Akm[k_,m_]:=Piecewise[{{0, (k != 4 && (k != 2 || m != 0)) || (m != -3 && m != 0 && m != 3)}, {A[2, 0], k == 2 && m == 0}, {-A[4, 3] + I*B[4, 3], k == 4 && m == -3}, {A[4, 0], k == 4 && m == 0}}, A[4, 3] + I*B[4, 3]]