In mathematics, theannulus theorem (formerly called theannulus conjecture) states roughly that the region between two well-behaved spheres is anannulus. It is closely related to thestable homeomorphism conjecture (now proved) which states that every orientation-preserving homeomorphism of Euclidean space is stable.

IfS andT are topological spheres in Euclidean space, withS contained inT, then it is not true in general that the region between them is anannulus, because of the existence ofwild spheres in dimension at least 3. So the annulus theorem has to be stated to exclude these examples, by adding some condition to ensure thatS andT are well behaved. There are several ways to do this.

The annulus theorem states that if any homeomorphismh ofRⁿ to itself maps the unit ballB into its interior, thenB −h(interior(B)) is homeomorphic to the annulusSⁿ⁻¹×[0,1].

The annulus theorem is trivial in dimensions 0 and 1. It was proved in dimension 2 byRadó (1924), in dimension 3 byMoise (1952), in dimension 4 byQuinn (1982), and in dimensions at least 5 byKirby (1969).

Robion Kirby's torus trick is a proof method employing an immersion of a punctured torus${\displaystyle {\mathbb{T}}^n-{\mathbb{D}}^n}$ into${\displaystyle {\mathbb{R}}^n}$, where then smooth structures can be pulled back along the immersion and be lifted to covers.The torus trick is used in Kirby's proof of the annulus theorem in dimensions${\displaystyle n\ge 5}$.It was also employed in further investigations of topological manifolds withLaurent C. Siebenmann^[1]

Here is a list of some further applications of the torus trick that appeared in the literature:

• Proving existence and uniqueness (up to isotopy) of smooth structures on surfaces^[2]
• Proving existence and uniqueness (up to isotopy) ofPL structures on 3-manifolds^[3]

A homeomorphism ofRⁿ is calledstable if it is the composite of (a finite family of) homeomorphisms each of which is the identity on some non-empty open set.

Thestable homeomorphism conjecture states that every orientation-preserving homeomorphism ofRⁿ is stable.Brown & Gluck (1964) previously showed that the stable homeomorphism conjecture is equivalent to the annulus conjecture, so it is true.

• Brown, Morton; Gluck, Herman (1964), "Stable structures on manifolds. II. Stable manifolds.",Annals of Mathematics, Second Series,79 (1):18–44,doi:10.2307/1970481,ISSN 0003-486X,JSTOR 1970482,MR 0158383
• Edwards, Robert D. (1984), "The solution of the 4-dimensional annulus conjecture (after Frank Quinn)",Four-manifold theory (Durham, N.H., 1982), Contemp. Math., vol. 35, Providence, R.I.: Amer. Math. Soc., pp. 211–264,doi:10.1090/conm/035/780581,ISBN 9780821850336,MR 0780581
• Kirby, Robion C. (1969), "Stable homeomorphisms and the annulus conjecture",Annals of Mathematics, Second Series,89 (3):575–582,doi:10.2307/1970652,ISSN 0003-486X,JSTOR 1970652,MR 0242165
• Moise, Edwin E. (1952), "Affine structures in 3-manifolds. V. The triangulation theorem and Hauptvermutung",Annals of Mathematics, Second Series,56 (1):96–114,doi:10.2307/1969769,ISSN 0003-486X,JSTOR 1969769,MR 0048805
• Quinn, Frank (1982),"Ends of maps. III. Dimensions 4 and 5",Journal of Differential Geometry,17 (3):503–521,doi:10.4310/jdg/1214437139,ISSN 0022-040X,MR 0679069
• Radó, T. (1924), "Über den Begriff der Riemannschen Fläche",Acta Univ. Szeged,2:101–121

• MathOverflow discussion on the Torus trick
• Video recording of interview with Robion Kirby
• Topological Manifolds Seminar (University of Bonn, 2021)

• Geometric topology
• Theorems in topology

• Articles with short description
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