Akm[k_,m_]:=Piecewise[{{A[0, 0], k == 0 && m == 0}, {A[2, 0], k == 2 && m == 0}, {A[4, 0], k == 4 && m == 0}, {I*B[5, 5], k == 5 && (m == -5 || m == 5)}, {A[6, 0], k == 6 && m == 0}}, 0]