http://www.eng.ucy.ac.cy/cpitris/courses/ece623/notes/SignalsAndSystems.pdf
http://users.ece.gatech.edu/~bonnie/book/worked_problems.html
http://florida.theorangegrove.org/og/file/e66b88b8-2801-ee81-1d88-cacd33185dd7/1/Signals.pdf
http://videoprocessing.ucsd.edu/~stanleychan/class/Su11/Book.pdf
http://books.google.com/books/about/Signals_And_Systems.html?id=lgx3o2zQm64C
http://www.dspguide.com/ch3/2.htm
http://books.google.com/books/about/Signals_And_Systems.html?id=VgUtRFHXQicC
http://books.google.com/books/about/Signals_And_Systems.html?id=LDadfi_t5xgC
http://books.google.com/books/about/An_Introduction_to_Signal_and_System_Ana.html?id=y5kKqwTHr9sC
http://books.google.com/books/about/Textbook_Of_Signals_And_Systems_Revised.html?id=42mmLVjCEkMC