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://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