Coupled differential equations generally present an important algebraic structure.
For example in the incompressible Navier-Stokes equations, the velocity is affected only by the selenoidal part of the applied force.
This structure can be translated naturally by the notion of complex.
One idea is then to exploit this complex structure at the discrete level in the creation of numerical methods.
The goal of the presentation is to expose the notion of complex by motivating its uses.
We will present in more detail the creation of a scheme for the Navier-Stokes equations and study its properties.