The Lusztig automorphism of the $q$-Onsager algebra

Abstract

Pascal Baseilhac and Stefan Kolb recently introduced the Lusztig automorphism $L$ of the $q$-Onsager algebra $\mathcal O_q$. In this paper, we express each of $L, L^{-1}$ as a formal sum involving some quantum adjoints. In addition, (i) we give a computer-free proof that $L$ exists; (ii) we establish the higher order $q$-Dolan/Grady relations previously conjectured by Baseilhac and Thao Vu; (iii) we obtain a Lusztig automorphism for the current algebra $\mathcal A_q$ associated with $\mathcal O_q$; (iv) we describe what happens when a finite-dimensional irreducible $\mathcal O_q$-module is twisted via $L$.