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Let $\mathbb{Z}_p$ be te ring of p-adic integers and let $T$,$T'$ be two free $\mathbb{Z}_p$-module with a continuous action of $G_{\mathbb{Q}_p}$ (the absolute Galois group of $\mathbb{Q}_p$).

It is not hard to see that if $T$ and T' are of rank one, then there is a non zero $G_{\mathbb{Q}_p}$-equivariant map T \to T' \otimes \mathbb{Q}_p / \mathbb{Z}_p if and only if T \simeq T' mod $p$ (just write down explicitely the characters).

Is this true in a more general case, namely when T' and T' mod $p$ are irreducible and $T$ is any $\mathbb{Z}_p$-representation ?

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Consider the exact sequence of trivial $G$-modules: $0 \rightarrow \mathbf{Z}/p \rightarrow \mathbf{Q}_p/\mathbf{Z}_p \rightarrow \mathbf{Q}_p/\mathbf{Z}_p \rightarrow 0,$ (where the second map is multiplication by $p$) and tensor with T'. Since T' is free over $\mathbf{Z}_p$, it is flat, and so the sequence remains exact. Thus T'/p injects into T' \otimes \mathbf{Q}_p/\mathbf{Z}_p. In particular, any non-trivial map T/p \rightarrow T'/p extends to a non-trivial map T \rightarrow T/p \rightarrow T'/p \rightarrow T' \otimes \mathbf{Q}_p/\mathbf{Z}_p.

In particular, for the "if" part, you only need the hypothesis that T' is flat and there is a non-zero $G$-equivariant map from $T/p$ to T'/p (for any $G$).

Conversely, any map T \rightarrow T' \otimes \mathbf{Q}_p/\mathbf{Z}_p factors through $T/p^n$ for some $n$, and hence as

T \rightarrow T/p^n \rightarrow (T' \otimes \mathbf{Q}_p/\mathbf{Z}_p)[p^n] = T'/p^n. If T'/p is irreducible, then the composition factors of the RHS are all of the form T'/p, and hence there is a surjective map T/p \rightarrow T'/p, which must be an isomorphism if $T/p$ is also irreducible.

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    Thanks a lot. I cannot vote up your answer...2010-11-23