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Can somone help me find an assymptotic formula for n, for fixed x , for this sum , perhaps an inequality would be even better, or some bound on the error. $\sum_{k=1}^n \frac{1}{\log(kx)}$

I need somthing better then the integral from 1 to n of ln(kx) with respect to k. Its also okay if you use special functions, like the logarithmic integral.

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    Use the Euler-Maclaurin series as suggested below. The expression doesn't in and of itself have a closed form that simplifies to elementary functions, nor have I found any special function that neatly fits your requirement.2012-11-17

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The Euler-Maclaurin series for $\sum_k 1/\log(kx)$ starts $ F(k) = \frac{\text{Li}(kx)}{x} - \frac{1}{2 \ln(kx)} - \frac{1}{12 \ln(kx)^2 k} + \frac{3 + 3 \ln(kx) + \ln(kx)^2}{360 \ln(kx)^4 k^3} + \ldots $

That is, we should have $\sum_{k=1}^n \frac{1}{\log(kx)} \approx F(n+1) + C$ where $C$ is a constant.