  The goal of this work is to introduce a local and a global interpolator in
Jacobi-weighted spaces, with optimal order of approximation in the context of
the $p$-version of finite element methods. Then, an a posteriori error
indicator of the residual type is proposed for a model problem in two
dimensions and, in the mathematical framework of the Jacobi-weighted spaces,
the equivalence between the estimator and the error is obtained on appropriate
weighted norm.
