When the Integral Converges
Upper Estimates
We have
f(2)≤∫21f(x)dx,
and, similarly,
f(3)≤∫32f(x)dx.
In general,
f(n)≤∫nn−1f(x)dx.
Therefore, we can conclude that
sn−f(1)≤∫n1f(x)dx,
which gives us
sn≤f(1)+∫n1f(x)dx.
Taking the limit as n→∞ yields
Converging Integral
If the integral converges, then we can conclude that ∑f(n) must also converge, as, if not, we would have found a number bigger than ∞!
Tail Estimates
Putting together the lower estimate from the previous page and the above upper estimate yields:
Tail Estimates:
∫∞1f(x)dx≤∞∑n=1f(n)≤f(1)+∫∞1f(x)dx
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