∗This work was done within the activities of Centro de Álgebra da Universidade de Lisboa, I.N.I.C. and partially supported by Projecto 87463 of J.N.I.C.T.
Notes
∗This work was done within the activities of Centro de Álgebra da Universidade de Lisboa, I.N.I.C. and partially supported by Projecto 87463 of J.N.I.C.T.