A two-variable refinement of the Stark conjecture in the function field case

Abstract

We propose a conjecture refining the Stark conjecture St(K/k,S) (Tate's formulation) in the function field case. Of course St(K/k,S) in this case is a theorem due to Deligne and independently to Hayes. The novel feature of our conjecture is that it involves two- rather than one-variable algebraic functions over finite fields. Because the conjecture is formulated in the language and framework of Tate's thesis we have powerful standard techniques at our disposal to study it. We build a case for our conjecture by (i) proving a parallel result in the framework of adelic harmonic analysis which we dub the adelic Stirling formula, (ii) proving the conjecture in the genus zero case and (iii) explaining in detail how to deduce St(K/k,S) from our conjecture. In the genus zero case the class of two-variable algebraic functions thus arising includes all the F_q[T]-solitons previously studied and applied by the author, collaborators and others. Ultimately the inspiration for this paper comes from striking examples given some years ago by R. Coleman and D. Thakur.