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