I am self-studying functional analysis. As far as I know, a best approximation of $X$ by a closed subspace $C \subseteq X$ exists and is unique if $X$ is a Hilbert space, a uniformly convex Banach space, or more generally, a reflexive space.
My question is: is there a even more general condition on the space $X$ for the existence and uniqueness of the best approximation of $X$ by a closed and convex subspace $C \subseteq X$?
Any comments or suggested readings are welcome.
Best Answer
Sharper results: Best Approximation by Closed Sets in Banach Spaces and On a sufficient condition for proximity by Ka Sing Lau. Quote from the first: