Alexander horned sphere

The Alexander horned sphere is a pathological object in topology discovered by J. W. Alexander (1924). It is a particular topological embedding of a two-dimensional sphere in three-dimensional space. Together with its inside, it is a topological 3-ball, the Alexander horned ball, and so is simply connected; i.e., every loop can be shrunk to a point while staying inside. However, the exterior is not simply connected, unlike the exterior of the usual round sphere.

Alexander horned sphere

Construction

Diagram of the first few iterative steps in the construction of Alexander's horned sphere, from Alexander's original 1924 paper

The Alexander horned sphere is the particular (topological) embedding of a sphere in 3-dimensional Euclidean space obtained by the following construction, starting with a standard torus:

1. Remove a radial slice of the torus.
2. Connect a standard punctured torus to each side of the cut, interlinked with the torus on the other side.
3. Repeat steps 1–2 on the two tori just added ad infinitum.

Animated construction of Alexander's sphere.

By considering only the points of the tori that are not removed at some stage, an embedding of the sphere with a Cantor set removed results. This embedding extends to a continuous map from the whole sphere, which is injective (hence a topological embedding since the sphere is compact) since points in the sphere approaching two different points of the Cantor set will end up in different 'horns' at some stage and therefore have different images.

Impact on theory

The horned sphere, together with its inside, is a topological 3-ball, the Alexander horned ball, and so is simply connected; i.e., every loop can be shrunk to a point while staying inside. The exterior is not simply connected, unlike the exterior of the usual round sphere; a loop linking a torus in the above construction cannot be shrunk to a point without touching the horned sphere. This shows that the Jordan–Schönflies theorem does not hold in three dimensions, as Alexander had originally thought. Alexander also proved that the theorem does hold in three dimensions for piecewise linear/smooth embeddings. This is one of the earliest examples where the need for distinction between the categories of topological manifolds, differentiable manifolds, and piecewise linear manifolds became apparent.

Now consider Alexander's horned sphere as an embedding into the 3-sphere, considered as the one-point compactification of the 3-dimensional Euclidean space R³. The closure of the non-simply connected domain is called the solid Alexander horned sphere. Although the solid horned sphere is not a manifold, R. H. Bing showed that its double (which is the 3-manifold obtained by gluing two copies of the horned sphere together along the corresponding points of their boundaries) is in fact the 3-sphere. One can consider other gluings of the solid horned sphere to a copy of itself, arising from different homeomorphisms of the boundary sphere to itself. This has also been shown to be the 3-sphere. The solid Alexander horned sphere is an example of a crumpled cube; i.e., a closed complementary domain of the embedding of a 2-sphere into the 3-sphere.

Generalizations

One can generalize Alexander's construction to generate other horned spheres by increasing the number of horns at each stage of Alexander's construction or considering the analogous construction in higher dimensions.

Other substantially different constructions exist for constructing such "wild" spheres. Another example, also found by Alexander, is Antoine's horned sphere, which is based on Antoine's necklace, a pathological embedding of the Cantor set into the 3-sphere.

See also

• Cantor tree surface
• Wild knot
• Wild arc, specifically the Fox–Artin arc

References

Citations

• Alexander, J. W. (1924), "An Example of a Simply Connected Surface Bounding a Region which is not Simply Connected", Proceedings of the National Academy of Sciences of the United States of America, 10 (1), National Academy of Sciences: 8–10, Bibcode:1924PNAS...10....8A, doi:10.1073/pnas.10.1.8, ISSN 0027-8424, JSTOR 84202, PMC 1085500, PMID 16576780
• Fuchs, Dmitry; Tabachnikov, Serge (2007), Mathematical Omnibus. 30 Lectures on Classical Mathematics, Providence, RI: American Mathematical Society, doi:10.1090/mbk/046, ISBN 978-0-8218-4316-1, MR 2350979
• Hatcher, Allen, Algebraic Topology, http://pi.math.cornell.edu/~hatcher/AT/ATpage.html
• Hocking, John Gilbert; Young, Gail Sellers (1988) [1961]. Topology. Dover. ISBN 0-486-65676-4.
• Spivak, Michael (1999). A comprehensive introduction to differential geometry (Volume 1). Publish or Perish. ISBN 0-914098-70-5.

External links

• Weisstein, Eric W. "Alexander's Horned Sphere". MathWorld.
• Zbigniew Fiedorowicz. Math 655 – Introduction to Topology. [1] Archived 2005-08-25 at the Wayback Machine – Lecture notes
• Construction of the Alexander sphere
• rotating animation
• PC OpenGL demo rendering and expanding the cusp