### Sobolev space: I

#### by Yi Zhang

Sobolev space is based on generalize derivatives and the corresponding norm induced. As a special case, space has the meaning of finite “intensity” in general physical sense. For example, if is the flow velocity in spatial domain , then means the function and its first order derivatives are all in , i.e.

Physically it means the** total intensity of the sources **within is finite, so is the

**within the domain. Obviously this requirement is fundamental for any engineering analysis.**

*total kinetic energy*Meyers and Serrin proved that is dense in under the Sobolev space norm. This means that the function in Sobolev space can be approximated using smooth functions, and in many occasions the properties of Sobolev space is obtained by first demonstrate it in .

The fundamental question of approximating the variational form of a PDE is twofold:

*How much the smoothness/differentiability one can have from the weak solution?*

*How close the boundary condition can be approximated?*

The first question is referred to as ** regularity problem**. The properties of embedding, and traces of the Sobolev spaces are fundamental to answer questions above. It must be emphasized that the regularity of a weak solution depends on the

*, the*

**data***, and the*

**geometry of domain***. All three factors must be examined in practical finite element convergence analysis.*

**boundary condition**