Akm[k_,m_]:=Piecewise[{{0, (k != 3 && k != 5) || (m != -3 && m != 3)}, {-A[3, 3], k == 3 && m == -3}, {A[3, 3], k == 3 && m == 3}, {-A[5, 3], k == 5 && m == -3}}, A[5, 3]]