Mathematics Institute, Universitetsparken 5
DK--2100 Copenhagen Ø, Denmark
e-mail: jakobsen@math.ku.dk ,zhang@math.ku.dkPermanent address 2: Dept. of applied Math, Tsinghua University, Beijing, 100084, P.R. China
Abstract: We compute the center in case $q$ is a primitive roor of unity.
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