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chore: fix bump
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pitmonticone committed Nov 30, 2024
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3 changes: 2 additions & 1 deletion .gitignore
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Expand Up @@ -9,6 +9,7 @@ blueprint/web/
blueprint/src/web.paux
blueprint/src/web.bbl
blueprint/print/
blueprint/lean_decls
# Files generated by LaTeX
*.aux
*.bbl
Expand All @@ -18,4 +19,4 @@ blueprint/print/
*.pdf
*.fls
*.fdb_latexmk
*.synctex.gz
*.synctex.gz
10 changes: 5 additions & 5 deletions blueprint/src/chapter/ch02reductions.tex
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Expand Up @@ -69,21 +69,21 @@ \section{Galois representations and elliptic curves}\label{twopointfour}

The group structure behaves well under change of field.

\begin{lemma}\label{EllipticCurve.Points.map}\lean{EllipticCurve.Points.map}\leanok If $E$ is an elliptic curve over a field $k$, and if $K$ and $L$ are two fields which are $k$-algebras, and if $f:K\to L$ is a $k$-algebra homomorphism, the map from $E(K)$ to $E(L)$ induced by $f$ is an additive group homomorphism.
\begin{lemma}\label{WeierstrassCurve.Points.map}\lean{WeierstrassCurve.Points.map}\leanok If $E$ is an elliptic curve over a field $k$, and if $K$ and $L$ are two fields which are $k$-algebras, and if $f:K\to L$ is a $k$-algebra homomorphism, the map from $E(K)$ to $E(L)$ induced by $f$ is an additive group homomorphism.
\end{lemma}
\begin{proof} The equations defining the group law are ratios of polynomials with coefficients in $k$, and such things behave well under $k$-algebra homomorphisms.
\end{proof}

This construction is functorial (it sends the identity to the identity, and compositions to compositions).

\begin{lemma}\label{EllipticCurve.Points.map_id}\lean{EllipticCurve.Points.map_id}\uses{EllipticCurve.Points.map}
\begin{lemma}\label{WeierstrassCurve.Points.map_id}\lean{WeierstrassCurve.Points.map_id}\uses{WeierstrassCurve.Points.map}
The group homomorphism $E(K)\to E(K)$ induced by the identity map $K\to K$ is the
identity group homomorphism.
\end{lemma}
\begin{proof} An easy calculation.
\end{proof}

\begin{lemma}\label{EllipticCurve.Points.map_comp}\lean{EllipticCurve.Points.map_comp}\uses{EllipticCurve.Points.map}
\begin{lemma}\label{WeierstrassCurve.Points.map_comp}\lean{WeierstrassCurve.Points.map_comp}\uses{WeierstrassCurve.Points.map}
If $K\to L\to M$ are $k$-algebra homomorphisms then the group homomorphism $E(K)\to E(M)$
induced by the map $K\to M$ is the composite of the map $E(K)\to E(L)$ induced by $K\to L$
and the map $E(L)\to E(M)$ induced by the map $L\to M$.
Expand All @@ -93,15 +93,15 @@ \section{Galois representations and elliptic curves}\label{twopointfour}

Thus if $f:K\to L$ is an isomorphism of fields, the induced map $E(K)\to E(L)$ is an isomorphism of groups, with the inverse isomorphism being the map $E(L)\to E(K)$ induced by $f^{-1}$. This construction thus gives us an action of the multiplicative group $\Aut_k(K)$ of automorphisms of the field $K$ on the additive abelian group $E(K)$.

\begin{definition}\label{EllipticCurve.galoisRepresentation}\lean{EllipticCurve.galoisRepresentation}\uses{EllipticCurve.Points.map_id,EllipticCurve.Points.map_comp}
\begin{definition}\label{WeierstrassCurve.galoisRepresentation}\lean{WeierstrassCurve.galoisRepresentation}\uses{WeierstrassCurve.Points.map_id,WeierstrassCurve.Points.map_comp}
If $E$ is an elliptic curve over a field $k$ and $K$ is a field and a $k$-algebra,
then the group of $k$-automorphisms of $K$ acts on the additive abelian group $E(K)$.
\end{definition}
In particular, if $\Qbar$ denotes an algebraic closure of the rationals (for example, the algebraic numbers in $\bbC$) and if $\GQ$ denotes the group of field isomorphisms $\Qbar\to\Qbar$, then for any elliptic curve $E$ over $\Q$ we have an action of $\GQ$ on the additive abelian group $E(\Qbar)$.

We need a variant of this construction where we only consider the $n$-torsion of the curve, for $n$ a positive integer. Recall that if $A$ is any additive abelian group, and if $n$ is a positive integer, then we can consider the subgroup $A[n]$ of elements $a$ such that $na=0$. If a group~$G$ acts on $A$ via additive group isomorphisms, then there will be an induced action of~$G$ on $A[n]$.

\begin{definition}\label{EllipticCurve.torsionGaloisRepresentation}\lean{EllipticCurve.torsionGaloisRepresentation}\uses{EllipticCurve.galoisRepresentation}
\begin{definition}\label{WeierstrassCurve.torsionGaloisRepresentation}\lean{WeierstrassCurve.torsionGaloisRepresentation}\uses{WeierstrassCurve.galoisRepresentation}
If $E$ is an elliptic curve over a field $k$ and $K$ is a field and a $k$-algebra,
and if $n$ is a natural number, then the group of $k$-automorphisms of $K$ acts on the additive abelian group $E(K)[n]$ of $n$-torsion points on the curve.
\end{definition}
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8 changes: 4 additions & 4 deletions blueprint/src/chapter/ch03frey.tex
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Expand Up @@ -10,7 +10,7 @@ \section{The arithmetic of elliptic curves}
We give an overview of the results we need, citing the literature for proofs. Everything here is
standard, and most of it dates back to the 1970s or before.

\begin{theorem}\label{EllipticCurve.n_torsion_card}\lean{EllipticCurve.n_torsion_card}\notready
\begin{theorem}\label{WeierstrassCurve.n_torsion_card}\lean{WeierstrassCurve.n_torsion_card}\notready
Let $n$ be a positive integer, let $F$ be a separably closed
field with $n$ nonzero in $F$, and let $E$ be an elliptic curve over $F$. Then the $n$-torsion $E(K)[n]$
in the $F$-points of $E$ is a finite group of size $n^2$.
Expand All @@ -29,14 +29,14 @@ \section{The arithmetic of elliptic curves}
with $a_i\mid a_{i+1}$ (this is possible by the structure theorem for finite abelian groups), and then to apply our hypothesis firstly with $d=a_1$ to deduce $t=r$ and then with $d=a_t$ to deduce $a_1=a_t$.
\end{proof}

\begin{corollary}\label{Elliptic_curve_n_torsion_2d}\lean{EllipticCurve.n_torsion_dimension}
\begin{corollary}\label{Elliptic_curve_n_torsion_2d}\lean{WeierstrassCurve.n_torsion_dimension}
Let $n$ be a positive integer, let $F$ be a separably closed
field with $n$ nonzero in $F$, and let $E$ be an elliptic curve over $F$. Then the $n$-torsion $E(F)[n]$
in the $F$-points of $E$ is a finite group isomorphic to $(\Z/n\Z)^2$.
\end{corollary}
\begin{proof}\uses{group_theory_lemma,EllipticCurve.n_torsion_card}
\begin{proof}\uses{group_theory_lemma,WeierstrassCurve.n_torsion_card}
This follows from the previous group-theoretic lemma~\ref{group_theory_lemma} and
theorem~\ref{EllipticCurve.n_torsion_card}.
theorem~\ref{WeierstrassCurve.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.
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