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Let $$\pi_0 (x)=\frac12 \lim_{h \to 0}[\pi (x+h)+\pi (x-h)]$$ where $\pi (x)$ is the prime counting function. Then $$\pi_0 (x)=\sum_{n=1}^\infty \frac{\mu (n)}{n}f(x^{1/n})$$ where $$f(x)=\operatorname{li}(x)-\sum_{\rho} \operatorname{li}(x^\rho)-\log (2)+\int_x^\infty \frac{dt}{t(t^2-1)\log t},$$ and the sum is over the non-trivial zeros $\rho$ of the Riemann zeta function. This is taken from https://en.wikipedia.org/wiki/Explicit_formulae_for_L-functions.

Euler's totient function $\varphi (n)$ is a number-theoretic function just as $\pi (n)$. Is there an analogous, analytic representation of $\varphi (n)$?

aeraw
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  • there's something just further down the page no? – tomos Mar 19 '22 at 17:21
  • @tomos How can you solve for $\varphi (n)$ from that formula? The formula you're referring to doesn't give $\varphi (n)$ explicitly. – aeraw Mar 19 '22 at 17:27
  • i have no idea, i just wondered if you'd missed it – tomos Mar 19 '22 at 20:56
  • The residue theorem for $\int_{(2)} \frac{\zeta(s-1)}{s(s-1)\zeta(s)}x^sds$ has good chances to work for $x> 1$ – reuns Mar 21 '22 at 00:11
  • @reuns I see that $\frac{\zeta (s-1)}{\zeta (s)}$ comes from the Dirichlet series but what exactly does this integral yield? Could you be more specific please? – aeraw Mar 21 '22 at 11:50

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The totient summatory function

$$\Phi(x)=\sum\limits_{n=1}^x\varphi(n)$$

associated with

$$\frac{\zeta(s-1)}{\zeta(s)}=\sum\limits_{n=1}^\infty\varphi(n)\,n^{-s},\quad\Re(s)>2$$

is analogous to the second Chebyshev function

$$\psi(x)=\sum\limits_{n=1}^x\Lambda(n)$$

associated with

$$\frac{\partial\log\zeta(s)}{\partial s}=-\frac{\zeta'(s)}{\zeta(s)}=\sum\limits_{n=1}^\infty\Lambda(n)\,n^{-s},\quad\Re(s)>1$$

and the Riemann prime-power counting function

$$\Pi(x)=\sum\limits_{n=2}^x\frac{\Lambda(n)}{\log(n)}$$

associated with

$$\log\zeta(s)=\sum\limits_{n=2}^\infty\frac{\Lambda(n)}{\log(n)}\,n^{-s},\quad\Re(s)>1$$

The analytic representation of $\pi_0(x)$ is based on the analytic representation for $\Pi_0(x)$ and the relationship $\pi(x)=\sum\limits_{n=1}^\infty\frac{\mu(n)}{n}\,\Pi\left(x^{\frac{1}{n}}\right)$ which is the Möbius inversion of the relationship $\Pi(x)=\sum\limits_{n=1}^\infty\frac{1}{n}\,\pi\left(x^{\frac{1}{n}}\right)$.

But an analytic formula for $\Phi(x)$ would perhaps yield an analytic formula for $\varphi(n)$ since $\varphi(n)=\Phi(n)-\Phi(n-1)$. For example, I believe $\Lambda(n)=\psi_o\left(x+\frac{1}{2}\right)-\psi_o\left(x-\frac{1}{2}\right)$ evaluates correctly at integer values of $x\ge 2$ since the explicit formula for $\psi_o(x)$ only converges for $x>1$.

The problem is I don't believe the analytic formula for $\Phi(x)$ at the referenced Wikipedia article converges.

PeterHumphries explains the reason for this in a comment on my my related question.

The explicit formula for $\sum_{n \leq x} \varphi(x)$ is wrong; when you shift the contour, the shifted contour integral is not small. One can use this to show that the error term for this sum is at least as large as a constant multiple of $x\sqrt{\log \log x}$ infinitely often.

Peter provides further clarification in another comment.

What I mean is that $\sum_{n \leq x} \varphi(x)$ has a main term, coming from the pole of $\zeta(s - 1)/\zeta(s)$ at $s = 2$, and an error term of size at least $x\sqrt{\log \log x}$, which does not come from the poles of $\zeta(s - 1)/\zeta(s)$ at the zeroes of $\zeta(s)$.

Note that in the two comments quoted above $\varphi(x)$ should have been $\varphi(n)$.

I've investigated a potential analytic representation of $\varphi(n)$ based on this answer I posted to a question about an entire function interpolating the Möbius function $\mu(n)$, but it wouldn't really be analogous to explicit formulas related to the Riemann prime-power counting function $\Pi(x)$ (referred to as $f(x)$ in the question above) or the second Chebyshev function $\psi(x)$.

In my answer linked in the paragraph above formulas (5) and (6) are only equivalent to formulas (7) and (8) when $F_a(s)=\sum\limits_{n=1}^\infty \frac{a(n)}{n^s}$ converges for $\Re(s)\ge 2$ which it doesn't in the case of $a(n)=\varphi(n)$ where $F_a(s)=\frac{\zeta(s-1)}{\zeta(s)}$. I believe as $K\to\infty$ formula (10) for $\tilde{a}(s)$ may still be valid at integer values of s when $|s|\le K$, but perhaps generally diverges at non-integer values of $s$ because this condition isn't met.

The following figure illustrates formula (10) for $\tilde{a}(s)$ corresponding to $a(n)=\varphi(n)$ where formula (10) is evaluated at $K=20$ and $f=1$. The red discrete portion of the plot represents the evaluation of $\varphi(|s|)$ at integer values of $s$. Note formula (10) for $\tilde{a}(s)$ converges to $0$ at $s=0$ and to $a(|s|)$ at positive and negative integer values of $s$.


Illustration of formula (10)

Figure (1): Illustration of formula (10) for $\tilde{a}(s)$ corresponding to $a(n)=\varphi(n)$

Steven Clark
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  • I don't think I understand the linked answer. What does "evaluation frequency" mean and what is $b(n)$ when $a(n)=\varphi (n)$? Or does $b(n)$ always involve calculating the Möbius function? – aeraw Mar 20 '22 at 03:49
  • I find the posts quite confusing. In https://math.stackexchange.com/questions/2737492/questions-on-convergence-of-explicit-formulas-for-fx-sum-limits-n-1x-an, is the formula for $\sum_{n=1}^x \varphi (n)$ correct? If so, how do you obtain $\varphi (k)$ explicitly from that? And what is the difference between $\Phi$ and $\Phi_o$? – aeraw Mar 20 '22 at 04:00
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    @aeraw $b(n)$ can always be calculated using the Möbius inversion formula $b(n)=\sum_{d|n} a(d),\mu(n/d)$, but in some cases it simplifies to another well-known arithmetic function such as it did in most of the rows in Table (1) of my linked answer. I'm not sure the Möbius inversion formula can be simplified for the case $a(n)=\varphi(n)$. The evaluation frequency $f$ is the parameter $f$ which appears in the formulas in my answer. In Figure (1) of my linked answer you can clearly see the small oscillation which is related to using $f=4$ for the evaluation frequency. – Steven Clark Mar 20 '22 at 04:20
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    In my linked question $\Phi(n)=\sum\limits_{n=1}^x \varphi(n)$ is the totient summatory function and $\Phi_o(x)$ refers to an explicit formula for $\underset{\epsilon\to 0}{\text{lim}}\left(\frac{\Phi(n-\epsilon)+\Phi(n+\epsilon)}{2}\right)$. The subscript on $\Phi_o(x)$ is used to distinguish the analytic representation from the standard function the same way you did with $\pi_0(x)$ versus $\pi(x)$ in the formulas in your question above (except I used the letter "o" whereas you used the number "0"). – Steven Clark Mar 20 '22 at 04:38
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    @aeraw As I said in my answer above, the formula for $\Phi_o(x)$ in my linked question doesn't converge for reasons explained by Peter Humphries. I was thinking if it did that perhaps the relationship $\varphi(n)=\Phi(n)-\Phi(n-1)$ could be used, but $\varphi(n)=\Phi_o(n)-\Phi_o(n-1)$ wouldn't quite work because analytic formulas typically converge to the half step on transitions (i.e. $\underset{\epsilon\to 0}{\text{lim}}\left(\frac{\Phi(n-\epsilon)+\Phi(n+\epsilon)}{2}\right)$. – Steven Clark Mar 20 '22 at 07:22