A new Plethystic Symmetric Function Operator and The rational Compositional Shuffle Conjecture at t=1/q

Our main result here is that the specialization at $t=1/q$ of the $Q_{km,kn}$ operators studied in [4] may be given a very simple plethystic form. This discovery yields elementary and direct derivations of several identities relating these operators at $t=1/q$ to the Rational Compositional Shuffle conjecture of [3]. In particular we show that if $m,n$ and $k$ are positive integers and $(m,n)$ is a coprime pair then $$q^{(km-1)(kn-1)+k-1\over 2} Q_{km,kn}(-1)^{kn}\Big|_{t=1/q} \,=\, \textstyle{[k]_q\over [km]_q} e_{km}\big[ X[km]_q\big]$$ where as customarily, for any integer $s \geq 0$ and indeterminate $u$ we set $[s]_u=1+u+\cdots +u^{s-1}$. We also show that the symmetric polynomial on the right hand side is always Schur positive. Moreover, using the Rational Compositional Shuffle conjecture, we derive a precise formula expressing this polynomial in terms of Parking functions in the $km\times kn$ lattice rectangle.