http://www.ece.jhu.edu/~cooper/courses/214/signalsandsystemsnotes.pdf

http://www.eas.uccs.edu/wickert/ece2610/lecture_notes/ece2610_chap9.pdf

https://engineering.purdue.edu/~mikedz/ee301/OW_LTI_SystemProps.pdf

http://www.stanford.edu/class/ee261/reader/chapter7.pdf