Chromatic symmetric functions in noncommuting variables revisited

Abstract

In 1995 Stanley introduced a generalization of the chromatic polynomial of a graph G, called the chromatic symmetric function, XG, which was generalized to noncommuting variables, YG, by Gebhard-Sagan in 2001. Recently there has been a renaissance in the study of XG, in particular in classifying when XG is a positive linear combination of elementary symmetric or Schur functions.We extend this study from XG to YG, including establishing the multiplicativity of YG, and showing YG satisfies the k-deletion property. Moreover, we completely classify when YG is a positive linear combination of elementary symmetric functions in noncommuting variables, and similarly for Schur functions in noncommuting variables, in the sense of Bergeron-Hohlweg-Rosas-Zabrocki. We further establish the natural multiplicative generalization of the fundamental theorem of symmetric functions, now in noncommuting variables, and obtain numerous new bases for this algebra whose generators are chromatic symmetric functions in noncommuting variables. Finally, we show that of all known symmetric functions in noncommuting variables, only all elementary and specified Schur ones can be realized as YG for some G.