I am a graduate student in theoretical computer science, and in particular, approximation algorithms. I find now that I am more interested in pure math (I can say this because I seem to have enjoyed math courses more than the CS courses). I would like to ask if there are areas in theoretical computer science which are pretty much pure math (to be more precise, an area that is of interest in pure math on it's own without considering the applications to CS), or if I need to consider a major switch. I am already two and a half years into the program, so I am not sure if a switch would be a good idea at this point.
The only such thing I could find was graph minor theory, from browsing acceptance lists of top conferences. But that doesn't count as an 'area' for me that I can just focus on.