Theta maps for combinatorial Hopf algebras

Abstract: There is a very natural and well-behaved Hopf algebra morphism from quasisymmetric functions to peak algebra, which we call it Theta map. This paper focuses on generalizing the peak algebra by constructing generalized Theta maps for an arbitrary combinatorial Hopf algebra. The image of Theta maps lies in the odd Hopf subalgebras, so we present a strategy to find odd Hopf subalgebra of any combinatorial Hopf algebra. We also give a combinatorial description of a family of Theta maps for Malvenuto-Reutenauer Hopf algebra of permutations $\operatorname{\mathsf{\mathfrak{S}Sym}}$ whose images are generalizations of the peak algebra. We also indicate a criterion to check whether a map is a Theta map. Moreover, precise descriptions of the Theta maps for the following Hopf algebras will be presented, Hopf subalgebras of quasisymmetric functions, commutative and co-commutative Hopf algebras, and theta maps for a Hopf algebra $\mathcal{V}$ on permutations.