Let E be a real normed linear space. A subset X\subset E is called a retract of E if there exists a continuous mapping
r:E\rightarrow X, a retraction, satisfying r(x)=x,\ x\in X. It is well known that every nonempty closed convex subset of E is a retract of E. Nonconvex retracts are studied in this paper.

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Li, Pengcheng, Zhang, Guowei