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Acta Mathematica Vietnamica

APPROXIMATING SOLUTIONS OF THE EQUATION x=T(x,x)

icon-email W. A. KIRK

Abstract

Let D be a bounded closed convex subset of a Banach space, and let T:D×DD be a continuous mapping which satisfies for all x,y,z,tD
T(x,y)T(z,t)max{xz,yt}
with strict inequality holding when xzyt. Suppose T condensing in the sense that
γ(T(U,V))<max{γ(U),γ(V)}
for subsets U,V of D for which  γ(UV)>0 (where γ denotes the usual Kuratowski set-measure of noncompactness). A projection-iteration method is shown to converge to a solution of x=T(x,x). The significance of this result is that it holds in arbitrary spaces.