I read [here][1]
the following:

> The parametrix is a useful concept in the study of elliptic
> differential operators and, more generally, of hypoelliptic
> pseudodifferential operators with variable coefficient, since for such
> operators over appropriate domains a parametrix can be shown to exist,
> can be somewhat easily constructed and be a smooth function away
> from the origin. Having found the analytic expression of the
> parametrix, it is possible to compute the solution of the associated
> fairly general elliptic partial differential equation by solving an
> associated Fredholm integral equation: also, the structure itself of
> the parametrix reveals properties of the solution of the problem
> without even calculating it, like its smoothness and other
> qualitative properties.

Does anyone know any reference (books/papers) where I can find more infomation about using parametrix and Fredholm integral equations for attacking hypoellipticity problems?

Thanks

  [1]: https://en.wikipedia.org/wiki/Parametrix