Integral Formula for Particular Dirichlet Series

The following is a formula for a Dirichlet series where sR and we satisfy a particular constraint on the sequence an.

Theorem

If A(x)=nxan=O(xδ). For s>δ,

n=1anns=s1A(t)ts+1dt.
Proof

We use Abel summation with f(n)=1ns to deduce that

nxanns=A(x)xs+s1xA(t)ts+1dtlimxnxanns=limx(A(x)xs+s1xA(t)ts+1dt)(1)n=1anns=limx(A(x)xs)+s1A(t)ts+1dt.

Then because A(x)=O(xδ) by assumption, there exists an x0 and constant C such that

x>x0|A(x)|<C|xδ||A(x)xs|<C|xδxs|=C|xδs|.

Because s>δ, δs<0 and hence as x, |xδs|0. Therefore we have by the pinching theorem that

limx|A(x)xs|=limx|A(x)|xs=0.

Therefore (1) becomes

n=1anns=+s1A(t)ts+1dt.