http://en.wikipedia.org/wiki/Nyquist_stability_criterion

http://www.ee.up.ac.za/main/_media/en/undergrad/subjects/ebb320/lecture20.pdf

http://en.wikibooks.org/wiki/Control_Systems/Nyquist_Stability_Criteria

http://courseware.ee.calpoly.edu/~fdepiero/fdepiero_dsp_notes/notes%20&%20materials/EE%20302%20Nyquist%20Stability.pdf

http://www.dcsc.tudelft.nl/~et2905/collegeslides/et2905_d2_lecture6_4up.pdf

http://bass.gmu.edu/~gbeale/ece_421/nyquist_01.html

http://www.chem.mtu.edu/~tbco/cm416/nyqsamp.html

http://www.stanford.edu/class/archive/ee/ee214/ee214.1032/Handouts/nyquist.pdf

http://control.ee.ethz.ch/~ifaatic/2012/references/IEEE_Trans._Automatic_Control_1980.pdf

http://www.google.com/url?sa=t&rct=j&q=problem%20for%20nyquist%20theorem%20stability&source=web&cd=33&cad=rja&ved=0CD0QFjACOB4&url=http%3A%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fdownload%3Fdoi%3D10.1.1.37.8068%26rep%3Drep1%26type%3Dpdf&ei=OwLoUMG4CszktQag5oHADA&usg=AFQjCNEepQYdqOHkupUq2ZRVKjFs8lprLw

http://www.google.com/url?sa=t&rct=j&q=problem%20for%20nyquist%20theorem%20stability&source=web&cd=46&cad=rja&ved=0CFUQFjAFOCg&url=http%3A%2F%2Fprofsite.um.ac.ir%2F~karimpor%2Fmulti%2FMultivariable_chapter4.ppt&ei=aQPoUImCGrD34QSOtIHIBQ&usg=AFQjCNE_iNai2oaV8iS-YJ9lzntJaIB1UQ