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