[itissid]

I think that most of the times they are particular to a domain to solve particular problem. For example Some researcher would have a grant for developing an optical inspection of a circuit board using a camera moving on the 2d euclidean space efficiently is a TSP. You can develop a Polynomial Time approximation scehne(PTAS) to solve it.

At first it's novel then st some point it appears in a book specific to polynomial time approximation schems in books dedicated to the cause. Look at the references in wikipedia. They often point to a larger body of work than what the wiki can possibly hope to cover.