How to find only negative root of any polynomial

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I am looking for a filter design. and from all possible roots of transfer function, i have to filter only left half s-plane (negative) roots. please suggest any code as "roots()" gives all +ve and _ve roots

Answers (2)

Azzi Abdelmalek
Azzi Abdelmalek on 7 Nov 2014
r=roots(your_array)
r=r(r<0))

Mikhail
Mikhail on 7 Nov 2014
If you have symbolic math toolbox, function vpasolve will find all roots. After that you can filter it with something like:
a=vpasolve(eqn);
b=a(find(real(a)<0));

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