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So I was reading a book in analytic number theory and there was this claim $$\sum_{a\leq x} 1/a =\log x+O(1)$$ and since we kinda skipped these big $O$ notation problems at early uni days I got wondering how I can be sure in something like this?

I mean I can see how $\sum_{a\leq x} 1/a \leq \log x$ holds but how can I be sure that the rest is $O(1)$?

Also if I take $\sum_{a\leq x} O(1)$, $O(1)$ I can get out of the sum and the sum is something that is less than $x$ so $O(x)$ so that gives me $O(1)O(x)$ which is $O(x)$, am I correct?

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    $\begingroup$ $$ \log n = \int_1^n {\frac{{dx}}{x}} < \sum\limits_{k = 1}^n {\frac{1}{k}} < 1 + \int_1^n {\frac{{dx}}{x}} = 1 + \log n $$ $\endgroup$
    – Gary
    Jun 15 at 11:04
  • $\begingroup$ Can you explain first how the first inequality holds and how this answers my question? $\endgroup$
    – MotionMath
    Jun 15 at 11:11
  • $\begingroup$ Note that $$ \sum\limits_{k = 1}^n {\frac{1}{k}} = \sum\limits_{k = 1}^n {\int_k^{k + 1} {\frac{{dx}}{k}} } \ge \sum\limits_{k = 1}^n {\int_k^{k + 1} {\frac{{dx}}{x}} } = \int_1^{n + 1} {\frac{{dx}}{x}} = \log (n + 1) > \log n. $$ The inequalities show that $\sum\nolimits_{k = 1}^n {\frac{1}{k}} - \log n$ is bounded, i.e., $$ \sum\limits_{k = 1}^n {\frac{1}{k}} = \log n + \mathcal{O}(1). $$ $\endgroup$
    – Gary
    Jun 15 at 11:16
  • $\begingroup$ Does this help math.stackexchange.com/questions/4139408/…? $\endgroup$
    – zkutch
    Jun 15 at 13:57
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To summarize the comments to this question and add one bit of extra info, the notation $$ f(x) = O(g(x)) $$ is defined to mean that $\left| f(x)\right| \leq C g(x)$, where $C$ is a constant independent of $x$. For your specific question, one needs to show that for some suitable constant $C$, we have $$ \left| \sum_{a \leq x} \frac{1}{a} - \log x\right| \leq C. $$ From the comments provided by Gary, we have precisely this inequality with $C = 1$. In fact, one can use partial summation to deduce that $$ \sum_{a \leq x} \frac{1}{a} = \log x + \gamma + O\left(\frac{1}{x}\right), $$ where $\gamma$ denotes the Euler-Mascheroni constant. In general, when dealing with the sums $$ \sum_{a\leq x} f(a) $$ where $f$ is differentiable (or, say, continuously differentiable $k$ times), you should expect the sum above to be comparable to the integral $$ \int_{1}^{x} f(t) dt. $$ Indeed, this is precisely the content of the Euler-Maclaurin summation formula.

By a more careful analysis, one can obtain an asymptotic expansion of the sum above with an arbitrarily small error term. For instance, if $\psi(x) = \left\{ x\right\} - \frac{1}{2}$, where $\left\{ x\right\}$ denotes the fractional part of $x$, then $$ \sum_{a \leq x} \frac{1}{a} = \log x + \gamma - \frac{\psi(x)}{x} + O\left(\frac{1}{x^2}\right). $$

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