http://digital.library.adelaide.edu.au/dspace/bitstream/2440/70793/1/02whole.pdf

http://www.dtic.mil/dtic/tr/fulltext/u2/a439395.pdf

http://www.analog.com/library/analogdialogue/archives/43-09/EDCh%208%20filter.pdf

http://www.google.com/url?sa=t&rct=j&q=positive%20real%20function&source=web&cd=13&cad=rja&ved=0CDsQFjACOAo&url=http%3A%2F%2Fwww.springer.com%2Fcda%2Fcontent%2Fdocument%2Fcda_downloaddocument%2F9781846285165-c1.pdf%3FSGWID%3D0-0-45-317047-p173662722&ei=3Xb5UKnnF8jC0QWLhIHoDw&usg=AFQjCNH4J6zeDyLDxfsJVBqz43h1WRjgsA

http://books.google.com/books?id=2DEft_wNLjQC&pg=SA8-PA13&lpg=SA8-PA13&dq=positive+real+function&source=bl&ots=ZxyO3UOg3z&sig=egRRPnOP6Qrs-V7EdmMDugYkgbE&hl=en&sa=X&ei=3Xb5UKnnF8jC0QWLhIHoDw&ved=0CGYQ6AEwCTgK

http://www.dtic.mil/dtic/tr/fulltext/u2/634764.pdf

http://inst.eecs.berkeley.edu/~ee247/fa05/lectures/L2_f05.pdf