Ramification in p-division fields associated to elliptic curves with good ordinary reduction - MathOverflow most recent 30 from http://mathoverflow.net 2013-06-19T16:47:39Z http://mathoverflow.net/feeds/question/69824 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/69824/ramification-in-p-division-fields-associated-to-elliptic-curves-with-good-ordinar Ramification in p-division fields associated to elliptic curves with good ordinary reduction Álvaro Lozano-Robledo 2011-07-08T20:02:47Z 2011-07-08T20:22:42Z <p>Dear MO,</p> <p>Let $p$ be a prime and let $E/\mathbb{Q}_p$ be an elliptic curve. Suppose that $E/\mathbb{Q}_p$ has good ordinary reduction at $p$. In his 1972 paper <em>Propriétés galoisiennes des points d'ordre fini des courbes elliptiques</em>'' (more specifically, see <em>Corollaire</em>, in p. 274), Serre shows along the way that the inertia subgroup of $\operatorname{Gal}(\mathbb{Q}_p(E[p])/\mathbb{Q}_p)$ is, with respect to a suitable basis of $E[p]$, isomorphic to either a matrix group of the form {$[\ast\ 0; 0\ 1]$} or {$[\ast\ \ast; 0\ 1]$} as a subgroup of $\operatorname{GL}(2,\mathbb{F}_p)$. After this result, Serre remarks that he doesn't know of any simple criterion that would determine whether one is in the first case or the second case.</p> <p><strong>Question:</strong> Nowadays, do we know of a criterion to tell whether one is in the first case or the second case?</p> <p><strong>A more concrete question:</strong> Here is the particular example that I am working with: Let $E/\mathbb{Q}$ be 1225h1'' in Cremona's tables, given by $$E : y^2 + xy + y = x^3 + x^2 - 8x + 6.$$ This curve has a rational $37$-isogeny and therefore $\operatorname{Gal}(\mathbb{Q}(E[37])/\mathbb{Q})$ is a Borel subgroup of $\operatorname{GL}(2,\mathbb{F}_{37})$. The curve $E$ has good ordinary reduction at $p=37$ and I am trying to find out whether the ramification index of $37$ in the extension $\mathbb{Q}(E[37])/\mathbb{Q}$ is just $\varphi(37)$ or rather $\varphi(37)\cdot 37$, where $\varphi$ is the Euler phi function.</p> <p>The $37$th division polynomial of $E/\mathbb{Q}$ has degree $684$ and it factors (over $\mathbb{Q}[x]$) as a product of $4$ polynomials of degrees $6$, $6$, $6$ and $666$, respectively. The extension of degree $666$ is, well, <em>diabolically</em> large and I can't find the ramification at $37$ computationally... or at least I don't know how to!</p> <p>Thanks for your help!</p> http://mathoverflow.net/questions/69824/ramification-in-p-division-fields-associated-to-elliptic-curves-with-good-ordinar/69825#69825 Answer by Felipe Voloch for Ramification in p-division fields associated to elliptic curves with good ordinary reduction Felipe Voloch 2011-07-08T20:20:44Z 2011-07-08T20:20:44Z <p>Assume $p \ne 2$. The condition for the representation to be tamely ramified (i.e $* = 0$ in the upper right entry of the matrix) is that $j(E) \equiv j_0 \mod p^2$ where $j(E)$ is the $j$-invariant of $E$ and $j_0$ is the $j$-invariant of the canonical lift of the reduction of $E$. This is proved in Gross "A tameness criterion for galois representations..." Duke J. 61 (1990) on page 514. For $p=2$ you need the congruence modulo $8$. Serre gives an algorithm for computing $j_0$ in Lubin-Serre-Tate.</p>