# Exotic **R**^{4}

__: A smooth 4-manifold homeomorphic yet not diffeomorphic to euclidean space__

**Short description**

In mathematics, an **exotic** [math]\displaystyle{ \R^4 }[/math] is a differentiable manifold that is homeomorphic (i.e. shape preserving) but not diffeomorphic (i.e. non smooth) to the Euclidean space [math]\displaystyle{ \R^4. }[/math] The first examples were found in 1982 by Michael Freedman and others, by using the contrast between Freedman's theorems about topological 4-manifolds, and Simon Donaldson's theorems about smooth 4-manifolds.^{[1]}^{[2]} There is a continuum of non-diffeomorphic differentiable structures of [math]\displaystyle{ \R^4, }[/math] as was shown first by Clifford Taubes.^{[3]}

Prior to this construction, non-diffeomorphic smooth structures on spheres – exotic spheres – were already known to exist, although the question of the existence of such structures for the particular case of the 4-sphere remained open (and still remains open as of 2023). For any positive integer *n* other than 4, there are no exotic smooth structures on [math]\displaystyle{ \R^n; }[/math] in other words, if *n* ≠ 4 then any smooth manifold homeomorphic to [math]\displaystyle{ \R^n }[/math] is diffeomorphic to [math]\displaystyle{ \R^n. }[/math]^{[4]}

## Small exotic R^{4}s

An exotic [math]\displaystyle{ \R^4 }[/math] is called **small** if it can be smoothly embedded as an open subset of the standard [math]\displaystyle{ \R^4. }[/math]

Small exotic [math]\displaystyle{ \R^4 }[/math] can be constructed by starting with a non-trivial smooth 5-dimensional *h*-cobordism (which exists by Donaldson's proof that the *h*-cobordism theorem fails in this dimension) and using Freedman's theorem that the topological *h*-cobordism theorem holds in this dimension.

## Large exotic R^{4}s

An exotic [math]\displaystyle{ \R^4 }[/math] is called **large** if it cannot be smoothly embedded as an open subset of the standard [math]\displaystyle{ \R^4. }[/math]

Examples of large exotic [math]\displaystyle{ \R^4 }[/math] can be constructed using the fact that compact 4-manifolds can often be split as a topological sum (by Freedman's work), but cannot be split as a smooth sum (by Donaldson's work).

Michael Hartley Freedman and Laurence R. Taylor (1986) showed that there is a maximal exotic [math]\displaystyle{ \R^4, }[/math] into which all other [math]\displaystyle{ \R^4 }[/math] can be smoothly embedded as open subsets.

## Related exotic structures

Casson handles are homeomorphic to [math]\displaystyle{ \mathbb{D}^2 \times \R^2 }[/math] by Freedman's theorem (where [math]\displaystyle{ \mathbb{D}^2 }[/math] is the closed unit disc) but it follows from Donaldson's theorem that they are not all diffeomorphic to [math]\displaystyle{ \mathbb{D}^2 \times \R^2. }[/math] In other words, some Casson handles are exotic [math]\displaystyle{ \mathbb{D}^2 \times \R^2. }[/math]

It is not known (as of 2022) whether or not there are any exotic 4-spheres; such an exotic 4-sphere would be a counterexample to the smooth generalized Poincaré conjecture in dimension 4. Some plausible candidates are given by Gluck twists.

## See also

- Akbulut cork - tool used to construct exotic [math]\displaystyle{ \R^4 }[/math]'s from classes in [math]\displaystyle{ H^3(S^3,\mathbb{R}) }[/math]
^{[5]} - Atlas (topology)

## Notes

## References

- Freedman, Michael H.; Quinn, Frank (1990).
*Topology of 4-manifolds*. Princeton Mathematical Series.**39**. Princeton, NJ: Princeton University Press. ISBN 0-691-08577-3. https://archive.org/details/topologyof4manif0000free. - Freedman, Michael H.; Taylor, Laurence R. (1986). "A universal smoothing of four-space".
*Journal of Differential Geometry***24**(1): 69–78. doi:10.4310/jdg/1214440258. ISSN 0022-040X. https://projecteuclid.org/euclid.jdg/1214440258. - Kirby, Robion C. (1989).
*The topology of 4-manifolds*. Lecture Notes in Mathematics.**1374**. Berlin: Springer-Verlag. ISBN 3-540-51148-2. - Scorpan, Alexandru (2005).
*The wild world of 4-manifolds*. Providence, RI: American Mathematical Society. ISBN 978-0-8218-3749-8. - Stallings, John (1962). "The piecewise-linear structure of Euclidean space".
*Proc. Cambridge Philos. Soc.***58**(3): 481–488. doi:10.1017/s0305004100036756. Bibcode: 1962PCPS...58..481S. http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=2053736&fulltextType=RA&fileId=S0305004100036756. MR0149457 - Gompf, Robert E.; Stipsicz, András I. (1999).
*4-manifolds and Kirby calculus*. Graduate Studies in Mathematics.**20**. Providence, RI: American Mathematical Society. ISBN 0-8218-0994-6. - Taubes, Clifford Henry (1987). "Gauge theory on asymptotically periodic 4-manifolds".
*Journal of Differential Geometry***25**(3): 363–430. doi:10.4310/jdg/1214440981. Template:Project Euclid. http://projecteuclid.org/euclid.jdg/1214440981.

Original source: https://en.wikipedia.org/wiki/Exotic R4.
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