 ### Connected choice and the Brouwer fixed point theorem / Vasco Brattka; Stéphane Le Roux; Joseph S. Miller; Arno Pauly

Journal of Mathematical Logic, Volume: 19, Issue: 01, Start page: 1950004

Swansea University Author:

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DOI (Published version): 10.1142/s0219061319500041

Abstract

We study the computational content of the Brouwer Fixed Point Theorem in the Weihrauch lattice. Connected choice is the operation that finds a point in a non-empty connected closed set given by negative information. One of our main results is that for any fixed dimension the Brouwer Fixed Point Theo...

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Published in: Journal of Mathematical Logic 0219-0613 1793-6691 World Scientific Pub Co Pte Lt 2019 https://cronfa.swan.ac.uk/Record/cronfa48651 No Tags, Be the first to tag this record!
Abstract: We study the computational content of the Brouwer Fixed Point Theorem in the Weihrauch lattice. Connected choice is the operation that finds a point in a non-empty connected closed set given by negative information. One of our main results is that for any fixed dimension the Brouwer Fixed Point Theorem of that dimension is computably equivalent to connected choice of the Euclidean unit cube of the same dimension. Another main result is that connected choice is complete for dimension greater than or equal to two in the sense that it is computably equivalent to Weak Kőnig's Lemma. While we can present two independent proofs for dimension three and upwards that are either based on a simple geometric construction or a combinatorial argument, the proof for dimension two is based on a more involved inverse limit construction. The connected choice operation in dimension one is known to be equivalent to the Intermediate Value Theorem; we prove that this problem is not idempotent in contrast to the case of dimension two and upwards. We also prove that Lipschitz continuity with Lipschitz constants strictly larger than one does not simplify finding fixed points. Finally, we prove that finding a connectedness component of a closed subset of the Euclidean unit cube of any dimension greater or equal to one is equivalent to Weak Kőnig's Lemma. In order to describe these results, we introduce a representation of closed subsets of the unit cube by trees of rational complexes. Computable analysis; Weihrauch lattice; reverse mathematics; choice principles; connected sets; fixed point theorems 01 1950004