Nonexpansive maps in nonlinear smooth spaces

Abstract

We introduce the notion of a nonlinear smooth space generalizing both CAT ⁡ ( 0 ) \operatorname {CAT}(0) spaces as well as smooth Banach spaces. We show that this notion allows for a unified treatment of several results in functional analysis. Namely, we substantiate the usefulness of this setting by establishing a nonlinear generalization of an important result due to Reich in Banach spaces. On par with the linear context, this nonlinear version entails a convergence proof of several other methods. Here, we establish the convergence of a general form of the Halpern-type schema for resolvent-like families of functions. We furthermore prove the convergence of the viscosity generalization of Halpern’s iteration (even for families of maps) generalizing a result due to Chang. This work is set in the context of the ‘proof mining’ program, and the results are complemented with quantitative information like rates of convergence and of metastability (in the sense of T. Tao).