Fermionic formula for double Kostka polynomials

Abstract

The $X=M$ conjecture asserts that the $1D$ sum and the fermionic formula coincide up to some constant power. In the case of type $A,$ both the $1D$ sum and the fermionic formula are closely related to Kostka polynomials. Double Kostka polynomials $K_{{\boldsymbol\lambda},{\boldsymbol\mu}}(t),$ indexed by two double partitions ${\boldsymbol\lambda},{\boldsymbol\mu},$ are polynomials in $t$ introduced as a generalization of Kostka polynomials. In the present paper, we consider $K_{{\boldsymbol\lambda},{\boldsymbol\mu}}(t)$ in the special case where ${\boldsymbol\mu}=(-,\mu'')$. We formulate a $1D$ sum and a fermionic formula for $K_{{\boldsymbol\lambda},{\boldsymbol\mu}}(t),$ as a generalization of the case of ordinary Kostka polynomials. Then we prove an analogue of the $X=M$ conjecture.