http://en.wikipedia.org/wiki/LTI_system_theory
http://158.132.21.81/class/EIE312/LS102_LTIS.pdf
http://web.eecs.umich.edu/~aey/eecs206/lectures/lti2.pdf
http://itee.uq.edu.au/~coms3100/Lecture%20Notes/lec04.pdf
http://melodi.ee.washington.edu/courses/ee518/notes/lec2.pdf
http://tintoretto.ucsd.edu/jorge/teaching/mae143a/lectures/3timedomainanalysis.pdf
http://www.cis.hut.fi/Opinnot/T-61.140/Luennot/Ch2.pdf
http://faemino.ucsd.edu/~soniamartinez/teaching/143a_SignalsSystems/data/impulse-response.pdf
http://www.csie.nctu.edu.tw/~cmliu/Courses/audio/zfreqmap.pdf
http://seit.unsw.adfa.edu.au/staff/sites/valu/TEACHING/SS/lect10-Ztransform.pdf