Representation theory in complex rank, I

P. Deligne defined interpolations of the tensor category of representations of the symmetric group S_n to complex values of n. Namely, he defined tensor categories Rep(S_t) for any complex t. This construction was generalized by F. Knop to the case of wreath products of S_n with a finite group. Generalizing these results, we propose a method of interpolating representations categories of various algebras containing S_n (such as degenerate affine Hecke algebras, symplectic reflection algebras, rational Cherednik algebras, etc.) to complex values of n. We also define the group algebra of S_n for complex n, study its properties, and propose a Schur-Weyl duality for Rep(S_t). In version 2, same more details have been added.