What is the hardest math you used in a project?

Ive done the C++ challenge to calculate pi deeply and quickly; also finding prime numbers. Those were fun to do as a beginner.

Much more recently, IBM qiskit has a bit too advanced math for me. Over my head; but theoretically i did use it lol. I cant say i understood it.

I imagine for example that some implementations were made fast enough to meet a requirement with some very hard algorithms.