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Chebyshev Type II analog lowpass filter prototype

`[z,p,k] = cheb2ap(n,Rs)`

`[z,p,k] = cheb2ap(n,Rs)` finds
the zeros, poles, and gain of an order `n` Chebyshev
Type II analog lowpass filter prototype with stopband ripple `Rs` dB down from the passband peak value. `cheb2ap` returns
the zeros and poles in length `n` column vectors `z` and `p` and
the gain in scalar `k`. If `n` is
odd, `z` is length `n-1`. The transfer
function is

Chebyshev Type II filters are monotonic in the passband and
equiripple in the stopband. The pole locations are the inverse of
the pole locations of `cheb1ap`,
whose poles are evenly spaced about an ellipse in the left half plane.
The Chebyshev Type II stopband edge angular frequency *ω*_{0} is
set to 1 for a normalized result. This is the frequency at which the
stopband begins and the filter has magnitude response of 10^{–Rs/20}.

[1] Parks, Thomas W., and C. Sidney Burrus. *Digital
Filter Design*. New York: John Wiley & Sons, 1987,
chap. 7.

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