More specifically, folk mathematics, or mathematical folklore, is the body of theorems, definitions, proofs, facts or techniques that circulate among mathematicians by word of mouth, but have not yet appeared in print, either in books or in scholarly journals.
[1] Quite important at times for researchers are folk theorems, which are results known, at least to experts in a field, and are considered to have established status, though not published in complete form.
The problems form the "folklore" of ring theory, and the solutions are given in as much detail as possible.
[2]Another distinct category is well-knowable mathematics, a term introduced by John Conway.
[3] These mathematical matters are known and factual, but not in active circulation in relation with current research (i.e., untrendy).