Porosity of perturbed optimization problems in Banach spaces ✩

Let X be a Banach space and Z a nonempty closed subset of X. Let J :Z → R be a lower semicontinuous function bounded from below. This paper is concerned with the perturbed optimization problem infz∈Z{J (z)+ ‖x − z‖}, denoted by (x, J )-inf for x ∈X. In the case when X is compactly fully 2-convex, it is proved in the present paper that the set of all points x in X for which there does not exist z0 ∈ Z such that J (z0)+‖x− z0‖ = infz∈Z{J (z)+‖x− z‖} is a σ -porous set in X. Furthermore, if X is assumed additionally to be compactly locally uniformly convex, we verify that the set of all points x ∈X \Z0 such that the problem (x, J )-inf fails to be approximately compact, is a σ -porous set in X \ Z0, where Z0 denotes the set of all z ∈Z such that z ∈ PZ(z). Moreover, a counterexample to which some results of Ni [R.X. Ni, Generic solutions for some perturbed optimization problem in nonreflexive Banach space, J. Math. Anal. Appl. 302 (2005) 417–424] fail is provided. © 2005 Elsevier Inc. All rights reserved.