I'm presently finishing up my undergraduate degree in Mathematics, and have written a number of papers, three of which have been published in reputable combinatorial journals (think journals published by Elsevier and Springer). From these three, I am the sole author on two of them.
Given that I have invested so much time and energy on 'extra curricular activities' (as my head of department calls them), can they serve as the basis to a PhD thesis? Which is to say, can a considerable part of a thesis consist of previous work which has not been used to obtain any other degree?
This final remark is important, I feel. A fourth paper with my supervisor is in the works, but the material presented there will appear in my undergraduate dissertation. I am then of the opinion that this material should not be used toward another degree.
If it helps, I attend a northern European university, and intend on doing a PhD in Europe/UK/USA.
Edit: I would like to perhaps mention the motivation for asking this question. I have heard stories of people doing research in their private capacity, then doing a PhD in quite a short period of time (a year or so), since they had a lot of material already. I cannot imagine this being the case however. What are your thoughts on this?