F_q[M_2], F_q[GL_2] and F_q[SL_2] as quantized hyperalgebras

Abstract

Let U_q(sl_2) be the standard Drinfeld-Jimbo quantized universal enveloping algebra over sl_2, let F_q[SL_2] be the corresponding quantum function algebra, and let R be the ring of Laurent polynomials in q with coefficients in the ring of integers. Let \Cal{U}_q(sl_2) be the unrestricted R-integer form of U_q(sl_2) introduced by De Concini, Kac and Procesi. Within the quantum function algebra F_q[SL_2], we study the subset \Cal{F}_q[SL_2] of all elements which give values in the ring R when paired with \Cal{U}_q(sl_2). In this paper we describe \Cal{F}_q[SL_2]. In particular we provide a presentation of it by generators and relations, and a nice R-spanning set (of PBW type). Moreover, we give a direct proof that \Cal{F}_q[SL_2] is a Hopf subalgebra of F_q[SL_2], and that the specialization of \Cal{F}_q[SL_2] at q=1 is the hyperalgebra U_Z(sl_2^*) associated to the Lie bialgebra sl_2^* dual to sl_2: in other words, \Cal{F}_q[SL_2] is a "quantum hyperalgebra". In fact, our description of \Cal{F}_q[SL_2] is much like the presentation of Lusztig's restricted R-integer form of U_q(sl_2). We describe explicitly also the specializations of \Cal{F}_q[SL_2] at roots of 1, and the associated quantum Frobenius (epi)morphism; these results again closely resemble Lusztig's ones. All this improve results proved in previous work by the first named author basing upon the results of De Concini, Kac and Procesi. The same analysis is done for the analogue algebra \Cal{F}_q[GL_2], with similar results, and also (as a key, intermediate step) for \Cal{F}_q[Mat_2], for which even stronger results hold, in particular a PBW-like theorem.