Hi All An LTI system is causal if and only if h(t)=0 for t<0. As derivation goes h(t)=h(t)sgn(t).Taking Freq transform both side and doing some manipulations.We get to point that H(jw) = Hilbert transform of itself. Then they say we can divide H(jw) into real and imaginary part.And real part will be even function and imaginary part will be odd function. Can you tell me why real part will be even function and imaginary part will be odd function? regards Piyush ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu