N-ball

This is a bizarre feature of geometery that I forgot about: the volume of a the unit n-dimensional ball tends to zero as n increases past 5.  A unit n-ball (or hypersphere) is the set of points that can be reached by going 1 unit in any direction.  I think this is really counter-intuitive.

Image from Wolfram on ball.

Wikipedia on the curse of dimensionality says:

Thus, in some sense, nearly all of the high-dimensional space is “far away” from the centre, or, to put it another way, the high-dimensional unit space can be said to consist almost entirely of the “corners” of the hypercube, with almost no “middle”. (This is an important intuition for understanding the chi-squared distribution.)

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1 thought on “N-ball”

  1. the unit ball gets very spikey — there are spikes pinned on the axes one unit away, but the surface connecting the spikes gets sucked towards the origin very strongly, so it looks like a sea-urchin or something like that. the the volume is esentially contained entirely within the spikes and is therefore quite small.

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