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:

- Partial differential equation | 0, 2k, 45
- Differential equation | 0, 3k, 66
- Calculus | 17, 7k, 159
- Mathematics | 17, 13k, 329
- Ciro Santilli's Homepage | 262, 181k, 3k

- FEniCS Project | 233, 372, 1
- Lagrangian mechanics | 907, 2k, 19
- Lagrangian vs Hamiltonian | 105