http://www.ece.ucsb.edu/courses/courses/ECE146/146A_W11Madhow/signals_systems_dec2010.pdf
http://books.google.com/books/about/Signals_And_Systems.html?id=NnlecwBlMZgC
http://courses.ece.ubc.ca/359/lectures/chapter2.pdf
http://www.dspguide.com/CH5.PDF