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Computing All Maps into a Sphere

Published: 02 June 2014 Publication History

Abstract

Given topological spaces X,Y, a fundamental problem of algebraic topology is understanding the structure of all continuous maps XY. We consider a computational version, where X,Y are given as finite simplicial complexes, and the goal is to compute [X,Y], that is, all homotopy classes of such maps. We solve this problem in the stable range, where for some d ≥ 2, we have dim X ≤ 2d−2 and Y is (d-1)-connected; in particular, Y can be the d-dimensional sphere Sd. The algorithm combines classical tools and ideas from homotopy theory (obstruction theory, Postnikov systems, and simplicial sets) with algorithmic tools from effective algebraic topology (locally effective simplicial sets and objects with effective homology). In contrast, [X,Y] is known to be uncomputable for general X,Y, since for X=S1 it includes a well known undecidable problem: testing triviality of the fundamental group of Y.
In follow-up papers, the algorithm is shown to run in polynomial time for d fixed, and extended to other problems, such as the extension problem, where we are given a subspace AX and a map AY and ask whether it extends to a map XY, or computing the ℤ2-index—everything in the stable range. Outside the stable range, the extension problem is undecidable.

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cover image Journal of the ACM
Journal of the ACM  Volume 61, Issue 3
May 2014
262 pages
ISSN:0004-5411
EISSN:1557-735X
DOI:10.1145/2628069
Issue’s Table of Contents
Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from [email protected].

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Publication History

Published: 02 June 2014
Accepted: 01 February 2014
Revised: 01 January 2014
Received: 01 September 2011
Published in JACM Volume 61, Issue 3

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Author Tags

  1. Computational topology
  2. homotopy groups
  3. postnikov systems

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