Log-concavity of characters of parabolic Verma modules, and of restricted Kostant partition functions

Abstract: In 2022, Huh-Matherne-Meszaros-St. Dizier showed that normalized Schur polynomials are Lorentzian, thereby yielding their continuous (resp. discrete) log-concavity on the positive orthant (resp. on their support, in type-$A$ root directions). A reinterpretation of this result is that the characters of finite-dimensional simple representations of $\mathfrak{sl}{n+1}(\mathbb{C})$ are denormalized Lorentzian. In the same paper, these authors also showed that shifted characters of Verma modules over $\mathfrak{sl}{n+1}(\mathbb{C})$ are denormalized Lorentzian. In this work we extend these results to a larger family of modules that subsumes both of the above: we show that shifted characters of all parabolic Verma modules over $\mathfrak{sl}{n+1}(\mathbb{C})$ are denormalized Lorentzian. The proof involves certain graphs on $[n+1]$; more strongly, we explain why the character (i.e., generating function) of the Kostant partition function of any loopless multigraph on $[n+1]$ is Lorentzian after shifting and normalizing. In contrast, we show that a larger universal family of highest weight modules, the higher order Verma modules, do not have discretely log-concave characters. Finally, we extend all of these results to parabolic (i.e. "first order") and higher order Verma modules over the semisimple Lie algebras $\oplus{t=1}^T \mathfrak{sl}_{n_t+1}(\mathbb{C})$.