Remarks on the derived center of small quantum groups

Let u(q)(g) be the small quantum group associated with a complex semisimple Lie algebra g and a primitive root of unity q, satisfying certain restrictions. We establish the equivalence between three different actions of g on the center of u(q)(g) and on the higher derived center of u(q)(g). Based on the triviality of this action for g = sl(2), sl(3), sl(4), we conjecture that, in finite type A, central elements of the small quantum group u(q)(Sl(n)) arise as the restriction of central elements in the big quantum group u(q)(sl(n)). We also study the role of an ideal z(Hig) known as the Higman ideal in the center of u(q)(g). We show that it coincides with the intersection of the Harish-Chandra center and its Fourier transform, and compute the dimension of z(Hig) in type A. As an illustration we provide a detailed explicit description of the derived center of u(q)(sl(2)) and its various symmetries.