A note on passing from a quasi-symmetric function expansion to a Schur function expansion of a symmetric function

Abstract: Egge, Loehr and Warrington gave in \cite{ELW} a combinatorial formula that permits to convert the expansion of a symmetric function, homogeneous of degree $n$, in terms of Gessel's fundamental quasisymmetric functions into an expansion in terms of Schur functions. Surprisingly the Egge, Loehr and Warrington result may be shown to be simply equivalent to replacing the Gessel fundamental by a Schur function indexed by the same composition. In this paper we give a direct proof of the validity of this replacement. This interpretation of the result in \cite{ELW} has already been successfully applied to Schur positivity problems.