Akm[k_,m_]:=Piecewise[{{0, (k != 5 && (((k != 1 || m != 0) && k != 3) || (m != -2 && m != 0 && m != 2))) || (m != -4 && m != -2 && m != 0 && m != 2 && m != 4)}, {A[1, 0], k == 1 && m == 0}, {A[3, 2], k == 3 && (m == -2 || m == 2)}, {A[3, 0], k == 3 && m == 0}, {A[5, 4], k == 5 && (m == -4 || m == 4)}, {A[5, 2], k == 5 && (m == -2 || m == 2)}}, A[5, 0]]