Almost all topics in Numerical analysis need to use mathematics and computer science simultaneously.
Implementation of mathematical algorithms required computations using computers to study their efficiency, complexity, and convergence.
Also, there are topics in applied mathematics as (Ordinary and partial differential equations) required algorithms to find their numerical solutions and hence computers are needed.
My favorite topic in this concern is to study primality tests in NUMBER THEORY ( Prime numbers, Elliptic curves, recurrences,etc.) and this required to determine their complexity which is very useful in the vital topic in computer science CRYPTOGRAPHY.