arXiv subjects
Publications and source records attributed to Arian Berdellima.
Weak topologies that yield weak convergence for bounded sequences and nets in CAT($0$) spaces have been studied in the past. We are here concerned with weak topologies that yield weak convergence of unbounded sequences and nets. We analyze two such topologies that generalize the weak topology on Hilbert spaces and that agree with the strong topology on a CAT($0$) space if and only if the space is locally compact.
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.
Let $(f^n),f$ be a sequence of proper closed convex functions defined on a Hadamard space. We show that the convergence of proximal mappings $J^n_{\lambda}x$ to $J_{\lambda}x$, under certain additional conditions, imply Mosco convergence of $f^n$ to $f$. This result is a converse to a theorem of Bacak about Mosco convergence in Hadamard spaces.