Sergiu Rudeanu Dragos Vaida Semirings in Operations Research and Computer Science: More Algebra. 61-85 2004 61 Fundam. Inform. 1 http://iospress.metapress.com/openurl.asp?genre=article&issn=0169-2968&volume=61&issue=1&spage=61 db/journals/fuin/fuin61.html#RudeanuV04