Zeta Function and Harmonic Series
Recently someone asked about ways of computing Euler's constant,
gamma. This started me thinking about the partial sums of the
harmonic series. Let h(n) denote the sum
h(n) = 1/1 + 1/2 + 1/3 + 1/4 + ... + 1/n
These sums are related to the zeta function by the formula
n zeta(k)
lim h(2n) - h(n) = SUM ---------
n->oo k=2 2^k
Since zeta(k) approaches 1 as k increases, we can get faster
convergence by re-writing this in the form
1 n zeta(k) - 1
lim h(2n) - h(n) = --- + SUM -------------
n->oo 2 k=2 2^k
It seems that it should be possible to compute gamma by making use of
this relation. This could also be written as a recurrence
2n zeta(k) - 1
lim h(4n) = 2 h(2n) - h(n) + SUM -------------
n->oo k=n+1 2^k
Notice that, for example, zeta(40) = 1.00000000000090949478..., so the
terms (zeta(k)-1)/2^k quickly become very small as k increases.
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