Arcadian Functor

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Marni D. Sheppeard

Wednesday, June 04, 2008

POW Riemann

Todd and Vishal's Problem of the Week number 3 (solution here) was to compute, for any n>1, the series (from k=0)

S(n)kB(n+k;k)-1

where B(n+k;k) is a binomial coefficient. In the case n=2 we see that the sum takes the form

S(2)=1+13+16+110+=2

which is a sum of reciprocals of triangular numbers 12k(k+1) (from k=1). For n=3 we obtain the reciprocals of the tetrahedral numbers, and S(3)=32. The tetrahedral number Tk=16k(k+1)(k+2) is the sum of the first k triangular numbers. By the way, only three tetrahedral numbers are perfect squares, namely 1, 4 and T48=19600. One guesses that in general S(n) is a series of reciprocals of tetrahedral numbers in dimension n. Indeed

S(n)=nn-1

But whenever discussing infinite series of simple polytopes, an M theorist cannot help thinking of the Riemann zeta function. Observe that for n=2

S(2)=k2k2+k=2ζ(2)-k2k3+k2
=2[ζ(2)-ζ(3)+ζ(4)-ζ(5)+]=2

from which one deduces, allowing cancellation of infinities (!), that

ζ(2)-ζ(3)+ζ(4)-ζ(5)+=1

What kind of zeta sums do we get in general?

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