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%I #32 Dec 22 2022 16:34:35
%S 1,2,3,5,6,12,56,61
%N Numbers k such that the topological k-sphere has a unique differentiable structure up to diffeomorphism.
%C Whether 4 is a term is an open question. - _Andrey Zabolotskiy_, Feb 02 2018
%C Except (possibly) for k=4, these are the numbers k such that A001676(k)=1. - _Jeppe Stig Nielsen_, May 22 2019
%C The list in Milnor's paper (p. 807, A358290) does not include the term 56 because it was only discovered later that it is actually a term of this list, see Wang & Xu, Theorem 1.14. - _Andrey Zabolotskiy_, Nov 29 2022
%C Dimensions without exotic spheres. - _Charles R Greathouse IV_, Dec 22 2022
%H Brady Haran and Ciprian Manolescu, <a href="https://www.youtube.com/watch?v=CVOr7f_VALc">The Puzzling Fourth Dimension (and exotic shapes)</a>, Numberphile video (2022).
%H John W. Milnor, <a href="https://www.ams.org/notices/201106/rtx110600804p.pdf">Differential Topology Forty-six Years Later</a>, Notices Amer. Math. Soc. 58 (2011), 804-809.
%H Guozhen Wang and Zhouli Xu, <a href="https://doi.org/10.4007/annals.2017.186.2.3">The triviality of the 61-stem in the stable homotopy groups of spheres</a>, Annals of Mathematics, 186 (2017), 501-580; arXiv:<a href="https://arxiv.org/abs/1601.02184">1601.02184</a> [math.AT], 2016-2017.
%Y Cf. A001676, A358290.
%K nonn,more,hard,nice
%O 1,2
%A _N. J. A. Sloane_, Jun 25 2011
%E Definition rewritten by _Jeppe Stig Nielsen_, May 22 2019
%E The term a(7) = 56 inserted by _Andrey Zabolotskiy_, Nov 27 2022
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