Fundamental and homogeneous bases of Hopf algebras built from nonsymmetric operads

Abstract

We introduce new partial order structures on the underlying sets of free nonsymmetric operads. These posets involve decorated ordered rooted trees, and their terminal intervals are lattices. These lattices are not graded, not self-dual, and not semi-distributive, but they are EL-shellable, and their M\"bius functions take values in $\{-1, 0, 1\}$. They admit sublattices on the families of $m$-Fuss-Catalan objects and of forests of trees. This latter order structure is used to construct two new bases for the natural Hopf algebras of free nonsymmetric operads: a fundamental basis and a homogeneous basis. Along with the already known elementary basis of these Hopf algebras, this yields a triple of bases. The situation is similar to what is observed in the Hopf algebras of Malvenuto-Reutenauer, Loday-Ronco, and noncommutative symmetric functions, each of which presents such triples of bases and basis changes involving, respectively, the right weak partial order, the Tamari partial order, and the Boolean lattice partial order.