On simple factors of modular jacobians: endomorphism ring and simplicity of mod p reduction - MathOverflow most recent 30 from http://mathoverflow.net 2013-06-19T11:21:16Z http://mathoverflow.net/feeds/question/109248 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/109248/on-simple-factors-of-modular-jacobians-endomorphism-ring-and-simplicity-of-mod-p On simple factors of modular jacobians: endomorphism ring and simplicity of mod p reduction Tommaso Centeleghe 2012-10-09T18:46:28Z 2012-10-22T12:38:26Z <p>Let $A_f$ be the abelian variety over $\mathbf{Q}$ arising as a $\mathbf{Q}$-simple factor of the Jacobian $J_0(N)$ of the modular curve associated to a normalized newform $f$ of weight $2$ on the congruence subgroup $\Gamma_0(N)$. There is a piece $\mathbf{T}_f$ of the Hecke ring acting on $A_f$. Such a ring is an order in the number field generated by the Fourier coefficients of $f$, whose degree equals the dimension of $A$. It is a result of Ribet (I think) that the natural map $\mathbf{T}_f\otimes\mathbf{Q}\rightarrow\textrm{End}(A)\otimes\mathbf{Q}$ is an isomorphism, where $\textrm{End}(A)$ is the ring of $\mathbf{Q}$-enndomorphisms of $A$ (I couldn't attach the index to End..).</p> <p>I have two questions, please:</p> <p>1) Can we find an abelian variety $A_f'$ over $\mathbf{Q}$, which is $\mathbf{Q}$-isogenous to $A_f$, and such that $\textrm{End}(A_f')$ is the maximal order of $\textrm{End}(A_f')\otimes\mathbf{Q}=\textrm{End}(A_f)\otimes\mathbf{Q}$? (Notice that in the one dimensional case $\textrm{End}(A_f)$ is already maximal, being it the ring of integers)</p> <p>2) Does the fact that $A_f$ has such a large endomorphism ring imply that the mod $p$ reduction of $A_f$ be simple over the prime field with $p$ elements? Here $p$ is a prime of good reduction for $A_f$.</p> <p>Thanks!</p> http://mathoverflow.net/questions/109248/on-simple-factors-of-modular-jacobians-endomorphism-ring-and-simplicity-of-mod-p/110146#110146 Answer by François Brunault for On simple factors of modular jacobians: endomorphism ring and simplicity of mod p reduction François Brunault 2012-10-20T12:23:53Z 2012-10-22T12:38:26Z <p>I found a reference for the first question in the following PhD thesis :</p> <p>J. Wilson, <em><a href="http://eprints.maths.ox.ac.uk/00000032/01/wilson.pdf" rel="nofollow">Curves of genus 2 with real multiplication by a square root of 5</a></em></p> <p>The answer is yes. This is a consequence of the following general fact about abelian varieties (see Prop 2.5.4 in the thesis).</p> <p>Let $A$ be an abelian variety over a field $k$. Let $R$ be an order in a number field $F$. Assume that $R$ embeds into $\operatorname{End}_k(A)$. Then there exists an abelian variety $B/k$ which is $k$-isogenous to $A$ and such that $\mathcal{O}_F$ embeds into $\operatorname{End}_k(B)$.</p> <p>The idea is to take $B=A/G$ with $G=(n\mathcal{O}_F) A[n^2]$, where $n$ is the index of $R$ in $\mathcal{O}_F$.</p> <p>The thesis also contains interesting examples of varieties $A_f$ with Hecke field $K_f=\mathbf{Q}(\sqrt{5})$. These are natural examples to try for Question 2 (although I have no idea how to compute the reduction of $A_f$ mod $p$).</p> <p><strong>EDIT.</strong> The answer to Question 2 is negative in general. There are newforms $f$ of weight $2$ on $\Gamma_0(N)$ such that $A_f$ splits over $\overline{\mathbf{Q}}$. This happens for example when $f$ has extra-twist. The first example appears at level $N=63$, see Table 1 p. 13 in</p> <p>MR1933828 (2003i:11078) González-Jiménez, Enrique ; González, Josep. <em>Modular curves of genus 2.</em> Math. Comp. 72 (2003), no. 241, 397--418 (electronic).</p> <p>Assume $A_f \sim E_1 \times E_2$ where everything is defined over some number field $K$. If $p$ is a prime of good reduction for $A_f$ which splits totally in $K$, then $A_f$ mod $p$ is $\mathbf{F}_p$-isogenous to a product of elliptic curves over $\mathbf{F}_p$, so it is not simple.</p>