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}}, 0]