Which noun express the action of solving a problem?

When I want to express the action or process of solving a (mathematical) problem, I always doubt among using "the problem resolution", "the problem solving" or "the problem solution". Which is the most suitable one?

For example, in the phrase "the problem resolution can be very challenging due to the expensive iterations of the algorithm", I believe that "problem solution" would not be correct: the solution is not challenging (it can be can be very simple, for example, the number 25), it is the process of finding the solution that is challenging.