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Wikipedia states under the entry for the von Mangoldt function:

The Fourier transform of the von Mangoldt function gives a spectrum with spikes at ordinates equal to imaginary part of the Riemann zeta function zeros.

(I believe "ordinates" should be changed to "abscissas".)

First, what does taking the Fourier transform of the von Mangoldt function mean?

Second, if meaningful, is it true?

Third, if true, how might this be related to the sum of the exponentials $e^{iImg(z_n)x}$ over the non-trivial zeros $z_n$ above and below the real axis, assuming the RH is true?

(There is some history behind this statement in an older MO-Q, but the analysis there is not clear to me either.)

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  • $\begingroup$ I suggested once that maybe the real part of the non trivial zeros of $\zeta$ could arise as the Fourier transform of the trivial zeros and their imaginary part as the Fourier transform of (powers of) primes or of the logarithms thereof. But I think we lack the relevant theoretical framework to adress these issues rigorously. $\endgroup$ Commented Sep 20, 2019 at 6:08
  • $\begingroup$ a very extensive MO posting that addresses these issues: mathoverflow.net/q/162076/11260 $\endgroup$ Commented Sep 20, 2019 at 6:12
  • $\begingroup$ @CarloBeenakker , yes, I found that, but the arguments and conclusions there are not clear to me. $\endgroup$ Commented Sep 20, 2019 at 13:19
  • $\begingroup$ @SylvainJULIEN , it appears Mangoldt was able to work out rigorously the basic relation in 1895, but Riemann, of course, had understood already. See Wikipedia "Explicit formulae for L functions" en.m.wikipedia.org/wiki/Explicit_formulae_for_L-functions and "The Riemann hypothesis" by Conrey ams.org/notices/200303/fea-conrey-web.pdf $\endgroup$ Commented Sep 23, 2019 at 21:07

1 Answer 1

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Start with the explicit formula

$$\sum_{n \le x}\Lambda(n) =\frac1{2i\pi} \int_{2-i\infty}^{2+i\infty} \frac{-\zeta'(s)}{\zeta(s)}\frac{x^s}s ds=1_{x > 1}\sum Res(\frac{-\zeta'(s)}{\zeta(s)}\frac{x^s}s)$$ $$=1_{x > 1}( x - \sum_\rho \frac{x^\rho}{\rho} - \frac12 \log 2\pi - \sum_{k=1}^\infty \frac{x^{-2k}}{-2k})$$

Since $\sum_\rho \frac1{|\rho|^2}<\infty$ the RHS converges in $L^1_{loc}$ thus we can differentiate both sides in the sense of distributions, the RHS being continuous at $1$ we get

$$\sum_n \Lambda(n) \delta(x-n) =1_{x > 1} - 1_{x > 1}\sum_\rho x^{\rho-1} +\frac{d}{dx} 1_{x > 1}\log(1-x^{-2})$$

If the RH is true, letting $x =e^u$ and multiplying both side by $e^{u/2}$, since $e^{u/2}\delta(e^u-n) = \frac{\delta(u-\log n)}{n^{1/2}}$

$$\sum_n \frac{\Lambda(n)}{n^{1/2}}\delta(u-\log n) =1_{u > 0}e^{u/2} - 1_{u > 0}\sum_t e^{itu} +e^{-u/2}\frac{d}{du}1_{u > 0} \log(1-e^{-2u}))$$

Making both side even $$\sum_n \frac{\Lambda(n)}{n^{1/2}} (\delta(u-\log n)+\delta(u+\log n))=e^{|u|/2} - \sum_t e^{itu} -e^{-|u|/2}(\frac{d}{d|u|} \log(1-e^{-2|u|})$$

Which means that we have the Fourier transform in the sense of distributions $$\mathcal{F}^{-1}\left[2 \pi \sum_t \delta(\omega-t)\right] = e^{|u|/2}-e^{-|u|/2}(\frac{d}{d|u|} \log(1-e^{-2|u|})-\sum_n \frac{\Lambda(n)}{n^{1/2}} (\delta(u-\log n)+\delta(u+\log n)) $$

If the RH is not true then those things are true only in the sense of analytic functionals, for example they hold when using Gaussians as test functions.

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  • $\begingroup$ Please identify $t$ ($\rho$ or $\rho-1$), $\omega$, the variables and normalization of the Fourier transform, $1_{u>0}$ (Heaviside step fct?), etc. If you take a FT over finite limits, i.e., multply the RHS by a rect fct. centered at the origin and apply the FT, then, if your analysis is correct, the delta fcts. will become convolved with a sinc fct (the FT of a rectangle fct.), or equivalently replaced by a sinc fct, which you can then plot. You should have sinc fcts normalized and distributed according to the LHS. $\endgroup$ Commented Sep 20, 2019 at 19:14
  • $\begingroup$ I see that you are borrrowing some of the notation in the analysis of en.m.wikipedia.org/wiki/Chebyshev_function $\endgroup$ Commented Sep 20, 2019 at 19:56
  • $\begingroup$ I'm not sure of what you are saying. Under RH $\sum_t$ is over the imaginary parts of non-trivial zeros. In the yellow part the normalization is $\mathcal{F}[f(u)](\omega) = \int_{-\infty}^\infty f(u) e^{-i \omega u} du$. If $\phi$ is Schwartz then $\int_{-\infty}^\infty e^{-i u \omega}\phi(u) ( e^{|u|/2}-e^{-|u|/2}(\frac{d}{d|u|} \log(1-e^{-2|u|})-\sum_n \frac{\Lambda(n)}{n^{1/2}} (\delta(u-\log n)+\delta(u+\log n)))du$ $ = 2\pi\sum_t \mathcal{F}[\phi](\omega-t)$ $\endgroup$
    – reuns
    Commented Sep 20, 2019 at 20:01
  • $\begingroup$ I plotted here the numerical version math.stackexchange.com/a/3122844/276986 $\endgroup$
    – reuns
    Commented Sep 20, 2019 at 20:11
  • $\begingroup$ Thanks, it'll take s little time to digest it. $\endgroup$ Commented Sep 21, 2019 at 15:43

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