On the exponent of finite-dimensional Hopf algebras
  One of the classical notions of group theory is the notion of the exponent of a group. The exponent of a group is the least common multiple of orders of its elements.   In this paper we generalize the notion of exponent to Hopf algebras. We give five equivalent definitions of the exponent. Two of them are: 1) the exponent of H equals the order of the Drinfeld element u of the quantum double D(H); 2) the exponent of H is the order of the squared brading acting in the tensor square of the regular representation of D(H).   We show that the exponent is invariant under twisting. We prove that for semisimple and cosemisimple Hopf algebras H, the exponent is finite and divides dim(H)^3. For triangular Hopf algebras in characteristic zero, we show that the exponent divides dim(H). We conjecture that if H is semisimple and cosemisimple then the exponent always divides dim(H).   At the end we formulate some open questions, in particular suggest a formulation for a possible Hopf algebraic analogue of Sylow's theorem. 
Pavel Etingof and Shlomo Gelaki