Nonexpansive retractions in Hilbert spaces
Kazimierz Goebel, Ewa Sędłak · Annales Universitatis Mariae Curie-Sklodowska sectio A – Mathematica · 2009
Let H be a Hilbert space and C H be closed and convex. The mapping P : H ! C known as the nearest point projection is nonexpansive (1-lipschitzian). We observed that, the natural question: Are there nonex- pansive projections Q : H! C other than P? is neglected in the literature. Also, the answer is not often present in the folklore of the Hilbert space theory. We provide here the answer and discuss some facts connected with the subject.