MathOverflow is a question and answer site for professional mathematicians. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Let $P$ be a pro-p group. Assume that there is a filtration of $P$ by normal subgroups $P_i$ such that $P_0=P$ and $P_{i+1} < P_i(i\in\mathbb N)$. Let $V$ be an $l$-adic representation of $P$, where the prime $l$ is prime to $p$.

Question: Is there a direct sum decomposition $V=\bigoplus_{i\geq 0,i\in\mathbb Z}V_i$ by $P$-submodules such that the $P_r$-fixed part is given by $V^{P_i}=\bigoplus_{i\leq r} V_i$ ?

share|cite|improve this question
Profinite groups are in particular compact, so all complex representations are unitarizabile and decompose uniquely into irreducible. For your particular problem, it suffices to define $V_0 = V^P$ and $$ V_i = V^{P_i} \ominus V_{P_{i-1}},$$ where the minus is the orthogonal difference in the sense of Hilbert spaces. In some sense, you need to copy this to the $\ell$-adic reps. – Marc Palm Apr 24 '13 at 13:58
Is the category of $\ell$-adic reps of pro-p-groups not semisimple? – Marc Palm Apr 24 '13 at 13:59
at least if $p \neq \ell$? – Marc Palm Apr 24 '13 at 14:00
The category of $l$-adic reps of pro-$p$ groups is indeed semisimple. This is because it factors through $GL_n(\mathbb Z_l)$, but the Sylow $p$-subgroups of that all inject into $GL_n(\mathbb F_l)$, so there's a bound on their size. – Will Sawin Apr 24 '13 at 17:33
Do you mean that l-adic representations of pro-p groups factors through a finite quotient.. so they are semisimple? By the way, Is there any good reference which discuss about this kind problems? – Int Apr 24 '13 at 23:15

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.