fix broken reference

Author: kbuzzard

blueprint/src/chapter/ch03frey.tex

@@ -36,7 +36,7 @@ \section{The arithmetic of elliptic curves}
 \end{corollary}
 \begin{proof}\uses{group_theory_lemma,EllipticCurve.n_torsion_card}
   This follows from the previous group-theoretic lemma~\ref{group_theory_lemma} and
-  theorem~\ref{EllipticCurve.torsion_card}.
+  theorem~\ref{EllipticCurve.n_torsion_card}.
 \end{proof}
 
 We saw in section~\ref{twopointfour} that if $E$ is an elliptic curve over a field $k$ and if $k^{\sep}$ is a separable closure of~$k$, then the group $\Gal(k^{\sep}/k)$ acts on $E(k^{\sep})[n]$. Now let $n$ be a positive integer which is nonzero in $k$. We have just seen that $E(k^{\sep})[n]$ is isomorphic to $(\Z/n\Z)^2$, and it inherits an action of $\Gal(k^{\sep}/k)$. If we fix an isomorphism $E(k^{\sep})[n]\cong(\Z/n\Z)^2$ then we get a representation $\Gal(k^{\sep}/k)\to\GL_2(\Z/n\Z)$. A fundamental fact about this Galois representation is that its determinant is the cyclotomic character.