[RE,IM] = NYQUIST(SYS,W) and [RE,IM,W] = NYQUIST(SYS) return the
real parts RE and imaginary parts IM of the frequency response
(along with the frequency vector W if unspecified). No plot is
drawn on the screen. If SYS has NY outputs and NU inputs,
RE and IM are arrays of size [NY NU LENGTH(W)] and the response
at the frequency W(k) is given by RE(:,:,k)+j*IM(:,:,k).
help nyquist得到的