The complex input u is multiplied by the complex transfer function (depending on the angular frequency input w) to obtain the complex output y (nb = dimension of b, na = dimension of a):
b[1]*(jw)^[nb-1] + b[2]*(jw)^[nb-2] + ... + b[nb] y(jw) = ------------------------------------------------- * u(jw) a[1]*(jw)^[na-1] + a[2]*(jw)^[na-2] + ... + a[na]