Game theory

The nineteenth entry is o teorema popular, whose English name is « the folk theorem », and the middle word is the one a Portuguese reader is almost certain to read wrongly. Popular and « popular » match everywhere else in the language, and here they do not match at all. « Folk » is an old Germanic word for a people, the cousin of German Volk, and in the phrases English now builds with it the sense is never widespread and never well liked. It means handed down with no author attached. « A folk song » is a song nobody signed, « folk medicine » is what was passed on without a book, « a folk etymology » is an explanation the speakers made up for themselves, and « folklore » is the whole body of such material. The theorem carries the name for exactly that reason: the result went round among the people working on repeated games long before anybody wrote a proof down, so no name could be put on it. Rendering the term back as « the popular theorem » turns a fact about authorship into a claim about fame.