Dear MO,


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 ``*Propriétés galoisiennes des points d'ordre fini des courbes elliptiques*'' (more specifically, see *Corollaire*, 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.


**Question:** Nowadays, do we know of a criterion to tell whether one is in the first case or the second case?


**A more concrete question:** 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.


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, *diabolically* large and I can't find the ramification at $37$ computationally... or at least I don't know how to!


Thanks for your help!



Ramification in p-division fields associated to elliptic curves with good ordinary reduction
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.


