Skip to content

Commit

Permalink
get ch3 LaTeX compiling
Browse files Browse the repository at this point in the history
  • Loading branch information
kbuzzard committed Mar 24, 2024
1 parent 4217608 commit 79ae73d
Show file tree
Hide file tree
Showing 2 changed files with 15 additions and 7 deletions.
19 changes: 12 additions & 7 deletions blueprint/src/chapter/ch03frey.tex
Original file line number Diff line number Diff line change
Expand Up @@ -174,14 +174,14 @@ \section{Multiplicative reduction}
and with perfect residue field, and if $E$ has multiplicative reduction, then there's
an unramified character $\chi$ of $\Gal(K^{\sep}/K)$ whose square is 1, such that for
all positive integers $n$ with $n\not=0$ in $K$, the
$n$-torsion $E(K^{\sep})[n]$ is an extension of $\chi$ by $\eps\chi$, where $\eps$ is the
$n$-torsion $E(K^{\sep})[n]$ is an extension of $\chi$ by $\epsilon\chi$, where $\epsilon$ is the
cyclotomic character.
\end{corollary}
\begin{proof} After a quadratic twist we may assume that $E$ has split multiplicative reduction.
The result then follows from the uniformisation theorem and an explicit computation.
Note that even if we do not prove surjectivity of Tate's uniformisation, we still know
that it's surjective on the $n$-torsion, because all $n^2$ point in the $n$-torsion of $E$
are accounted for by the $n$-torsion in $K^{\sep}^\times/q^{\mathbb{Z}}$.
are accounted for by the $n$-torsion in $(K^{\sep})^\times/q^{\mathbb{Z}}$.
\end{proof}

\section{Hardly ramified representations}
Expand All @@ -208,15 +208,16 @@ \section{Hardly ramified representations}
devoted to a proof of this. The proof is standard; for another reference, see Theorem~2.15
of~\cite{ddt}.

\section{The $\ell$-torsion in the Frey curve is hardly ramified.}
\section{The l-torsion in the Frey curve is hardly ramified.}

Let $(a,b,c,\ell)$ be a Frey package, with associated Frey curve $E$ and mod $\ell$ Galois
representation $\rho=E[\ell]$. We now work through a proof that $\rho$ is hardly ramified.

\begin{theorem}\label{Frey_curve_good} If $p\not=\ell$ is a prime not dividing $abc$ then
$\rho$ is unramified at~$p$.
\end{theorem}
\begin{proof} Indeed, $E$ has good reduction at $p$, and hence $\rho$ is unramified at $p$ by~ref{good_reduction_implies_unramified}.
\begin{proof} Indeed, $E$ has good reduction at $p$, and hence $\rho$ is unramified at $p$
by~\ref{good_reduction_implies_unramified}.
\end{proof}

If however $p$ divides $abc$ then $E$ has multiplicative
Expand Down Expand Up @@ -265,8 +266,12 @@ \section{The $\ell$-torsion in the Frey curve is hardly ramified.}
\begin{theorem}\label{Frey_curve_mod_ell_rep_at_ell} Let $\rho$ be the $\ell$-torsion in the
Frey curve associated to a Frey package $(a,b,c,\ell)$. Then the restriction of $\rho$ to $\GQl$ comes from a finite flat group scheme.
\end{theorem}
\begin{proof} The Frey curve either has good reduction at $\ell$ (case 1 of FLT) or multiplicative reduction at $\ell$ (case 2 of FLT). In the first case the $\ell$-torsion is finite and flat
at $\ell$ by theorem~ref{good_reduction_implies_flat}. In the second case the theory of the Tate curve shows that the $\ell$-torsion is (up to quadratic twist) an \emph{unramified} extension of the trivial character by the cyclotomic character, and furthermore that the extension is controlled
\begin{proof} The Frey curve either has good reduction at $\ell$ (case 1 of FLT) or multiplicative
reduction at $\ell$ (case 2 of FLT). In the first case the $\ell$-torsion is finite and flat
at $\ell$ by theorem~\ref{good_reduction_implies_flat}. In the second case the theory of the Tate
curve shows that the $\ell$-torsion is (up to quadratic twist) an \emph{unramified} extension of
the trivial character by the cyclotomic character, and furthermore that the extension is
controlled
by the $\ell$th power of an $\ell$-adic unit. This extension is known to be finite and flat;
see for example Proposition~8.2 of~\cite{edix}.
\end{proof}
Expand All @@ -284,7 +289,7 @@ \section{The $\ell$-torsion in the Frey curve is hardly ramified.}
theorem~\ref{frey_curve_at_2}, and the fourth is theorem~\ref{Frey_curve_mod_ell_rep_at_ell}.
\end{proof}

\section{The $\ell$-torsion in the Frey curve is irreducible.}
\section{The l-torsion in the Frey curve is irreducible.}

We finish this chapter by showing that Mazur's theorem implies that the $\ell$-torsion in the Frey
curve is irreducible.
Expand Down
3 changes: 3 additions & 0 deletions blueprint/src/macro/common.tex
Original file line number Diff line number Diff line change
Expand Up @@ -3,11 +3,14 @@
\newcommand{\Z}{\mathbb{Z}}
\newcommand{\Q}{\mathbb{Q}}
\newcommand{\Qp}{\mathbb{Q}_p}
\newcommand{\Ql}{\mathbb{Q}_\ell}
\newcommand{\Qbar}{\overline{\Q}}
\newcommand{\Qpbar}{\overline{\Q}_p}
\newcommand{\Qlbar}{\overline{\Q}_\ell}
\newcommand{\bbC}{\mathbb{C}}
\newcommand{\GQ}{\Gal(\Qbar/\Q)}
\newcommand{\GQp}{\Gal(\Qpbar/\Qp)}
\newcommand{\GQl}{\Gal(\Qlbar/\Ql)}
\newcommand{\m}{\mathfrak{m}}
\DeclareMathOperator{\Gal}{Gal}
\DeclareMathOperator{\Aut}{Aut}
Expand Down

0 comments on commit 79ae73d

Please sign in to comment.