http://en.wikipedia.org/wiki/Positive-real_function

https://ccrma.stanford.edu/~jos/pasp/Positive_Real_Functions.html

https://ccrma.stanford.edu/~jos/prf/

http://personal.strath.ac.uk/yixiang.xu/chen_smith.pdf

http://www.egr.msu.edu/~khalil/NonlinearSystems/Sample/Lect_15.pdf

http://www-bcf.usc.edu/~ioannou/2003update/d12.pdf

http://www.ece.unb.ca/jtaylor/Publications/ieee_tcs74.pdf

https://ccrma.stanford.edu/~jos/prf/prf.pdf

http://vmg.pp.ua/books/%D0%A2%D0%B5%D1%85%D0%BD%D0%B8%D1%87%D0%B5%D1%81%D0%BA%D0%B8%D0%B5%D0%9D%D0%B0%D1%83%D0%BA%D0%B8/%D0%A6%D0%B8%D1%84%D1%80%D0%BE%D0%B2%D0%B0%D1%8F%20%D0%BE%D0%B1%D1%80%D0%B0%D0%B1%D0%BE%D1%82%D0%BA%D0%B0%20%D1%81%D0%B8%D0%B3%D0%BD%D0%B0%D0%BB%D0%BE%D0%B2/%D0%90%D0%BD%D0%B3%D0%BB/The_CircuitsANDFilters_Handbook,2003/0912%20ch73%20.pdf

http://icta05.teithe.gr/papers/06.pdf

http://dml.cz/bitstream/handle/10338.dmlcz/103253/AplMat_14-1969-6_1.pdf

http://www-control.eng.cam.ac.uk/Homepage/papers/cued_control_1075.pdf