Javier Ignacio Carbó Rubiera José M. Molina Jorge Dávila Muro Fuzzy referral based cooperation in social networks of agents. 1-13 2005 18 AI Commun. 1 http://iospress.metapress.com/openurl.asp?genre=article&issn=0921-7126&volume=18&issue=1&spage=1 db/journals/aicom/aicom18.html#CarboMM05