Weights and characters over Borcherds-Kac-Moody algebras

TL;DR

This paper derives explicit weight formulas for highest weight modules over Borcherds-Kac-Moody algebras, generalizing Kac-Moody results and introducing new concepts like holes and $P^{ ext{±}}$-dominant weights, with applications to character formulas and module structures.

Contribution

It extends weight formulas and character theories from Kac-Moody to Borcherds-Kac-Moody algebras, introducing novel concepts and covering a broader class of modules.

Findings

Explicit weight formulas for highest weight modules over BKM algebras.

Introduction of holes and $P^{ ext{±}}$-dominant weights concepts.

New character formulas and module structure analyses for negative rank-2 and $A_n$ type BKM algebras.

Abstract

Fix any Borcherds-Kac-Moody $\mathbb{C}$-Lie algebra (BKM LA) $\mathfrak{g}=\mathfrak{g}(A)$ of BKM-Cartan matrix $A$, and Cartan subalgebra $\mathfrak{h}\subset \mathfrak{g}$. In this paper, we obtain explicit weight formulas of any highest weight $\mathfrak{g}$-module $V$ with top weight $\lambda\in \mathfrak{h}^*$ : 1) Generalizing and extending those in one stroke from Kac-Moody (KM) case, of simples $V=L(\lambda)$ by Khare [Trans. Amer. Math. Soc. 2017] and Dhillon-Khare [Adv. Math. 2017 \& J. Algebra. 2022] and recently of all $V$ by Khare-Teja; via parabolic and higher order Verma $V$. 2) Uniform for all $(\mathfrak{g}, \lambda, V)$; seemingly novel even for integrable ($L(\lambda)$ and all intermediate) $V$ for dominant integral $\lambda\in P^+$. 3) As Weyl-orbit formulas (of finite-dim. $L(\lambda)$s) for several $V$; and for our candidates of parabolic Vermas over BKM LAs. 4) Using our concepts of holes (1-dim. weight-spaces lost) in $V$, and $P^{\pm}$-dominant weights to cover Chevalley-Serre relations in generic simple $V$s. We define $P^{\pm}$ to be the set of $\mu\in \mathfrak{h}^*$ paired with simple co-roots for $A_{ii}\geq 0$ as usual, but notably by negative multiples of $\frac{|A_{ii}|}{2}$ if $A_{ii}<0$. By-products of working with $P^{\pm}$: study of simples $L(\lambda)\ \forall\ \lambda\in P^{\pm}$, notably of $L(\rho)$ for Weyl vectors $\rho\in P^{\pm} \setminus P^+$, and their Verma covers; all novel to our best knowledge. For Weyl-Kac-Borcherds character type formulas of these $L(\lambda)$s over negative rank-2 $\mathfrak{g}$ and of $L(\rho)$ over negative $A_n$ type $\mathfrak{g}$ $\forall$ $n\in \mathbb{N}$, we explore : i) their presentations; ii) their Verma modules' structures; iii) problems on maximal vectors or Verma embeddings, from Kac-Kazhdan [Adv. Math. 1979], for our BKM $P^{\pm}$ setting.