Used a lot in quantum mechanics, where the equations are really hard to solve. There's even a dedicated wiki page for it: https://en.wikipedia.org/wiki/Perturbation_theory_(quantum_mechanics). Notably, Feynman diagrams are a way to represent perturbation calculations in quantum field theory.

Let's gather some of the best results we come across here:

- Numerical analysis | 97, 169, 1
- Mathematics | 17, 10k, 239
- Ciro Santilli's Homepage | 238, 147k, 2k

- Dirac equation solution for the hydrogen atom | 115
- Feynman diagram | 129, 136, 2
- Millennium Prize Problems | 244, 256, 1
- Lecture 1 | 240
- Schrodinger equation solution for the helium atom | 95, 188, 4