What is the hardest math problem?

There is no definitive answer to what the hardest math problem is, as it largely depends on the individual's level of expertise and familiarity with certain mathematical concepts. However, some of the most notoriously difficult math problems include:

1. The Riemann Hypothesis: A famous conjecture in number theory that deals with the distribution of prime numbers.

2. The Birch and Swinnerton-Dyer Conjecture: A problem in algebraic number theory related to elliptic curves.

3. The Collatz Conjecture: A simple yet unsolved problem in number theory that involves iterating a specific mathematical operation.

4. The P vs NP Problem: A major open question in theoretical computer science that deals with the efficiency of algorithms.

5. The Navier-Stokes Existence and Smoothness Problem: A problem in fluid dynamics that seeks to understand the behavior of viscous fluids.

These are just a few examples of difficult mathematical problems that have yet to be fully solved. Each problem presents its own unique challenges and has stumped mathematicians for decades, if not centuries.