Whittaker Limits of Difference Spherical Functions

The q-Whittaker function is introduced as a limit at t = 0 of the global q,t-spherical function, which extends the symmetric Macdonald polynomials to arbitrary eigenvalues. The limiting procedure generalizes that due to Etingof. The construction heavily depends on the technique of the q-Gaussians developed by the author (and Stokman in the non-reduced case). In this approach, the q-Whittaker function is given by a series convergent everywhere. One of the applications is a q-version of the Shintani–Casselman–Shalika formula, which is directly connected with the q,t-Mehta–Macdonald identities in terms of the Jackson integral. In type A, this formula generalizes that due to Gerasimov et al. The Harish-Chandra-type asymptotic formula is established for the global q,t-spherical functions, including the Whittaker limit.