http://www.ece.jhu.edu/~cooper/courses/214/signalsandsystemsnotes.pdf
http://www3.nd.edu/~mhaenggi/ee344-04/overview.html
http://www.ee.ic.ac.uk/hp/staff/dmb/courses/DSPDF/00400_Systems.pdf
http://tintoretto.ucsd.edu/jorge/teaching/mae143a/lectures/5fourierseries.pdf
http://en.wikipedia.org/wiki/Discrete_Fourier_transform
http://mathworld.wolfram.com/DiscreteFourierTransform.html
http://www.robots.ox.ac.uk/~sjrob/Teaching/SP/l7.pdf
http://en.wikipedia.org/wiki/Discrete-time_Fourier_transform
https://engineering.purdue.edu/~bouman/ece438/lecture/module_1/1.3_fourier_analysis/1.3.3_dtft.pdf
http://www.ee.cityu.edu.hk/~hcso/ee3202_6.pdf
http://www.cems.uvm.edu/~gmirchan/classes/EE275/Mitra_4/examples/AddEx_Ch3.pdf
http://cnx.org/content/m0524/latest/
http://en.wikibooks.org/wiki/Engineering_Tables/DTFT_Transform_Table
http://ocw.usu.edu/electrical_and_computer_engineering/signals_and_systems/8_2node2.html