Abstract:
Let (U,ρ) be a complete metric space and let Rp(R,U),p⩾1, and R(R,U) be the spaces of (strongly) measurable functions f:R→U for which the Bochner transforms R∋t↦fBl(t;⋅)=f(t+⋅) are recurrent functions with ranges in the metric spaces Lp([−l,l],U) and L1([−l,l],(U,ρ′)) where l>0, and (U,ρ′) is the complete metric space with the metric ρ′(x,y)=min{1,ρ(x,y)}, x,y∈U. Analogously, we define the spaces Rp(R,clbU) and R(R,clbU) of functions (multivalued mappings) F:R→clbU with ranges in the complete metric space (clbU,dist) of nonempty closed bounded subsets of the metric space (U,ρ) with the Hausdorff metric dist (while defining the multivalued mappings F∈R(R,clbU) the metric dist′(X,Y)=min{1,dist(X,Y)}, X,Y∈clbU, is also considered). We prove the existence of selectors f∈R(R,U) (accordingly f∈Rp(R,U)) of multivalued maps F∈R(R,clbU) (accordingly F∈Rp(R,clbU)) for which the sets of almost periods are subordinated to the sets of almost periods of multivalued maps F. For functions g∈R(R,U), the conditions for the existence of selectors f∈R(R,U) and f∈Rp(R,U) such that ρ(f(t),g(t))=ρ(g(t),F(t)) for a.e. t∈R are obtained. On the assumption that the function g and the multivalued map F have relatively dense sets of common ε-almost periods for all ε>0, we also prove the existence of selectors f∈R(R,U) such that ρ(f(t),g(t))⩽ρ(g(t),F(t))+η(ρ(g(t),F(t))) for a.e. t∈R, where η:[0,+∞)→[0,+∞) is an arbitrary nondecreasing function for which η(0)=0 and η(ξ)>0 for all ξ>0, and, moreover, f∈Rp(R,U) if F∈Rp(R,clbU). To prove the results we use the uniform approximation of functions f∈R(R,U) by elementary functions belonging to the space R(R,U) which have the sets of almost periods subordinated to the sets of almost periods of the functions f.
Citation:
L. I. Danilov, “Recurrent and almost recurrent multivalued maps and their selections. III”, Vestn. Udmurtsk. Univ. Mat. Mekh. Komp. Nauki, 2014, no. 4, 25–52