directly to the desired enumerative
classification of elliptic curves defined over $\Q$\@, but it does
bring to the fore the notion of \emph{modular form}, which is central
in the study of elliptic curves defined over $\Q$\@. What can be said
easily is that, according to the Shimura-Taniyama-Weil conjecture, the
\emph{isogeny} classes of elliptic curves defined over $\Q$ are
parameterized by certain modular forms.
\section{Modular forms}
The group $\Sl{2}{\Z}$ is an infinite group that is generated by the
two elements
\[ \mxtwo{0}{1}{-1}{0} \ , \ \ \mxtwo{0}{-1}{1}{1} \ , \]
which have orders 4 and 6 respectively. The action of $\Sl{2}{\Z}$ on
the upper-half plane $H$ by linear fractional transformations has kernel
\[ \balbr{ \pm \mxtwo{1}{0}{0}{1} } \ , \]
and the quotient of $\Sl{2}{\Z}$ by this kernel is the group
$\PSl{2}{\Z}$\@. It is not difficult to see that the set
\[\{ \tau \in H \ | \ -1/2 \leq \func{Re}(\tau) \leq 1/2, \ \ \abs{\tau} \geq 1 \}\]
is a ``fundamental domain'' for the action of $\PSl{2}{\Z}$ on $H$\@.
More precisely, this set meets each orbit, and the only redundancies
are the boundary identifications arising from the maps
\(\tau \mapsto \tau + 1\) and \(\tau \mapsto -1/\tau\)\@.
The quotient $H/\PSl{2}{\Z}$ is not compact since the fundamental
domain is ``open at the top''. Beyond that the modular invariant
$j$ induces a bicontinuous biholomorphic isomorphism of the quotient
$H/\PSl{2}{\Z}$ with the affine line over $\C$\@. Since
\( \jf{\tau + 1} = \jf{\tau}\), \ and since for
\( q = e^{2 \pi i \tau} \) one has $\abs{q} < 1$ for $\tau \in H$,
there is a holomorphic function $\Jtil$ in the punctured unit
disk such that $\jftil{q} = \jf{\tau}$\@. Likewise $\Delta$ may
be regarded as function of $q$, and one may use the calculus of residues
to show that $\Delta$ has a simple zero at $q = 0$; hence,
$\Jtil$ has a simple pole at $q = 0$, or, equivalently, $\J$
has a simple pole at $\infty$ (the ``missing top'' of the fundamental
domain). Thus, $\J$ gives rise to a bicontinuous biholomorphic
isomorphism
\[ H/\PSl{2}{\Z} \cup \{\infty\} \longrightarrow \proj{1}{\C} \ . \]
% \renewcommand{\thefootnote}{\fnsymbol{footnote}}
A non-trivial element of $\PSl{2}{\Z}$ has a fixed point in $H$ if and
only if it has finite order, and one's explicit knowledge of the
fundamental domain makes it possible to see that the only elements of
finite order are of order $2$ or $3$\footnote{Thus, one sees that the
primes $2$ and $3$ play a special role in the study of the group
$\Sl{2}{\Z}$.}\@.
% \renewcommand{\thefootnote}{\arabic{footnote}}
A \emph{congruence subgroup} of $\Sl{2}{\Z}$ is a subgroup $\Gamma$
that contains one of the principal congruence subgroups; the
\emph{principal congruence subgroup} $\Gamma(N)$ of \emph{level} $N$ is
the set of all elements $\gamma$ of $\Sl{2}{\Z}$ that are congruent
$\pmod{N}$ to the identity matrix. The group $\Gamma_0(N)$
is the congruence subgroup of $\Sl{2}{\Z}$ consisting of all elements
\[ \mxtwo{a}{b}{c}{d} \]
for which
\(c \equiv 0 \pmod{N}\).
It is obvious that each congruence group $\Gamma$ has finite index in
$\Sl{2}{\Z}$, and, consequently the quotient $H/\Gamma$ is a non-compact
Riemann surface.
Observe that for each level $N$ the
group $\Gamma_0(N)$ contains the parabolic element
\[ T = \mxtwo{1}{1}{0}{1} \ , \]
which gives rise to the holomorphic map
\( \tau \mapsto \tau + 1 \)
that fixes the point $\infty$\@.
A modular form\footnote{Details concerning the discussion
in this section may be found in Shimura's book \ct{shimiaf}.}
of \emph{weight} $w$ for $\Gamma$ is a holomorphic
function $f$ in $H$ that satisfies the functional equation
\beq
\label{eq.modform}
f(\gamma \cdot \tau) = (c \tau + d)^{w} f(\tau), \ \gamma \in \Gamma
\eeq
and that is holomorphic at each \emph{cusp} of $\Gamma$\@. The role
of \emph{cusps} for $\Gamma$ is to provide a slightly larger set $H^*$
than $H$,
\[ H^* = H \cup \{\text{cusps}\} \ , \]
where $\Gamma$ acts such that $H^*/\Gamma$ is a compact Riemann
surface containing $H/\Gamma$ as the open complement of a finite set
of points arising from cusps. The cusps of $\Gamma$ are the points of
the closure of the boundary of $H$ in $\proj{1}{\C}$ =
\(\C \cup \{\infty\}\) that are fixed by some non-trivial
parabolic element of $\Gamma$\@. When $\Gamma$ = $\Sl{2}{\Z}$, the
set of cusps is
\(\Q \cup \{\infty\}\).
In view of (\ref{eq.modform}) applied to the case
\( \gamma = T \) one sees that a modular form $f$ of any weight
for the group $\Gamma_0(N)$ satisfies
\beq
\label{eq.period}
f(\tau + 1) = f(\tau) \ ,
\eeq
and, therefore, $f$ defines a holomorphic function in the variable $q
= e^{2 \pi i \tau}$ for $0 < q < 1$\@. The condition in the
definition of \emph{modular form} that $f$ should be \emph{holomorphic
at} $\infty$ means that $f$ as a function of $q$ is holomorphic at $q
= 0$\@. Consequently, $f$ admits an absolutely convergent Fourier
expansion
\beq
\label{eq.fourier}
f(\tau) = \sum_{m = 0}^{\infty} {c_m e^{2 \pi i m \tau}}\sum: \ ,
\eeq
which is a Taylor series in $q$\@.
For any cusp $\rho$ of a congruence group $\Gamma$ one may define the
notion \emph{holomorphic} at $\rho$ for a modular form $f$ by an
analogous procedure using an arbitrary parabolic element of $\Gamma$
that fixes $\rho$ instead of $T$\@. For a given
congruence group $\Gamma$ two cusps $\rho$ and $\sigma$ are
\emph{equivalent} if there is some element $\gamma$ in $\Gamma$ such that
\(\sigma = \gamma \cdot \rho\). A modular form $f$ is holomorphic
at any cusp that is equivalent to another where it is holomorphic.
The modular form $f$ is a \emph{cuspform} if, in addition to being
holomorphic at each cusp, $f$ vanishes at each cusp.
For a given congruence group $\Gamma$ a modular form vanishes at
any cusp that is equivalent to another where it vanishes.
The set of modular forms of given weight $w$ forms a
finite-dimensional vector space over $\C$ in which the set of
cuspforms is a linear subspace of codimension bounded by the
number of equivalence classes of cusps. In fact, using
``Eisenstein series'' one may show that the codimension of the
space of cuspforms in the space of modular forms is often equal to
the number of equivalence classes of cusps. For example, with the group
\(\Gamma(1) = \Sl{2}{\Z}\) there are no modular forms of odd
weight, there is an Eisenstein series of every even weight greater
than $2$ that is not a cuspform, and every cusp is equivalent to
$\infty$\@. Furthermore, since $\infty$ is the only zero of the cusp
form $\Delta$ (of the preceding section) in the quotient
\(H^{*}/\Gamma(1)\)
and since $\infty$ is a simple zero of $\Delta$\@, every cuspform for
$\Gamma(1)$ is divisible by $\Delta$\@. Thus, in this case, there are
no cuspforms of weight less than $12$\@.
It is not difficult to see that the cuspforms of weight $2$ for a
congruence group $\Gamma$ correspond to holomorphic differential
$1$-forms (differentials of the first kind) on the compact Riemann
surface
\( X = H^{*}/\Gamma\).
Thus, the dimension of the space of cuspforms of weight $2$ is the
\emph{genus} of $X$\@. The fact that there are no cuspforms of weight
$2$ for the group $\Gamma(1)$ matches the previously mentioned fact
that $X$ is $\Proj{1}$\@. It is certain of the cuspforms of weight
two for the groups $\Gamma_0(N)$ that, according to the
Shimura-Taniyama-Weil conjecture, parameterize the isogeny classes of
elliptic curves defined over $\Q$\@.
\section{Euler products}
It will be recalled that the infinite series
\[ \sum_{n = 1}^{\infty} {\frac{1}{n^s}} \sum:\]
converges for $\func{Re}(s) > 1$ and gives rise by analytic continuation
to a meromorphic function $\zeta(s)$ in $\C$\@. For $\func{Re}(s) > 1$
$\zeta(s)$ admits the absolutely convergent infinite product
expansion
\[ \prod_p {\frac{1}{1 - p^{-s}}}\prod:\ , \]
taken over the set of primes. This ``Euler product'' may be regarded
as an analtyic formulation of the principle of unique factorization
in the ring $\Z$ of integers. It is, as well, the product taken
over all the non-archimedean completions of the rational field $\Q$
(which completions $\Q_p$ are indexed by the set of primes) of the
``Mellin transform''\footnote{The Mellin transform is, more or less,
Fourier transform on the multiplicative group.
Classically, the Mellin transform $\varphi$ of $f$ is given formally by
\[ \varphi(s) = \int_0^{\infty} {f(x) x^s (dx/x)}\int: \ \eos \]}
in $\Q_p$
\[ \xi_p(s) = \frac{1}{1 - p^{-s}} \ , \]
of the canonical ``Gaussian
density''
\[
\Phi_p(x) = \lbalbr{
\begin{array}{ll}
1 & \text{if\ } x \in \text{ closure of}\ \Z \ \text{in} \ \Q_p \,\eos \\
0 & \text{otherwise\eos}
\end{array}
} \ ,
\]
which Gaussian density is equal to its own Fourier transform.
For the archimedean completion
\( \Q_{\infty} = \R \)
of the rational field $\Q$ one forms the classical Mellin transform
\[ \xi_{\infty}(s) =\pi^{-(s/2)} \Gamma(s/2) \]
of the classical Gaussian density
\[ \Phi_{\infty}(x) = e^{- \pi x^2} \ , \]
(which also is equal to its own Fourier transform).
Then the function
\[ \xi(s) = \xi_{\infty}(s) \zeta(s) =
\prod_{p \leq \infty} {\xi_p(s)} \prod: \]
is meromorphic in $\C$, and satisfies the functional equation
\beq
\label{eq.zetafnl}
\xi(1-s) = \xi(s) \ .
\eeq
The connection of Riemann's $\zeta$-function with the subject of
modular forms begins with the observation that $\zeta(2s)$ is essentially
the Mellin transform of $\theta_{I}(x) = \theta(ix) - 1$\@, where $\theta$,
which is a modular form of weight $1/2$ and level $8$, is defined in the
upper-half plane $H$ by the formula
\[ \theta(\tau) = \sum_{m \in \Z} {\func{exp}(\pi i \tau m^2)} \sum: \ . \]
In fact, one of the classical proofs of the functional equation
(\ref{eq.zetafnl}) is given by applying the Poisson summation
formula\footnote{On the other hand, (\ref{eq.zetafnl}) may be regarded
directly as a \emph{divergent} model of the Poisson summation formula.}
to the function
\( x \mapsto \func{exp}(\pi i \tau x^2) \),
while observing that the substitution
\( s \mapsto (1/2) -s \)
for $\zeta(2s)$ corresponds in the upper-half plane to
the substitution
\( \tau \mapsto -1/\tau \)
for the theta series.
If $f$ is a cuspform for a congruence group $\Gamma$ containing
\[ T = \mxtwo{1}{1}{0}{1} \ ,\]
and so, consequently,
\(f(\tau + 1) = f(\tau)\), then, as previously explained, one has
the Fourier expansion (\ref{eq.fourier})
\[ f(\tau) = \sum_{m = 1}^{\infty} {c_m e^{2 \pi i m \tau}}\sum: \ .\]
The Mellin transform $\varphi(s)$ of $f_I$ leads to the Dirichlet series
\beq
\label{eq.dirseries}
\varphi(s) = \sum_{m = 1}^{\infty} {c_m m^{-s}}\sum:\ ,
\eeq
which may be seen to have a positive abscissa of convergence.
One is led to the questions:
\be
\item For which cuspforms $f$ does the associated Dirichlet series
$\varphi(s)$ admit an analytic continuation with functional equation?
\item For which cuspforms $f$ does the associated Dirichlet series
$\varphi(s)$ have an Euler product expansion?
\ee
For the ``modular group'' $\Gamma(1)$ the Dirichlet series associated
to every cuspform of weight $w$ admits an analtyic continution with
functional equation under the substitution
\( s \mapsto w-s\).
Since $\Gamma(1)$ is generated by the two matrices $T$ and
\[ W = \mxtwo{0}{1}{-1}{0} \]
and since the functional equation of a modular form $f$ relative to
$T$ is reflected in the formation of the Fourier series
(\ref{eq.fourier}), the condition that an absolutely convergent series
(\ref{eq.fourier}) is a modular form for $\Gamma(1)$ is the functional
equation for a modular form relative solely to $W$\@. This is
equivalent to the (properly formulated) functional equation for the
associated Dirichlet series $\varphi$ together with a ``growth
condition''. For the group $\Gamma_0(N)$, with $N > 1$, the question
of a functional equation is more complicated since, although $T$ is
available, there is no reason for a cuspform to satisfy a law of
transformation relative to $W$\@. But note that for any $\Gamma$ the
set of cuspforms of given weight for which the associated Dirichlet
series have analytic continuations satisfying a given finite set of
functional equations is a vector space. On the other hand, there is
no reason to believe, even for level $1$, that the cuspforms admitting
an Euler product expansion form a vector space.
In a nutshell the cuspforms admitting Euler products are those which
arise as eigenforms for an arithmetically defined commutative algebra
of semi-simple operators on the space of cuspforms of a given weight
introduced by E. Hecke. The theory of Hecke operators is reasonably
simple for level $1$ but somewhat more complicated in general (see, e.g.,
Shimura's book \ct{shimiaf}).
Observing that the formula
\[ds^2 = \frac{dx^2 + dy^2}{y^2}\ ,\ \text{for}\ \tau=x+iy \in H\ ,\]
gives a (the hyperbolic) $\Sl{2}{\R}$-invariant metric in $H$ with
associated invariant measure
\[d\mu = \frac{dx dy}{y^2}\ ,\]
one introduces the Petersson (Hermitian) inner product in the space of
cuspforms of weight $w$ for $\Gamma$ with the definition:
\beq
\label{eq.innerprod}
\langle f,g \rangle = \int_{H/\Gamma}\
{f(\tau)\, \ovbar{g}(\tau)\, {\func{Im}(\tau)}^w\, d\mu(\tau)}\int: \ .
\eeq
(Integration over the quotient $H/\Gamma$ makes sense since the
integrand
\[ f(\tau)\, \ovbar{g}(\tau)\, y^w \]
is $\Gamma$-invariant.)
For the modular group $\Gamma(1)$ the $n^{th}$ Hecke operator
\(T(n) = T_w(n)\) is the linear endomorphism of the space
of cuspforms of weight $w$ arising from the following considerations.
Let $S_n$ be the set of $2 \times 2$ matrices in $\Z$ with determinant
$n$. For
\[ M = \mxtwo{a}{b}{c}{d} \in S_n \]
and for a function $f$ in $H$ one defines
\beq
\label{eq.actonfns}
(M \cdot_w f)(\tau) \, = \, \det(M)^{w-1} (c \tau + d)^{-w} f(\tau) \ ,
\eeq
and then, observing that $\Gamma(1)$ under $\cdot_w$ acts trivially on the
modular forms of weight $w$,
one may define the Hecke operator $T_w(n)$ by
\beq
\label{eq.heckeop}
{T_w(n)}(f) = \sum_{M \in S_n/\Gamma(1)} {(M \cdot_w f)(\tau)} \sum: \ ,
\eeq
where the quotient $S_n/\Gamma(1)$ refers to the action of $\Gamma(1)$
by left multiplication on the set $S_n$\@.
One finds for $m$, $n$ coprime that
\[ T(m n) = T(m) T(n) \ ,\]
and furthermore one has
\[ T(p^{e+1}) = T(p^e)T(p) - p^{w-1}T(p^{e-1}) \ . \]
Consequently, the operators $T(n)$ commute with each other, and, therefore,
generate a commutative algebra of endomorphisms of the space of cusp
forms of weight $w$ for $\Gamma(1)$\@. It is not difficult to see that
the Hecke operators are self-adjoint for the Petersson inner product
on the space of cuspforms. Consequently, the space of cuspforms of
weight $w$ admits a basis of simultaneous eigenforms for the Hecke
algebra. A ``Hecke eigencuspform'' is said to be
\emph{normalized} if its Fourier coefficient $c_1 = 1$\@. If $f$ is
a normalized Hecke eigencuspform, then
\bi
\item The Fourier coefficient $c_m$ of $f$ is the eigenvalue of $f$
for $T(m)$\@.
\item The Fourier coefficients $c(m) = c_m$ of $f$ satisfy
\begin{menu}
\item $c(m n) = c(m) c(n)$\ \ for $m, n$ coprime, and
\item $c(p^{e+1}) = c(p^e)c(p)-p^{w-1}c(p^{e-1})$\ \ for $p$ prime.
\end{menu}
\ei
Consequently, the Dirichlet series associated with a simultaneous
Hecke eigencuspform of level $1$ and weight $w$ admits an Euler product
\beq
\label{eq.modeulerp}
\varphi(s) = \prod_p {\frac{1}{1 - c_p p^{-s} + p^{w-1-2s}}} \prod: \ .
\eeq
For example, when $f$ is the unique normalized cuspform $\Delta$ of
level $1$ and weight $12$\@, one has
\[ \varphi(s) = \prod_p {\frac{1}{1 - \tau(p) p^{-s} + p^{11-2s}}}\prod: \ , \]
where $c_p = \tau(p)$ is the function $\tau$ of Ramanujan.
For the congruence group $\Gamma_0(N)$ a Hecke eigencuspform
of weight $w$ gives rise to a Dirichlet series $\varphi(s)$ that
admits an Euler product expansion whose factors at primes $p$ coprime
to $N$ resemble those given by (\ref{eq.modeulerp}). In order for
$\varphi(s)$ to satisfy a functional equation under the substitution
\( s \mapsto w -s \),\ one needs to require that the eigencuspform
$f$ admits a functional equation not only with respect to each element
of the group $\Gamma_0(N)$ but also with respect to the substitution
in the upper-half plane $H$ given by the matrix
\[ W_N = \mxtwo{0}{-1}{N}{0} \ . \]
A. Weil (\ct{weilmathann}) showed that the cuspforms of weight $2$
for the group $\Gamma_0(N)$ satisfying the appropriate functional
equation under the mapping of $H$ given by $W_N$ correspond precisely
to Dirichlet series with certain growth conditions that admit analytic
continuations as meromorphic functions in $\C$ satisfying a finite
number of ``twisted'' functional equations.
The reader will have noticed that it is not extremely easy to
characterize the cuspforms of weight $2$ that conjecturally
(Shimura-Taniyama-Weil) parameterize the isogeny classes of elliptic
curves defined over the rational field $\Q$\@. The Euler product is
an extremely important part of the characterization since the
Dirichlet series given by such an elliptic curve, as will be made
explicit in the next section, is, by its very nature, an Euler
product. Weil conjectures explicitly that the Dirichlet series with
Euler product given by each elliptic curve defined over $\Q$ satisfies
these conditions, i.e., is the Dirichlet series associated to some
$W_N$-compatible Hecke eigencuspform for the group $\Gamma_0(N)$,
where $N$ is the ``conductor'' of $E$\@. This has led to efforts,
related to the ``Langlands program'' to understand the
$W_N$-compatible Hecke eigencuspforms in a more intrinsic way as
objects of representation theory over $\Q$ (see, e.g., the survey of
Gelbart \ct{gelbartadv}).
\section{Elliptic curves over the rational field $\Q$}
Let $E$ be an elliptic curve defined over $\Q$\@. One may clear
denominators from its cubic equation, if necessary, in order to arrive
at an equation with integer coefficients having no common factor.
While the Weierstrass normal form (\ref{eq.swnormform}) is available to
represent the isomorphism class of any elliptic curve over a field of
characteristic different from $2$ and $3$, one needs the generalized
Weierstrass form
\beq
\label{eq.gwnormform}
y^2 + a_1 x y + a_3 y = x^3 + a_2 x^2 + a_4 x + a_6
\eeq
over an arbitrary field, and, moreover, for each elliptic curve $E$
defined over $\Q$ there is a ``best possible'' equation (e.g., see
Silverman \ct{silverman}) of the form (\ref{eq.gwnormform}) with
integer coefficients called the \emph{Neron model} of $E$\@. With an
abuse of notation $E$ will denote the Neron model, which may be regarded
as a ``curve over $\Z$''. (One would want to call it an ``elliptic curve
over $\Z$'' if it were ``smooth over $\Z$'', i.e., if it had good
reduction at each prime $p$; the fact that every Neron model has bad
reduction at least once corresponds under the ``dictionary'' to the
fact that there are no cuspforms of weight two and level 1.)
It then may be observed that for each prime $p$ the Neron model gives
rise to a cubic equation over the finite field $\ff{p}$\@. For all
but a finite number of $p$ the equation over $\ff{p}$ is non-singular
over $\ffbar{p}$, i.e., determines an elliptic curve $E_p$ defined
over $\ff{p}$\@. One says in this case that $E$ has ``good
reduction'' mod $p$\@. Following Tate (\ct{tatesurvey}) one
introduces
\[ b_2 = a_1^2 + 4 a_2 \ , \]
\[ b_4 = a_1 a_3 + 2 a_4 \ , \]
\[ b_6 = a_3^2 + 4 a_6 \ \text{, and} \]
\[ b_8 = b_2 a_6 - a_1 a_3 a_4 + a_2 a_3^2 - a_4^2 \ . \]
Then one has
\[ \Delta = - b_2^2 b_8 - 8 b_4^3 - 27 b_6^2 + 9 b_2 b_4 b_6 \ . \]
The non-vanishing of $\Delta$ mod $p$ is necessary and sufficient for
$E$ to have good reduction mod $p$\@. It follows that a prime $p$
divides $\Delta$ if and only if $E$ does not have good reduction mod
$p$\@. If $p$ is a prime for which $E$ has ``bad reduction'', then
there is a single singular point of the reduced curve $E_p$\@, and
either (a) $E_p$ has distinct tangent lines at the singular point
(\emph{semi-stable} reduction) or (b) $E_p$ has a single tangent line
occurring with multiplicity $2$\@. $E$ is called \emph{semi-stable} if
it has either good or semi-stable reduction at each prime. The
\emph{conductor} of $E$ is the integer $N$ defined by
\[ N = \prod_p {p^{\nu_p}}\prod: \ , \]
where
\[
\nu_p = \lbalbr{
\begin{array}{ll}
0 & \text{if\ }E\text{\ has good reduction at\ } p \text{\,.} \\
1 & \text{if\ }E\text{\ has semi-stable reduction at\ }p\text{\,.} \\
2 + \lambda_p \geq 2 & \text{otherwise.}
\end{array}
}
\]
The non-negative integer $\lambda_p$ cannot be positive unless $p$ is
$2$ or $3$\@. Tautologically, $E$ is semi-stable if and only if its
conductor $N$ is square-free.
One defines the ``L-series'' of $E$ by
\beq
\label{Lseries}
L(E,s) \ = \ \prod_{p | N} {\frac{1}{1 - c_p p^{-s}}}\prod:
\ \prod_{p \not\, | N} {\frac{1}{1 - c_p p^{-s} + p^{1 - 2s}}}\prod: \ ,
\eeq
where $c_p$ is defined when $E$ has good reduction mod $p$ by the
formula
\[ c_p = p + 1 - \abs{E(\ff{p})} \ , \]
and $c_p$ is defined when $E$ has bad reduction mod $p$ by
\[
c_p = \lbalbr{
\begin{array}{ll}
1 & \text{if\ }\nu_p = 1\text{\ and the tangents are defined over\ }
\ff{p} \,\eos \\
-1 & \text{if\ } \nu_p = 1 \text{\ with ``irrational'' tangents\eos} \\
0 & \text{if\ } \nu_p > 1 \,\eos
\end{array}
}
\]
One observes readily that the L-function of $E$ codifies information
about the number of points of $E$ in the finite field $\ff{p}$. Quite
generally for an algebraic variety defined over $\Q$ the analogous
codification of information obtained by counting points in the various
reductions mod $p$ of the variety yields the ``Hasse-Weil zeta
function'', which reflects ``cohomological'' information about $E$.
The L-series of $E$ is the essential part, corresponding to cohomology
in dimension $1$, of the Hasse-Weil zeta function of $E$\@. The
Hasse-Weil zeta function is a special case of the general notion
(Serre \ct{serrepurdue}) of ``zeta function'' for a \emph{scheme of
finite type} over $\Z$\@.
One observes that $L(E,s)$ resembles, at least insofar as one
considers its Euler factors for primes $p$ corresponding to good
reductions of $E$, the Dirichlet series associated to a cuspform of
weight $2$ that admits an Euler product expansion. The observation of
this resemblance is the beginning of an appreciation of the
Shimura-Taniyama-Weil conjecture. One is led to ask to what extent
the two classes of Dirichlet series with Euler products coincide. The
conjecture states that the L-function of an elliptic curve defined
over $\Q$ with conductor $N$ arises from a cuspform for the group
$\Gamma_0(N)$ that is compatible with the substitution in the
upper-half plane $H$ given by $W_N$\@. Isogenous elliptic curves have
the same $L$-function, and, conversely (cf. Tate \ct{tateendom} and
Faltings \ct{storrsconf}) two elliptic curves with the same
$L$-function must be isogenous. Thus, the idea of the conjecture is
that the isogeny classes of elliptic curves defined over $\Q$ with
conductor $N$ are in bijective correspondence with the set of Hecke
eigencuspforms for the group $\Gamma_0(N)$ of level $N$\@, compatible
with the extension of that group by the substitution arising from
$W_N$\@, having rational Fourier coefficients and not arising from
levels dividing $N$\@.
\section{The Shimura map}
Shimura (\ct{shimnagoya}, \ct{shimiaf}, \ct{shimjmsj}) showed
for a given $W_N$-compatible Hecke eigencuspform $f$ of weight $2$ for
the group $\Gamma_0(N)$ with rational Fourier coefficients how to
construct how to construct an elliptic curve $E_f$ defined over $\Q$
such that the Dirichlet series $\varphi(s)$ associated with $f$ is the
same as the L-function $L(E_f,s)$\@. Thus, the Shimura-Taniyama-Weil
conjecture becomes the statement that Shimura's map from the set of
such cuspforms to the set of elliptic curves defined over $\Q$ is
surjective up to isogeny. A rough description of the Shimura map
follows.
Let $\Gamma$ be a congruence subgroup of $\Sl{2}{\Z}$\@, and let
$X(\Gamma)$ denote the compact Riemann surface $H^{*}/\Gamma$. The
inclusion of $\Gamma$ in $\Gamma(1)$ induces a ``branched covering''
\[ X(\Gamma) \longrightarrow X(1) \cong \Proj{1} \ . \]
One may use the elementary Riemann-Hurwitz formula from combinatorial
topology to determine the Euler number, and consequently the genus,
of $X(\Gamma)$\@. The genus is the dimension of the space of cuspforms
of weight $2$. Even when the genus is zero one obtains embeddings
of $X(\Gamma)$ in projective spaces $\Proj{r}$ through holomorphic maps
\[ \tau \longmapsto (f_0(\tau), f_1(\tau), \ldots, f_r(\tau)) \ , \]
where
\(f_0, f_1, \ldots, f_r\)
is a basis of the space of modular forms of weight $w$ with $w$
sufficiently large. For example, any $w \geq 12$ will suffice for
$\Gamma(1)$\@. For $\Gamma_0(N)$ (but not for arbitrary $\Gamma$) one
may find a basis of the space of modular forms of weight $w$ having
rational Fourier coefficients. Using the corresponding projective
embedding one finds a \emph{model} for \(X_0(N)=X(\Gamma_0(N))\)
over $\Q$\@, i.e., an algebraic curve defined over $\Q$ in projective
space that is isomorphic as a compact Riemann surface to $X_0(N)$\@.
Associated with any ``complete non-singular'' algebraic curve (i.e.,
after Weyl \ct{weyl}, any compact Riemann surface) $X$ of genus $g$
is a complex torus, the ``Jacobian'' $J(X)$ of $X$, that is the
quotient of $g$-dimensional complex vector space $\C^g$ by the lattice
$\Omega$ generated by the ``period matrix'', which is the
$g \times 2g$ matrix in $\C$ obtained by integrating each of the
$g$ members $\omega_i$ of a basis of the space of holomorphic
differentials over each of the $2g$ loops in $X$ representing the
members of a homology basis in dimension $1$\@. Furthermore, if one
picks a base point $z_0$ in $X$, then for any $z$ in $X$, the path
integral from $z_0$ to $z$ of each of the $g$ holomorphic
differentials is well-defined modulo the periods of the differential.
One obtains a holomorphic map
\(X \rightarrow J(X)\) from the formula
\[ z \longmapsto (\int_{z_0}^{z} {\omega_1}\int:, \ldots,
\int_{z_0}^{z} {\omega_g}\int:) \ \bmod \Omega \ . \]
This map is, in fact, universal for pointed holomorphic maps from $X$
to complex tori. Furthermore, the Jacobian $J(X)$ is an algebraic
variety that admits definition over any field of definition for $X$
and $z_0$, and the universal map also admits definition over any such
field. The complex tori that admit embeddings in projective space are
the abelian group objects in the category of projective varieties.
They are called \emph{abelian varieties}. Every abelian variety is
isogenous to the product of ``simple'' abelian varieties: abelian
varieties having no abelian subvarieties. Shimura showed that one of
the simple isogeny factors of $J(X_0(N))$ is an elliptic curve $E_f$
defined over $\Q$ characterized by the fact that its
one-dimensional space of holomorphic differentials induces on $X_0(N)$\aoc
via the composition of the universal map with projection on $E_f$\aoc
the one-dimensional
space of differentials on $X_0(N)$ determined by the cuspform $f$\@.
He showed further that $L(E_f, s)$ is the Dirichlet series
$\varphi(s)$ with Euler product given by $f$\@. An elliptic curve $E$
defined over $\Q$ is said to be \emph{modular} if it is isogenous to
$E_f$ for some $W_N$-compatible Hecke eigencuspform of weight $2$ for
$\Gamma_0(N)$\@. Equivalently $E$ is modular if and only if $L(E,s)$
is the Dirichlet series given by such a cuspform. The
Shimura-Taniyama-Weil Conjecture states that every elliptic curve
defined over $\Q$ is modular. Shimura \ct{shimnagoya} showed that
this conjecture is true in the special case where the $\Z$-module rank
of the ring of endomorphisms of $E$ is greater than one. In this case
the point $\tau$ (notation of section \ref{sec.ellipC}) of the
upper-half plane corresponding to $E(\C)$ is a quadratic imaginary
number, and $L(E, s)$ is a number-theoretic $L$-function associated
with the corresponding imaginary quadratic number field.
\section{The hypothetical Frey curve}
Let $p \geq 5$ be a prime. Based on the assumption, which presumably
is false, that there are non-zero integers $a, b, c$ such that
\( a^p + b^p + c^p = 0 \)\@,
G. Frey observed that the elliptic curve given by the equation
\beq
\label{eq.frey}
y^2 = x(x - a^p)(x + b^p) \ ,
\eeq
which is certainly defined over $\Q$, would not be likely to be
modular. Thus, if the Shimura-Taniyama-Weil Conjecture were true,
then ``Fermat's Last Theorem'' would also be true. By 1987 it had
been shown through the efforts of Frey, Ribet and Serre that the Frey
curve (\ref{eq.frey}) is not modular. The proof involves the
systematic study of what is known as the ``$\ell$-adic representation''
of an elliptic curve defined over $\Q$\@, which is described in the
next section. This same technique is what has been reported to be the
basis of the proof of Wiles that every semi-stable elliptic curve
defined over $\Q$ is modular. The Frey curve (\ref{eq.frey}) has
discriminant $\Delta = (abc)^p$\@. It is only slightly difficult to
see that it is semi-stable, and, therefore, that its conductor $N$ is
the square-free integer $abc$\@. If the Frey curve is modular, one
is led to a cuspform of weight $2$ for $\Gamma_0(abc)$\@. The theory
of $\ell$-adic representations leads one along a path of reductions
of the level $N$ from the initial level $abc$
that enables one to conclude that there is a cuspform
of weight $2$ for $\Gamma_0(2)$\@; but the genus of $X_0(2)$ is $0$,
and, consequently, there is no such cuspform.
\section{$\ell$-adic representations of $\GalQ$}
Let $E$ be an elliptic curve defined over $\Q$\@. Inasmuch as the group
law
\( E \times E \rightarrow E \)
is defined over $\Q$ it follows that for each integer $m$ the group
(scheme) $E[m]$ of $m$-torsion points, i.e., for any field $K$
containing $\Q$ the group $E[m](K)$ consisting of all $x$ in $E(K)$
such that $mx = 0$, is defined by equations with rational
coefficients. Consequently, any automorphism of $K$ must carry the
group $E[m](K)$ into itself. Since $E(\C)$ is the quotient of $\C$ by
a lattice, it is clear that $E[m](\C)$ is isomorphic to
\( \Z/m\Z \times \Z/m\Z \); in fact, this latter group is
isomorphic to $E[m](K)$ for each algebraically closed field of
characteristic $0$\@. There is a unique ring homomorphism
\( \Z/mn\Z \rightarrow \Z/m\Z \) for each integer $n \geq 1$,
and the family of these ring homomorphisms gives rise to an inverse
system in the category of commutative rings. If one specializes to
the case $m = \ell^r$\@, where $\ell$ is prime, the projective limit
is the ring $\Zl$ of \ladic integers. The groups $E[m]$
form a direct system with respect to the inclusions
\( E[m] \subseteq E[mn] \), but, corresponding to the inverse
system of the groups $\Z/m\Z$\@, form an inverse system (the Tate
system) with respect to the family of homorphisms
\( E[mn] \rightarrow E[m] \)
defined by
\( x \mapsto nx \). If one specializes to the case
$m = \ell^r$\@, where $\ell$ is prime, one obtains the projective limit
\beq
\label{eq.tategp}
\Tate(E) = \projlim_{r \rightarrow \infty} E[\ell^r](\Qbar) \cong
\Zl \times \Zl \ ,
\eeq
which is isomorphic to the cohomology module
\[ H^1(E, \Zl) \ . \]
The action of $\GalQ$ on the torsion groups $E[m]$ induces an action
of $\GalQ$ on the projective limit $\Tate(E)$\@. This action gives
rise to a representation
\[ \rpl : \GalQ \longrightarrow \Gl{2}{\Zl} \ ,\]
which is called the \ladic representation of $E$\@. In considering
$\rpl$ one is reminded of the action of the automorphism group of a
manifold $M$ on the cohomology $H^*(M)$ and, more particularly, the
action of Gal($\C/\R$) on the cohomology of $M$ when $M$ is an
algebraic manifold in $\proj{n}{\C}$ defined by equations with \emph{real}
coefficients, but one must keep in mind that the transformations
of $E(\Qbar)$ arising from the elements of $\GalQ$ are not even
remotely continuous in the classical topology on $E(\C)$\@. More
generally, there is an \emph{algebraic} way of defining the cohomology
ring $H^*(M, \Zl)$ (see Tate \ct{tatepurdue}) when $M$ is an
algebraic variety with the property that automorphisms fixing the
field of definition act on $H^*(M, \Zl)$\@. An introduction to the
study of $\rpl$ may be found in Serre's ``Montreal Notes''
\ct{serremontreal}.
The canonical ring homomorphism from the ring $\Zl$ of \ladic integers
to the field $\Zlm$ induces a group homomorphism
\( A \rightarrow \ovbar{A} \),
called \emph{reduction} mod $\ell$\@, from the group $\Gl{2}{\Zl}$ to
the finite group $\Gl{2}{\Zlm}$\@. An \ladic representation $\rho$ of
$\GalQ$ is called \emph{modular} if it is isomorphic to the
representation $\rpl$ arising from the elliptic curve $E_f$ that is
the image under the Shimura map of a modular form $f$\@. A
representation
\[ \GalQ \rightarrow \Gl{2}{\Zlm} \]
is called \emph{modular} if
it is isomorphic to $\rpm$ for some modular \ladic representation
$\rpl$\@. In the extensive detailed study of representations of
$\GalQ$ particular attention has been paid to the question of when a
representation in $\Gl{2}{\Zl}$ is modular and also to the question of
when a representation of $\Gl{2}{\Zlm}$ is modular. Under certain
conditions (see Serre \ct{serreduke} and Ribet \ct{ribinv},
\ct{ribtoul}) one can show that $\rpl$ is modular if $\rpm$ is
modular, i.e., $\rpl$ is modular if it is \emph{congruent} mod $\ell$
to a modular \ladic representation. Such arguments are central both
to the work of Ribet in showing that the Shimura-Taniyama-Weil
conjecture implies ``Fermat's Last Theorem'' and to the reported work
of Wiles in proving that semi-stable elliptic curves are modular. In
the work of Ribet the basic idea is that the modularity of the Frey
curve, which has square-free conductor $N = abc$, implies the
existence of a cusp form of weight $2$ and level $N$. By using an
argument at the scene of the mod $\ell$ representations, Ribet shows
that one may split each odd prime divisor out of the level $N$ and
arrive at the conclusion that there is a cusp form of weight $2$ and
level $2$, which is not possible.
\newpage
\appendix{Late 1993/early 1994 Status}
Andrew Wiles posted the following announcement in the ``UseNet''
electronic news group called ``sci.math'':
\begin{verbatim}
From: wiles@rugola.Princeton.EDU (Andrew Wiles)
Newsgroups: sci.math
Subject: Fermat status
Message-ID: <1993Dec4.013650.12700@Princeton.EDU>
Date: 4 Dec 93 01:36:50 GMT
Sender: news@Princeton.EDU (USENET News System)
Organization: Princeton University
Lines: 21
Originator: news@nimaster
Nntp-Posting-Host: rugola.princeton.edu
In view of the speculation on the status of my work on the
Taniyama-Shimura conjecture and Fermat's Last Theorem I will give a
brief account of the situation. During the review process a number of
problems emerged, most of which have been resolved, but one in
particular I have not yet settled. The key reduction of (most cases
of ) the Taniyama-Shimura conjecture to the calculation of the Selmer
group is correct. However the final calculation of a precise upper
bound for the Selmer group in the semistable case (of the symmetric
square representation associated to a modular form) is not yet
complete as it stands. I believe that I will be able to finish this
in the near future using the ideas explained in my Cambridge
lectures.
The fact that a lot of work remains to be done on the
manuscript makes it still unsuitable for release as a preprint . In
my course in Princeton beginning in February I will give a full
account of this work.
Andrew Wiles.
\end{verbatim}
\newpage
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\end{document}