Eike Neumann - Computational Problems in Metric Fixed Point Theory and their Weihrauch Degrees

lmcs:1621 - Logical Methods in Computer Science, December 29, 2015, Volume 11, Issue 4 - https://doi.org/10.2168/LMCS-11(4:20)2015
Computational Problems in Metric Fixed Point Theory and their Weihrauch Degrees

Authors: Eike Neumann

    We study the computational difficulty of the problem of finding fixed points of nonexpansive mappings in uniformly convex Banach spaces. We show that the fixed point sets of computable nonexpansive self-maps of a nonempty, computably weakly closed, convex and bounded subset of a computable real Hilbert space are precisely the nonempty, co-r.e. weakly closed, convex subsets of the domain. A uniform version of this result allows us to determine the Weihrauch degree of the Browder-Goehde-Kirk theorem in computable real Hilbert space: it is equivalent to a closed choice principle, which receives as input a closed, convex and bounded set via negative information in the weak topology and outputs a point in the set, represented in the strong topology. While in finite dimensional uniformly convex Banach spaces, computable nonexpansive mappings always have computable fixed points, on the unit ball in infinite-dimensional separable Hilbert space the Browder-Goehde-Kirk theorem becomes Weihrauch-equivalent to the limit operator, and on the Hilbert cube it is equivalent to Weak Koenig's Lemma. In particular, computable nonexpansive mappings may not have any computable fixed points in infinite dimension. We also study the computational difficulty of the problem of finding rates of convergence for a large class of fixed point iterations, which generalise both Halpern- and Mann-iterations, and prove that the problem of finding rates of convergence already on the unit interval is equivalent to the limit operator.


    Volume: Volume 11, Issue 4
    Published on: December 29, 2015
    Submitted on: December 8, 2013
    Keywords: Mathematics - Logic,Computer Science - Logic in Computer Science

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    Source : ScholeXplorer IsReferencedBy DOI 10.1007/978-3-030-14812-6_23
    • 10.1007/978-3-030-14812-6_23
    • 10.1007/978-3-030-14812-6_23
    Finite Choice, Convex Choice and Sorting
    Kihara, Takayuki ; Pauly, Arno ;

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