Some details about function "buttord"

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feng Winner
feng Winner 2012-12-9
As we all know, function "buttord" is used to returns the order N and the "3 dB frequency" of the lowest digital Butterworth filter. In the function prototype, at first, prewarp frequencies from digital (unit circle) to analog. In this part, there is:
if strcmp(opt,'z') % digital
WP=tan(pi*wp/2);
WS=tan(pi*ws/2);
However, I remember that word in my book. "Usually,In order to make the frequency of analog filter has one-to-one correspondence with any frequency of digital filter, we need to improt a constant c. which is:
Ω=c*tan(Ω1*T/2);
So, in the function "buttord", c is used in the value of 1. That confuses me so much. How could this happen or what does the writer mean?

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