A general Chevalley formula for semi-infinite flag manifolds and quantum K-theory

We give a Chevalley formula for an arbitrary weight for the torus-equivariant K-group of semi-infinite flag manifolds, which is expressed in terms of the quantum alcove model. As an application, we prove the Chevalley formula for an anti-dominant fundamental weight for the (small) torus-equivariant quantum K-theory QK(T)(G/B) of an (ordinary) flag manifold G/B; this has been a longstanding conjecture about the multiplicative structure of QK(T)(G/B). In type A(n-1), we prove that the so-called quantum Grothendieck polynomials indeed represent (opposite) Schubert classes in the (non-equivariant) quantum K-theory QK(SLn/B); we also obtain very explicit information about the coefficients in the respective Chevalley formula.