pℓpℓ-torsion points in finite abelian groups and combinatorial identities

The main aim of this article is to compute all the moments of the number of pℓpℓ-torsion elements in some type of finite abelian groups. The averages involved in these moments are those defined for the Cohen–Lenstra heuristics for class groups and their adaptation for Tate–Shafarevich groups. In particular, we prove that the heuristic model for Tate–Shafarevich groups is compatible with the recent conjecture of Poonen and Rains about the moments of the orders of p-Selmer groups of elliptic curves. For our purpose, we are led to define certain polynomials indexed by integer partitions and to study them in a combinatorial way. Moreover, from our probabilistic model, we derive combinatorial identities, some of which appearing to be new, the others being related to the theory of symmetric functions. In some sense, our method therefore gives for these identities a somehow natural algebraic context.