On continuous choice of retractions onto nonconvex subsets

Details On continuous choice of retractions onto nonconvex subsets On continuous choice of retractions onto nonconvex subsets

2008-10-21

arxiv.org/abs/0810.3895v2

Topology Appl. 157:8 (2010), 1510-1517.

Mathematics

General Topology

Scientific paper

10.1016/j.topol.2009.04.066

For a Banach space $B$ and for a class $\A$ of its bounded closed retracts, endowed with the Hausdorff metric, we prove that retractions on elements $A \in \A$ can be chosen to depend continuously on $A$, whenever nonconvexity of each $A \in \A$ is less than $\f{1}{2}$. The key geometric argument is that the set of all uniform retractions onto an $\a-$paraconvex set (in the spirit of E. Michael) is $\frac{\a}{1-\a}-$paraconvex subset in the space of continuous mappings of $B$ into itself. For a Hilbert space $H$ the estimate $\frac{\a}{1-\a}$ can be improved to $\frac{\a (1+\a^{2})}{1-\a^{2}}$ and the constant $\f{1}{2}$ can be reduced to the root of the equation $\a+ \a^{2}+a^{3}=1$.

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