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Tuesday, April 26, 2016

Wallis sieve, and lp n-balls III

This is the third installment, do read the previous posts. I left some standing questions in the last post, and I am going to answer at least one of them: Can we squeeze an l_p ball´s volume out of a generalized Wallis sieve?. At the same time I will generalize and simplify the proof from the xkcd post. First, lets generalize the Wallis sieve.

In d dimensions, starting with a p sided hypercube and cutting it appropriately (I leave to the reader to draw it), we get the product,
A_n^d = \prod_{n=1}^\infty \frac{p^d n^{d-1} \Big(n+\frac{d}{p}\Big)} {(pn+1)^d},
which can be rewritten as the limit
A_n^d = lim_{n\to\infty} \frac{p^{nd}\Gamma(n+1)^{d-1}\Gamma\Big(n+1+\frac{d}{p}\Big)\Gamma\Big(1+\frac{d}{p}\Big)^d}{p^{nd}\Gamma(1+\frac{d}{p})\Gamma\Big(n+1+\frac{1}{p}\Big)^d}
where I have made use of the Euler gamma function recurrent properties (see previous posts).
Note that the d in A_n^d is an index, not an exponent.
 The limit can be separated into two factors (after cancelling the p^{nd}),
 A_n^d = lim_{n\to\infty}\Bigg[ \frac{\Gamma(n+1)^{d-1}\Gamma\Big(n+1+\frac{d}{p}\Big)}{\Gamma\Big(n+1+\frac{1}{p}\Big)^d} \Bigg]\Bigg[\frac{\Gamma\Big(1+\frac{1}{p}\Big)^d}{\Gamma(1+\frac{d}{p})}\Bigg].
Remember that \frac{\Gamma(n+a)}{\Gamma(n)}\sim n^a, so the first term when n\to\infty
\frac{\Gamma(n+1)^{d-1}\Gamma\Big(n+1+\frac{d}{p}\Big)}{\Gamma\Big(n+1+\frac{1}{p}\Big)^d} = \frac{\Gamma(n+1)^{d-1}\Gamma\Big(n+1+\frac{d}{p}\Big)}{\Gamma\Big(n+1+\frac{1}{p}\Big)^{d-1}\Gamma\Big(n+1+\frac{1}{p}\Big)} \sim \frac{n^{\frac{d-1}{p}}}{n^{\frac{d-1}{p}}}\sim 1.
So we obtain, finally,
 A_n^d = lim_{n\to\infty} \frac{\Gamma\Big(1+\frac{1}{p}\Big)^d}{\Gamma(1+\frac{d}{p})} = V_d^p\Big(\frac{1}{2}\Big).
The value of the limit is the volume of the l_p ball, with the case of the hypersphere p=2 being a particular case.
Another remarkable case happens when p=1 and we obtain the volume of the d-dimensional cross-polytope,  of radius R=\frac{1}{2}, which is \frac{1}{d!}, . The cross-polytope is the generalization of the octahedron to n dimensions and is the dual of the hypercube we start with.

This formula lets us also interpret the volume of various fat Cantor and other Smith-Cantor-Volterra sets.
The question still standing from last post is: Is there a geometrical interpretation for the intermediate A_n^d? And of course, What more can we learn from this relation between l_p and these sets?



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