Positive real lemma: necessary and sufficient conditions for the existence of solutions under virtually no assumptions. Ferrante, A. IEEE Trans. Automat. Contr., 50(5):720-724, 2005.  
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Paper  bibtex   @article{journals/tac/Ferrante05,
  added-at = {2012-11-19T00:00:00.000+0100},
  author = {Ferrante, Augusto},
  biburl = {http://www.bibsonomy.org/bibtex/2b5b52dabbdd255d6dced2b241057d265/dblp},
  ee = {http://dx.doi.org/10.1109/TAC.2005.847036},
  interhash = {cff6d354c1c410aad4f61c19373e508e},
  intrahash = {b5b52dabbdd255d6dced2b241057d265},
  journal = {IEEE Trans. Automat. Contr.},
  keywords = {dblp},
  number = 5,
  pages = {720-724},
  timestamp = {2012-11-20T11:45:54.000+0100},
  title = {Positive real lemma: necessary and sufficient conditions for the existence of solutions under virtually no assumptions.},
  url = {http://dblp.uni-trier.de/db/journals/tac/tac50.html#Ferrante05},
  volume = 50,
  year = 2005
} 
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