Blocks for mod $p$ representations of $GL_2(Q_p)$

Let $π_1$ and $π_2$ be absolutely irreducible smooth representations of $G=GL_2(Q_p)$ with a central character, defined over a finite field of characteristic $p$. We show that if there exists a non-split extension between $π_1$ and $π_2$ then they both appear as subquotients of the reduction modulo $p$ of a unit ball in a crystalline Banach space representation of $G$. The results of Berger-Breuil describe such reductions and allow us to organize the irreducible representation into blocks. The result is new for $p=2$, the proof, which works for all $p$, is new.

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