The Bailey lemma and Kostka polynomials

Using the theory of Kostka polynomials, we prove an A_{n-1} version of Bailey’s lemma at integral level. Exploiting a new, conjectural expansion for Kostka numbers, this is then generalized to fractional levels, leading to a new expression for admissible characters of A_{n-1}^{(1)} and to identities for A-type branching functions.