Akm[k_,m_]:=Piecewise[{{0, (k != 3 && (k != 1 || m != 0)) || (m != -3 && m != 0 && m != 3)}, {A[1, 0], k == 1 && m == 0}, {-A[3, 3] + I*B[3, 3], k == 3 && m == -3}, {A[3, 0], k == 3 && m == 0}}, A[3, 3] + I*B[3, 3]]