Akm[k_,m_]:=Piecewise[{{0, k == 0 && m == 0}, {A[2, 2] - I*B[2, 2], k == 2 && m == -2}, {-A[2, 1] + I*B[2, 1], k == 2 && m == -1}, {A[2, 0], k == 2 && m == 0}, {A[2, 1] + I*B[2, 1], k == 2 && m == 1}, {A[2, 2] + I*B[2, 2], k == 2 && m == 2}, {A[4, 4] - I*B[4, 4], k == 4 && m == -4}, {-A[4, 3] + I*B[4, 3], k == 4 && m == -3}, {A[4, 2] - I*B[4, 2], k == 4 && m == -2}, {-A[4, 1] + I*B[4, 1], k == 4 && m == -1}, {A[4, 0], k == 4 && m == 0}, {A[4, 1] + I*B[4, 1], k == 4 && m == 1}, {A[4, 2] + I*B[4, 2], k == 4 && m == 2}, {A[4, 3] + I*B[4, 3], k == 4 && m == 3}, {A[4, 4] + I*B[4, 4], k == 4 && m == 4}}, 0]