23/02/05 00:17:40.68 XfMj3WNk.net
>>166 追加
URLﾘﾝｸ(ja.wikipedia.org)
エキゾチック R^4
球面上の非微分同形の滑らかな構造(エキゾチックな球体) が存在することが既に知られていたが、 4-球体 の特定のケースに対するそのような構造の存在の問題は未解決のままであった (2022 年現在も未解決のままである)。
関連するエキゾチックな構造
Casson ハンドルはフリードマンの定理により
D^2 Ｘ R^2と同相であるが、ドナルドソンの定理から、それらはすべて
D^2 Ｘ R^2と微分同相ではない。言い換えれば、一部の Casson ハンドルはエキゾチック
D^2 Ｘ R^2である。
en.wikipediaより
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 Poincare conjecture in dimension 4. Some plausible candidates are given by Gluck twists.
URLﾘﾝｸ(en.wikipedia.org)
Exotic sphere
4-dimensional exotic spheres and Gluck twists
In 4 dimensions it is not known whether there are any exotic smooth structures on the 4-sphere. The statement that they do not exist is known as the "smooth Poincare conjecture", and is discussed by Michael Freedman, Robert Gompf, and Scott Morrison et al. (2010) who say that it is believed to be false.
Some candidates proposed for exotic 4-spheres are the Cappell?Shaneson spheres (Sylvain Cappell and Julius Shaneson (1976)) and those derived by Gluck twists (Gluck 1962). Gluck twist spheres are constructed by cutting out a tubular neighborhood of a 2-sphere S in S4 and gluing it back in using a diffeomorphism of its boundary S2×S1. The result is always homeomorphic to S4. Many cases over the years were ruled out as possible counterexamples to the smooth 4 dimensional Poincare conjecture. For example, Cameron Gordon (1976), Jose Montesinos (1983), Steven P. Plotnick (1984), Gompf (1991), Habiro, Marumoto & Yamada (2000), Selman Akbulut (2010), Gompf (2010), Kim & Yamada (2017).