I am writing my PhD thesis in mathematics.
It happens frequently that I use results that I don't have a reference for except
- textbooks stating "it is easily seen that ..." or "it is well-known that..."
- it is stated on Wikipedia (without proof)
- I have derived it with colleagues or people from Q&A sites
Then I try to find a source for some time, before I write down a proof by myself. However, most probably, there has been an earlier mention of this result that I could reference instead.
How should I proceed in such cases?