Prove that ## \sum_{n\leq x}\frac{\phi(n)}{\sqrt{n}}=Cx^{3/2}+.... ##.

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The discussion focuses on proving that the sum of the Euler's totient function divided by the square root of n converges to a specific form. It establishes that for x ≥ 2, the sum can be expressed as Cx^(3/2) plus a smaller order term, where C is a constant equal to 4/π². The proof involves manipulating summations and applying properties of the Möbius function and big-O notation. The conversation also addresses how to factor out big-O terms, specifically relating to the expression involving √x and log(x). Ultimately, the conclusion reinforces the relationship between the sums and their asymptotic behavior.
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Homework Statement
Prove that, for ## x\geq 2 ##, ## \sum_{n\leq x}\frac{\phi(n)}{\sqrt{n}}=Cx^{3\2}+O(x^{1/2}\log {x}) ##, where ## C ## is a constant that you should determine.
Relevant Equations
If ## \alpha\leq 1 ## and ## x\geq 2 ##, then ## \sum_{n\leq x}\frac{\phi(n)}{n^{\alpha}}=\frac{x^{2-\alpha}}{2-\alpha}\frac{1}{\zeta(2)}+O(x^{1-\alpha}\log {x}) ##.
Proof:

Let ## x\geq 2 ##.
Then ## \sum_{n\leq x}\frac{\phi(n)}{n^{\alpha}}=\sum_{d\leq x}\frac{\mu(d)}{d^{\alpha}}\sum_{q\leq \frac{x}{d}}\frac{1}{q^{\alpha-1}} ##.
Observe that
\begin{align*}
&\sum_{n\leq x}\frac{\phi(n)}{\sqrt{n}}=\sum_{d\leq x}\frac{\mu(d)}{\sqrt{d}}\sum_{q\leq \frac{x}{d}}\sqrt{q}\\
&=\sum_{d\leq x}\frac{\mu(d)}{\sqrt{d}}(\frac{2}{3}(x/d)^{3/2}+O(\sqrt{x/d}))\\
&=\frac{2}{3}x^{3/2}\sum_{d\leq x}\frac{\mu(d)}{d^{2}}+O(\sqrt{x}\sum_{d\leq x}\frac{\mu(d)}{d})\\
&=\frac{2}{3}x^{3/2}\frac{1}{\zeta(2)}-\frac{2}{3}x^{3/2}\sum_{d>x}\frac{\mu(d)}{d^{2}}+O(\sqrt{x}\log {x})\\
&=\frac{2}{3}x^{3/2}\frac{1}{\zeta(2)}+O(\sqrt{x})+O(\sqrt{x}\log {x})\\
&=\frac{2}{3}x^{3/2}\frac{6}{\pi^{2}}+O(\sqrt{x}+\sqrt{x}\log {x})\\
&=\frac{4}{\pi^{2}}x^{3/2}+O(x^{1/2}\log {x}).\\
\end{align*}
Therefore, ## \sum_{n\leq x}\frac{\phi(n)}{\sqrt{n}}=Cx^{3/2}+O(x^{1/2}\log {x}) ##, where ## C=\frac{4}{\pi^{2}} ## is a constant.
 
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After factoring out the big-O notation, how does ## \sqrt{x}+\sqrt{x}\log {x}=x^{1/2}\log {x} ##?
 
Math100 said:
After factoring out the big-O notation, how does ## \sqrt{x}+\sqrt{x}\log {x}=x^{1/2}\log {x} ##?
I assume it's factored as ##O(\sqrt(1 +logx)=O(x^{1/2}(1+log x)) ##, and the ##1## by itself is absorbed into ##log x ##.
 
WWGD said:
I assume it's factored as ##O(\sqrt(1 +logx)=O(x^{1/2}(1+log x)) ##, and the ##1## by itself is absorbed into ##log x ##.
It is easier to consider it as ##0\cdot \sqrt{x}\log(x)< \sqrt{x}+\sqrt{x}\log(x) < 2\sqrt{x}\log(x).##
 
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