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hairy ball
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topology A result in topology stating that a continuous
vector field on a sphere is always zero somewhere. The name
comes from the fact that you can't flatten all the hair on a
hairy ball, like a tennis ball, there will always be a tuft
somewhere (where the tangential projection of the hair is
zero). An immediate corollary to this theorem is that for any
continuous map f of the sphere into itself there is a point
x such that f(x)=x or f(x) is the antipode of x. Another
corollary is that at any moment somewhere on the Earth there
is no wind.