POW Riemann
Todd and Vishal's Problem of the Week number 3 (solution here) was to compute, for any , the series (from )
where is a binomial coefficient. In the case we see that the sum takes the form
which is a sum of reciprocals of triangular numbers (from ). For we obtain the reciprocals of the tetrahedral numbers, and . The tetrahedral number is the sum of the first triangular numbers. By the way, only three tetrahedral numbers are perfect squares, namely 1, 4 and . One guesses that in general is a series of reciprocals of tetrahedral numbers in dimension . Indeed
But whenever discussing infinite series of simple polytopes, an M theorist cannot help thinking of the Riemann zeta function. Observe that for
from which one deduces, allowing cancellation of infinities (!), that
What kind of zeta sums do we get in general?
where is a binomial coefficient. In the case we see that the sum takes the form
which is a sum of reciprocals of triangular numbers (from ). For we obtain the reciprocals of the tetrahedral numbers, and . The tetrahedral number is the sum of the first triangular numbers. By the way, only three tetrahedral numbers are perfect squares, namely 1, 4 and . One guesses that in general is a series of reciprocals of tetrahedral numbers in dimension . Indeed
But whenever discussing infinite series of simple polytopes, an M theorist cannot help thinking of the Riemann zeta function. Observe that for
from which one deduces, allowing cancellation of infinities (!), that
What kind of zeta sums do we get in general?
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