%I
%S 0,1,3,7
%N Dimensions of spheres that admit continuous multiplications with unit element.
%C Adams's (1960) Hopf invariant one theorem states that S^0, S^1, S^3, S^7 are the only spheres that are Hspaces, i.e., that admit continuous multiplications with unit element.
%C This is related to the fact that nontrivial cross products only exist in vector spaces of 3 or 7 dimensions. [_Jonathan Vos Post_, Feb 09 2013]
%H J. F. Adams, <a href="https://www.maths.ed.ac.uk/~v1ranick/papers/adams1.pdf">On the nonexistence of elements of Hopf invariant one</a>, Ann. Math., 72 (1960), 20104.
%H Peter F. McLoughlin, <a href="http://arxiv.org/abs/1212.3515">When does a cross product on R^{n} exist?</a>, arXiv:1212.3515 [math.HO], 20122013.
%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Hspace">Hspace</a>
%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Hopf_invariant_one">Hopf invariant one</a>
%F a(n) = 2^n  1 for n = 0, 1, 2, 3.
%F a(n) = A222011(n)  1.
%e 0, 1, 3, 7 are members because multiplications on S^0, S^1, S^3, S^7 are defined by regarding them as the unit spheres in the real, complex, quaternion, and Cayley numbers, respectively.
%Y Cf. A222011.
%K nonn,fini,full,nice
%O 0,3
%A _Jonathan Sondow_, Feb 06 2013
