In many important applications, what you have to solve is not just a single partial differential equation, but multiple partial differential equations coupled to each other. This is the case for many key PDEs including:

- Differential equation | 0, 2k, 48
- Calculus | 11, 4k, 96
- Mathematics | 17, 10k, 239
- Ciro Santilli's Homepage | 238, 147k, 2k

- FEniCS Project | 221, 329, 1