Are there any topics in theoretical CS that are more about pure math? 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.
Here are three more fields that fit your criteria.


Category theory. This is clearly interesting to most pure maths
fields, but also has been very influential in the theory of
(functional, sequential) programming languages.
Logic, particularly proof theory. The connections with computer
science are too many to name, but logic is not only a rich field of
pure mathematical, but the foundation of mathematics.
Number theory, the "queen of mathematics", which was deemed to
be devoid of applications ... until cryptography came along.