>>11607668not him, but it depends on what you want to do. Generally now most mathematics is useful in TCS in some way, but the most mainstream topics will benefit from the following:
Combinatorics
Graph Theory
Number theory
Probability theory
Stochastic processes
Recursion theory and classic logic
Abstract Algebra (ring theory, representation theory, algebraic number theory are most used)
Linear Algebra (obviously proof based)
Less 'mainstream' but also very useful / interesting / well studied subfields:
Analysis (complex, sometimes real as well) (theory of probabilistic algorithms)
Analytic combinatorics (complicated structures, average case analysis)
Algebraic topology (comp topology, distributed networks, graphics and noise)
Analytic number theory (cryptography, codes, but there needs to be more connective work here)
Algebraic geometry (communication complexity, recently biggest lead on separation bounds between P vs NP)
Differential geometry (Vision and learning theory, animation and graphics, computational geometry)
What are your interests anon?
