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Here $s=\sigma+it$ denotes the complex variable. We denote the Ramanujan's zeta function $$\varphi(s)=\sum_{n=1}^\infty\frac{\tau(n)}{n^s}$$

for $\Re(s)>7$, where $\tau(n)$ is the Ramanujan tau function.

While I was studying a video by Harper, a video from YouTube with title The Riemann zeta function in short intervals - Harper - Bourbaki - 30/03/19 from the official channel Institut Henri Poincaré, I wondered if a similar principle than the professor shows as Principle I (see also PRINCIPLE 1.3 from [2]) works in some suitable sense for the Ramanujan's zeta function $\varphi(s)$.

Principle. For any $s$ with $\Re(s)\geq \frac{11}{6}$ (or at least $\Re(s)> \frac{11}{6}$) and $|\Im(s)|\geq 1$ we have $$\varphi(s)\cdot\prod_{\substack{p\text{ prime }\\p\leq X(s)}}\left(1-\frac{\tau(p)}{p^s}+\frac{p^{11}}{p^{2s}}\right)\simeq 1\tag{1}$$ in some "suitable sense".

Question. Does previous Principle involving the identity $(1)$ work for any suitable sense? Explain your words. Many thanks.

Thus I am asking about the explanation of the meaning of previous Principle involving the identity $(1)$. I just wrote it as a similar statement than the Principle I from Harper's colloquium (at few first minutes of the colloquium), but I have no knowledges to know the meaning of it.

References:

[1] G. H. Hardy, Ramanujan: Twelve lectures on subjects suggested by his life and work, AMS Chelsea Publishing (2002).

[2] Adam J. Harper, The Riemann zeta function in short intervals [after Najnudel, and Arguin, Belius, Bourgade, Radziwiłł, and Soundararajan], Séminaire BOURBAKI, 71e année, 2018–2019, no 1159 (Mars 2019).

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    $\begingroup$ What is $X(s)$? $\endgroup$
    – lcv
    Commented Nov 7, 2019 at 12:07
  • $\begingroup$ My apologizes @lcv I'm an amateur and I'm asking if this expression $(1)$ makes sense for the Ramanujan's zeta function after I was studying the cited colloquium from YouTube, thus I can not provide an answer for your question about the meaning of this formula $(1)$, in particular what is $X(s)$. I evoke that my Principle can be true since the Ramanujan's zeta function shares with the Riemann zeta function some properties, but my post was ask about the meaning of previous expression. Many thanks. $\endgroup$
    – user142929
    Commented Nov 7, 2019 at 12:52
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    $\begingroup$ what is "ponent"? $\endgroup$
    – kodlu
    Commented Nov 10, 2019 at 20:43
  • $\begingroup$ Many thanks for your attention, then it seems a bad translation of the Spanish word "ponente" (the professor who is invited to the colloquium). Thanks again @kodlu I removed such word. $\endgroup$
    – user142929
    Commented Nov 10, 2019 at 22:56
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    $\begingroup$ If I'm not mistaken, the values of the Ramanujan function $\tau$ are the Fourier coefficients of the only cusp form of weight 12, which may explain the approximate equality you ask about. As such this cusp form should belong to the Selberg class and hence have a(n) Euler product whose reciprocal has a limited development of the considered shape. $\endgroup$ Commented Nov 14, 2019 at 19:18

1 Answer 1

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It seems, essentially, that you want to estimate the $L$-function

$L(s,\Delta)=\sum_{n=1}^{\infty}\tau(n) n^{-s}=\prod_p (1-\tau(p)p^{-s}+p^{11-2s})^{-1}$

as a short Euler product, in the sense that there exists a reasonably small integer $X(s)>0$ such that

$L(s,\Delta)\approx\prod_{p\leq X(s)} (1-\tau(p)p^{-s}+p^{11-2s})^{-1}$

with a reasonably small error. This question has been well-studied for $L$-functions in various families when $s$ lies on the line $\mathrm{Re}(s)=1$, though one can place $s$ elsewhere in the critical strip. The literature focusing on $\mathrm{Re}(s)=1$ is very extensive, so I'll focus on that for now.

Rather than studying this problem for a single $L$-function, it is often a bit more interesting to study the problem for, say, the family of $L$-functions associated to weight $k$ level 1 holomorphic cuspidal newforms, and understand the average and limiting behavior as $k\to\infty$. Here is an excellent paper on this matter by Lau and Wu: https://hal.archives-ouvertes.fr/hal-00097046/document. One could also fix the weight, say $k=2$, and pursue similar questions as the level $N$ varies. Here is an excellent paper on this matter by Cogdell and Michel: http://www.math.osu.edu/~cogdell.1/moments-www.ps. Both of these papers were inspired in part by the work of Granville and Soundararajan, where they considered the same problem for Dirichlet $L$-functions associated to real characters: https://arxiv.org/abs/math/0206031.

ADDED: I was mixing normalizations. For the normalization in the original question, $\mathrm{Re}(s)=1$ should be changed to $\mathrm{Re}(s)=13/2$

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  • $\begingroup$ Many thanks for your answer. $\endgroup$
    – user142929
    Commented Nov 19, 2019 at 19:58

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