Whitney embedding theorem
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In differential topology, the Whitney embedding theorem states that any smooth second-countable <math>m<math>-dimensional manifold can be embedded in Euclidean <math>2m<math>-space. The result is sharp, in particular the projective <math>m<math>-space cannot be embedded into the Euclidean (<math>2m-1<math>)-space.
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A little about the proof
Cases <math>m=1<math> and <math>2<math> can be done by hand. For <math>m\ge 3<math> a general position argument show that there is an immersion <math>f:M\to\mathbb R^{2m}<math> with transversal self-intersections. Then apply the Whitney trick, i.e. the following procedure which removes self-intersections one by one.
Whitney trick
Suppose <math>p\in\mathbb R^{2m}<math> is a point of self-intersection and <math>x,y\in M<math> such that <math>f(x)=f(y)=p<math>. Connect <math>x<math> and <math>y<math> by a smooth curve
- <math>c:[0,1]\to M<math>
so that <math>f\circ c<math> is a simple closed curve in <math>\mathbb R^{2m}<math>. Construct an embedding of a <math>2<math>-disc <math>h:D^2\to\mathbb R^{2m}<math> with boundary <math>f\circ c<math>.
By a general position argument it can be constructed with no self-intersections and with no intersections with <math>f(M)<math> (here we use that <math>m\ge 3<math>). Then one can deform <math>f<math> in a little neighborhood of <math>h(D^2)<math> so that the self-intersection disappears. The last statement is very easy to see once you visualize this picture properly.
Other things coming from Whitney trick
Whitney trick is used to prove h-cobordism theorem, it also shows that two oriented submanifolds of complimentary dimensions in a simply connected manifold of dimension <math>\ge 5<math> are isotopic to submanifolds such that all points of intersections have the same sign.
History
The occasion of the proof by Hassler Whitney of the embedding theorem for smooth manifolds is said (rather surprisingly) to have been the first complete exposition of the manifold concept (which had been implicit in Riemann's work, Lie group theory, and general relativity for many years), building on Hermann Weyl's book The Idea of a Riemann surface.