1-dimensional semi-stable Galois representations with coefficients - MathOverflow

1-dimensional semi-stable Galois representations with coefficients

2012-05-27T11:51:20Z

For any p-adic local field K, all 1-dim semi-stable Galois repn: $G_K \to Q_p^{*}$ are just $Q_p(n)\otimes \mu$, where $Q_p(n)$ is the Tate twist of cyclotomic character, and $\mu$ an unramified charater.

My question is what if we replace the coefficient field to $E \neq Q_p$?

In fact, at the end of the paper by Gerasimos Dousmanis "Rank two filtered $(φ, N)$-modules with Galois descent data and coefficients", the filtered $(\varphi, N)$ modules of all such 1-dim repns are all classified. My question really is, how do we write out the representations explicitly?

Answer by Laurent Berger for 1-dimensional semi-stable Galois representations with coefficients

2012-05-27T14:14:23Z

If $E \neq Q_p$ then there may be more $1$-dimensional crystalline representations that the ones you mention. By Lubin-Tate theory, every character of $G_K$ can be written as an unramified character times a character of $O_K^\times$ (after making proper choices and identifications). The algebraic characters of $O_K^\times$ are then crystalline and if $E$ contains $K$, then these provide examples of crystalline characters.

EDIT : oops sorry, I did not read the question carefully enough, I did not see that you were asking for an explicit description of all such representations. As David Loeffler pointed out, the answer is in Brian Conrad's paper (now in appendix B, the paper having grown since last time I mentioned it).

Here is what it basically says, using the same identifications as above: assume that $E$ contains $K^{gal}$ (it's usually harmless to assume that the coefficient field is large enough). If $s$ runs through the set of embeddings $s : K \to E$ and the $a_s$ are integers, then $x \mapsto \prod_s s(x)^{a_s}$ gives rise to a crystalline character of $G_K$ and they're all of this type times an unramified character.

Answer by 750am for 1-dimensional semi-stable Galois representations with coefficients

2012-05-27T14:43:23Z

@Berger, just a quick question, so these $a_i$ will be the Hodge-Tate weights? Thanks!