Akm[k_,m_]:=Piecewise[{{0, (k != 5 && ((k != 1 && k != 3) || m != 0)) || (m != -4 && m != 0 && m != 4)}, {A[1, 0], k == 1 && m == 0}, {A[3, 0], k == 3 && m == 0}, {A[5, 4] - I*B[5, 4], k == 5 && m == -4}, {A[5, 0], k == 5 && m == 0}}, A[5, 4] + I*B[5, 4]]