If I were you, I would start to independently learn matroid theory (I think Günther Ziegler has a book or notes on this), topology (I guess Munkres for basic topology) and algebraic geometry (try Hartshorne's book). If you do theoretical CS, I'm sure that with enough motivation you can know the relevant areas of these fields as well as math students studying these fields (I know many math students who don't know any of these fields besides some basic topology). I like the idea of learning things as you encounter them in research in order to understand their significance. If there is a math course which is relevant to your research, maybe you can take it. Often times courses provide good structure.

Problem solved: several researchers from my institute doing my field are willing to take me on as a student (I did write to someone outside my institute but he declined). It seems that I just got out of the bad timing (one of them has a student who is graduating, another one is close to finishing an administrative position, and another solved a personal matter). My reluctance to leave really did stem from the name of the institute (highly ranked in my field by qs and shanghai rankings and has several award winning professors and professors with large grants), the lack of other viable options for a phd in my country which has a very small academic community especially in my field, and the fact that I really enjoy being there (sorry this is all I can disclose without giving up anonymity and the exact name of the university and field). I guess this goes to show that even great institutes can fall on bad timing (which sometimes lasts just a few weeks or months). As a side note, I can say that a multidisciplinary PhD would mean changing my field of research entirely (probably gave up my field of research there but oh well). I did consider doing a multidisciplinary in departmental PhD with 2 advisors from my department in different fields. Thanks for your comments anyways and I hope someone here can use my experience to learn something 🙂

Hi all! I recently graduated my Msc at a top university and after graduating my advisor told me that I cannot do a phd with her since she took an administrative position but she will recommend me to anyone I choose. I started to look for an advisor in my field in my university (which is considered to be one of the best in the world in my field). There are 5 people in my exact field at my university and all of them have a legitimate reason not to take students now, whether it be an administrative position, too many students, no good phd research problems or personal problems. So I have to either move to another field or to another university, both of which I really don't want to do since I really like the research group of my field at my university and I know that no other group like this exists in my country (my university is the best in my country and better than all others in my country by a long shot). I also really enjoy my field of research and am good at it, and I don't have good background for other fields. I thought of going abroad, but I don't have the money (not an EU citizen and I don't live in the USA... I cannot disclose more than this to keep some degree of anonymity) and will have to wait 6 months to 1.5 years to start a PhD. I know that no one is to blame for this situation, not me and not any professor, but it seems like such a shame that I will have to leave a field that I love or a research group that I love due to things that have nothing to do with me. Has anyone else been in this situation? Do you have tips on what to do?