Take the 2-minute tour ×  In this post, Justin gives a quote
about Raoul Bott that has this line in it:  He talked about
'folk' theorems... theorems everyone knew, but were never
written down.  What are some good/interesting examples of these
types of theorems?  share|improve this question I gave a sort of
weak example on page 1 of
math.uga.edu/~pete/coveringnumbersv2.pdf -- this is a simple
linear algebra result that often gets asked as a problem (e.g.
on MO!) but is rarely discussed in standard texts. –  Pete L.
Clark Jul 18 '10 at 23:18 Any response to this question ceases
to be an example! –  Qiaochu Yuan Sep 12 '11 at 1:01  11 Answers
11  The example I first learned was the following: a 2-D TQFT is
equivalent to a Frobenius algebra.  This is discussed and stated
as a folk theorem by Voronov; later, a careful proof was written
up and published by Lowell Abrams. See also the book by Joachim
Kock.  share|improve this answer I think it was also proved in a
paper of Sawin: ams.org/mathscinet-getitem?mr=135965