# A quartet of fermionic expressions for $M(k,2k\pm1)$ Virasoro characters via half-lattice paths

## TL;DR

This paper introduces new fermionic and bosonic expressions for Virasoro minimal model characters using half-lattice paths, revealing a quasiparticle structure and refining combinatorial proofs for these identities.

## Contribution

It provides novel fermionic formulas for $M(k,2k ext{±}1)$ characters based on half-lattice path analysis, extending Melzer's identities with combinatorial refinements.

## Key findings

- Four fermionic expressions per character derived from half-lattice paths
- Bosonic generating functions involve $q$-trinomial coefficients
- Infinite path limits recover Virasoro characters

## Abstract

We derive new fermionic expressions for the characters of the Virasoro minimal models $M(k,2k\pm1)$ by analysing the recently introduced half-lattice paths. These fermionic expressions display a quasiparticle formulation characteristic of the $\phi_{2,1}$ and $\phi_{1,5}$ 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 $\chi^{k,2k\pm1}_{r,s}$. 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 $\chi^{k,2k\pm1}_{r,s}$.