How free are you to take mathematics courses as a computer science graduate student?
This is frequently very easy: the boundary between mathematics and computer science is pretty blurry, especially for the more theoretical side of computer science.  In many universities, you will even find classes that are shared between math and computer science departments.  

I suppose that If you are a masters' student, you might run into problems fitting in the classes that you want to take if there are a lot of other program requirements.  For Ph.D. students, however, there are typically less constraints.