Further results for classical and universal characters twisted by roots of unity
Arvind Ayyer, Nishu Kumari
TL;DR
The paper studies factorization of irreducible characters of classical groups $GL_n$, $SO_n$, $Sp_{2n}$ under root-of-unity specializations, including twisted inputs $X_ω$. It develops explicit factorization formulas for Schur polynomials and for universal characters $\mathfrak{o}_\lambda$, $\mathfrak{sp}_\lambda$, $\mathfrak{so}_\lambda$ using partition cores/quotients, plus independence results for twisted variables. A key finding is that universal characters evaluated at roots of unity take values in $\{0, \pm1, \pm2\}$, while classical characters can be unbounded, with numerous equalities and identities across group types. Together, these results deepen structural connections among classical and universal characters and provide new tools for analyzing representations at roots of unity.
Abstract
We revisit factorizations of classical characters under various specializations, some old and some new. We first show that all characters of classical families of groups twisted by odd powers of an even primitive root of unity factorize into products of characters of smaller groups. Motivated by conjectures of Wagh and Prasad (Manuscr. Math. 2020), we then observe that certain specializations of Schur polynomials factor into products of two characters of other groups. We next show, via a detour through hook Schur polynomials, that certain Schur polynomials indexed by staircase shapes factorize into linear pieces. Lastly, we consider classical and universal characters specialized at roots of unity. One of our results, in parallel with Schur polynomials, is that universal characters take values only in $\{0, \pm 1, \pm 2\}$ at roots of unity.