ABSTRACT
In this paper we discuss from a mathematical point of view some well known estimators. We compare these estimators for a simple two dimensional problem. In particular we consider the highly popular Z2 estimator deviced by Zienkiewicz and Zhu. Very probably, this is the most used one to date. Lots of papers report applications of this estimator to many different problems (see [ 19] , [20] for some references) . However, its mathematical foundations were not so solid . In this paper, we show that this estimator shares essentially the same good mathematical properties of the others .
