$P$-partitions with flags and back stable quasisymmetric functions

Abstract: Stanley's theory of $(P,\omega)$-partitions is a standard tool in combinatorics. It can be extended to allow for the presence of a restriction, that is a given maximal value for partitions at each vertex of the poset, as was shown by Assaf and Bergeron. Here we present a variation on their approach, which applies more generally. The enumerative side of the theory is more naturally expressed in terms of back stable quasisymmetric functions. We study the space of such functions, following the work of Lam, Lee and Shimozono on back stable symmetric functions. As applications we describe a new basis for the ring of polynomials that we call forest polynomials. Additionally we give a signed multiplicity-free expansion for any monomial expressed in the basis of slide polynomials.