On a notion of averaged operators in CAT(0) spaces

Abstract

Averaged operators have played an important role in fixed point theory in Hilbert spaces. They emerged as a necessity to obtain solutions to fixed point problems where the underlying operator is not contractive and thus renders Banach fixed point theorem inaccessible. We introduce a notion of averaged operator in the broader class of $\text{CAT}(0)$ spaces. We call these operators $\alpha$-firmly nonexpansive and develop basic calculus rules for the quasi $\alpha$-firmly nonexpansive operators. In particular compositions of quasi $\alpha$-firmly nonexpansive operators is quasi $\alpha$-firmly nonexpansive and convex combination of a finite family of quasi $\alpha$-firmly nonexpansive operators is again quasi $\alpha$-firmly nonexpansive. For a nonexpansive operator $T:X\to X$ acting on a $\text{CAT}(0)$ space $X$ we show that the iterates $x_n:=Tx_{n-1}$ converge weakly to some element in the fixed point set $\text{Fix} T$ whenever $T$ is quasi $\alpha$-firmly nonexpansive. Moreover under a certain regularity condition the projections $P_{\text{Fix} T}x_n$ converge strongly to this weak limit. Our theory is illustrated with two classical examples of cyclic and averaged projections.