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A couple of weeks ago, when Miodrag Milenkovic posed an interesting general problem in connection with POW-7, I was reminded of the “hairy ball theorem” (obviously a phrase invented during a more innocent era!), and a surprisingly easy proof of same that John Baez once told me over beers in an English pub. John is quite a good story-teller, and I wasn’t able to guess the punch line of the proof before it came out of his mouth –when it came, I was so surprised that I nearly fell off my stool! Well, I happened to run across the original source of this proof recently, and though it may be “old news” for some, it’s such a nice proof that I thought it was worth sharing.
The hairy ball theorem says: every continuous tangent vector field on a sphere of even dimension must vanish somewhere (at some point of the sphere, the tangent vector is zero). In the case of an ordinary 2-dimensional sphere, if you think of a vector at a point as a little “hair” emanating from that point, then the theorem says that you can’t comb the hairs of a sphere so that they all lie flat against the sphere: there will be a cowlick sticking straight out somewhere.
The classical proofs usually make some kind of appeal to homology theory: for example, a deep result is that the Euler characteristic of a compact manifold can be computed in terms of any continuously differentiable tangent vector field, by adding up the so-called “indices” of the vector field in the neighborhoods of critical points, where the vector field vanishes (a technical result shows there is no loss of generality in assuming the vector field is continuously differentiable). If the vector field vanishes nowhere, then the Euler characteristic is the empty sum 0; this cannot be in the case of an even-dimensional sphere, because its Euler characteristic is 2. The hairy ball theorem follows.
Some of these homology-based proofs are quite slick, but generally speaking, homology theory requires some heavy infrastructure; the question is whether a more elementary proof exists. The following “analytic” proof is due to John Milnor and uses very little machinery, basically nothing beyond advanced calculus. I will follow his exposition (American Mathematical Monthly, July 1978, pp. 521-524) pretty closely.
For the first step, suppose that we have a continuously differentiable vector field defined in a compact region of space , . For any real and for , define a function
The matrix of first partial derivatives of is , where denotes the identity matrix. For sufficiently small, the determinant of this matrix is strictly positive over all of .
Lemma 1: If is sufficiently small, then is a one-to-one function of onto its image, and is a polynomial function of .
Proof: Since is continuously differentiable over a compact region, there is [using e.g. the mean value theorem, evidently a red rag for some people😉 ] a Lipschitz constant so that
We have only if ; if we assume , this can happen only if . So is one-to-one for such .
The determinant of the matrix of first partials above is of the form
where the are continuous functions in . By the first part of the lemma, we may take so small that is a continuously differentiable embedding, and then by a change-of-variables formula in multivariate calculus we have that
where This completes the proof.
Next, suppose that on we have a continuously differentiable non-vanishing field of tangent vectors. Applying the continuously differentiable map , we assume the vector field consists of unit tangent vectors. For each , the vector is thus of length , hence maps the unit sphere to the sphere of radius .
Lemma 2: For sufficiently small , the map maps onto the sphere of radius .
Proof: Extend the vector field on (and therefore also ) to the compact region between two concentric spheres, , by homothety, i.e., put , for and . There is a Lipschitz constant such that
Take , and let be any unit vector. The function
maps the complete metric space to itself (because and — just use triangle inequalities), and satisfies a Lipschitz condition where . By a classical fixed-point theorem, has under these conditions a (unique) fixed point , so that . Rescaling and by the factor , the statement of lemma 2 follows.
Now we prove the hairy ball theorem. If is a continuous non-vanishing vector field of tangent vectors on , let be the absolute minimum of . By standard techniques (e.g., using the Stone-Weierstrass approximation theorem), there is a continuously differentiable vector field such that , and then so that is also non-vanishing. As above, we may then substitute for , i.e., assume that consists of unit vectors.
Given , we extend to the region by homothety. For sufficiently small , the map defined above maps a spherical shell in this region bijectively onto the shell . Hence maps to the region , and we get a dilation factor:
By lemma 1, is polynomial in . So must be even; therefore admits a non-vanishing vector field only if is odd. This gives the hairy ball theorem.
Man, what an awesome proof. That John Milnor is just a master of technique.
Just a quick note on how any of this bears on Milenkovic’s problem. He asked whether for any topological embedding of in and any point in the region interior to the embedding, there exists a hyperplane through such that the barycenter of the -dimensional region coincides with .
Under the further simplifying assumption that the barycenter varies continuously with , the answer is ‘yes’ for even-dimensional spheres. For (taking to be the origin) we can define a tangent vector field whose value at is the vector from to the barycenter of . For even, this vector vanishes for some , hence coincides with the barycenter for that particular .