A fixed-point theorem of Krasnoselskii
Krasnosel'skij's fixed-point theorem asks for a convex set \(M\) and a mapping \(Pz=Bz+Az\) such that (i) \(Bx+Ay\in M\) for each \(x,y\in M\), (ii) \(A\) is continuous and compact, (iii) \(B\) is a contraction. Then \(P\) has a fixed point. A careful reading of the proof reveals that (i) need only ask that \(Bx+Ay\in M\) when \(x=Bx+Ay\). The proof also yields a technique for showing that such \(x\) is in \(M\).
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