POLYNOMIALS RELATED TO HALL NUMBERS

Abstract

Let Λ be a finite dimensional algebra of finite representation type over a finite field k. For any modules A, B and Pin mod Λ with P projective, we prove that there exists a polynomial ϕ B (P)Λ over Z whose evaluation at |E| for any conservative finite field extension E of Λ is the sum of Hall numbers F B E C E A E where C E runs through isoclasses in mod Λ E and P E is the projective cover of C E . As a consequence of this result and its dual version, Hall polynomials ϕ E CA exist when C or A is semisimple. As applications of the main result, we obtain the existence of Hall polynomials for Nakayama algebras and some selfinjective algebras. *Supported by the Volkswagen Foundation.