Projectors and Projection Methods

The projectors are considered as simple but important type of matrices and operators. Their basic theory can be found in many books, among which Hal­ mas [177], [178] are of particular significance. The projectors or projections became an active research area in the last two decades due to ideas generated from linear algebra, statistics and various areas of algorithmic mathematics. There has also grown up a great and increasing number of projection meth­ ods for different purposes. The aim of th