Weighted posets and the enriched monomial basis of QSym

Gessel’s fundamental and Stembridge’s peak functions are the generating functions for (enriched) P -partitions on labelled chains. They are also the bases of two significant subalgebras of formal power series, respectively, the ring of quasisymmetric functions (QSym) and the algebra of peaks. Hsiao introduced the monomial peak functions, a basis of the algebra of peaks indexed by odd integer compositions whose relation to peak functions mimics the one between the monomial and fundamental bases of QSym. We show that the extension of monomial peaks to any composition is a new basis of QSym and generalise Hsiao’s results including the product rule. To this end, we introduce a weighted variant of posets and study their generating functions.