A quartet of fermionic expressions for $M(k,2k\pm 1)$ Virasoro characters via half-lattice paths

Abstract

We derive new fermionic expressions for the characters of the Virasoro minimal models $M(k,2k\pm 1)$ by analysing the recently introduced half-lattice paths. These fermionic expressions display a quasiparticle formulation characteristic of the ${\varphi }_{2,1}$ and ${\varphi }_{1,5}$ integrable perturbations. We find that they arise by imposing a simple restriction on the RSOS quasiparticle states of the unitary models $M(p,p+1)$. In fact, four fermionic expressions are obtained for each generating function of half-lattice paths of finite length $L$, and these lead to four distinct expressions for most characters ${\chi }_{r,s}^{k,2k\pm 1}$. These are direct analogues of Melzer's expressions for $M(p,p+1)$, and their proof entails revisiting, reworking and refining a proof of Melzer's identities which used combinatorial transforms on lattice paths. We also derive a bosonic version of the generating functions of length $L$ half-lattice paths, this expression being notable in that it involves $q$-trinomial coefficients. Taking the $L\to \infty$ limit shows that the generating functions for infinite length half-lattice paths are indeed the Virasoro characters ${\chi }_{r,s}^{k,2k\pm 1}$.

29 pages. v2: minor improvements, references added