“…More precisely, we set I to be the two-sided ideal generated by the elements K i − K ′−1 i , which is also a Z 2 -graded Hopf ideal, and we have canonical isomorphism D(U 0 , U 0 )/I ∼ = U q (g) as Z 2 -graded Hopf algebras. Recently, Drinfeld doubles have been studied by various authors as a useful tool to recover the quantum groups (see, e.g., [5,12,13,20,21,22]).…”