Whitney embedding theorem
In differential topology, the Whitney embedding theorem states that
Any smooth second countable -dimensional manifold can be embedded in Euclidean 2m-space.
The result is sharp, in particular the projective m-space can not be embeded into Euclidean (2m − 1)-space
A little about the proof
Cases and can be done by hand. For a general position argument show that there is an immersion R2m with transversal self-intersections. Then apply the Whitney trick, i.e. the following procedure which removes self-inersections one by one.
Whitney trick
Suppose R2m is a point of self-intersection and such that . Connect and by a smooth curve
so that is a simple closed curve in R2m. Construct an embedding of a 2-disc R2m with boundary .
By a general position argument it can be constructed with no self-intersections and with no intersections with (here we use that ). Then one can deform in a little neighborhood of so that the self-intersecton 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 are isotopic to submanifolds such that all points of intesections 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.