Li's criterion

From Wikipedia, the free encyclopedia

In mathematics, in the area of number theory, Li's criterion is a particular statement about the positivity of a certain series that is completely equivalent to the Riemann hypothesis. The criterion is named after Xian-Jin Li, who presented it in 1997. Recently, Enrico Bombieri and Jeffrey C. Lagarias provided a generalization, showing that Li's positivity condition applies to any collection of points that lie on the Re s = 1/2 axis.

[edit] Definition

The Riemann ξ function is given by

$\xi (s)=\frac{1}{2}s(s-1) \pi^{-s/2} \Gamma \left(\frac{s}{2}\right) \zeta(s)$

where ζ is the Riemann zeta function. Consider the sequence

$\lambda_n = \frac{1}{(n-1)!} \left. \frac{d^n}{ds^n} \left[s^{n-1} \log \xi(s) \right] \right|_{s=1}$

Li's criterion is then the statement that

the Riemann hypothesis is completely equivalent to the statement that $λ n > 0$ for every positive integer n.

The numbers $λ n$ may also be expressed in terms of the non-trivial zeros of the Riemann zeta function:

$\lambda_n=\sum_{\rho} \left[1- \left(1-\frac{1}{\rho}\right)^n\right]$

where the sum extends over ρ, the non-trivial zeros of the zeta function. This conditionally convergent sum should be understood in the sense that is usually used in number theory, namely, that

$\sum_\rho = \lim_{N\to\infty} \sum_{|\Im(\rho)|\le N}$

[edit] A generalization

Bombieri and Lagarias demonstrate that a similar criterion holds for any collection of complex numbers, and is thus not restricted to the Riemann hypothesis. More precisely, let $R = {ρ}$ be any collection of complex numbers ρ, not containing $ρ = 1$ , which satisfies

$\sum_\rho \frac{1+\left|\Re(\rho)\right|}{(1+|\rho|)^2} < \infty$

Then one may make several equivalent statements about such a set. One such statement is the following:

One has $\Re(\rho) \le 1/2$ for every ρ if and only if

$\sum_\rho\Re\left[1-\left(1-\frac{1}{\rho}\right)^{-n}\right] \ge 0$

One may make a more interesting statement, if the set R obeys a certain functional equation under the replacement of $s \mapsto (1-s)$ . Namely, if, whenever ρ is in R, then both the complex conjugate $\overline{\rho}$ and $1 - ρ$ are in R, then Li's criterion can be stated as:

One has $\Re(\rho) = 1/2$ for every ρ if and only if

$\sum_\rho\left[1-\left(1-\frac{1}{\rho}\right)^{n}\right] \ge 0$

Bombieri and Lagarias also show that Li's criterion follows from Weil's criterion for the Riemann hypothesis.

[edit] References

Bombieri, Enrico; Lagarias, Jeffrey C. (1999). "Complements to Li's criterion for the Riemann hypothesis". Journal of Number Theory 77 (2): 274–287. DOI:10.1006/jnth.1999.2392. MR 1702145.

Lagarias, Jeffrey C. (2004). "Li coefficients for automorphic L-functions". arXiv:math.MG/0404394.

Li, Xian-Jin (1997). "The positivity of a sequence of numbers and the Riemann hypothesis". Journal of Number Theory 65 (2): 325–333. DOI:10.1006/jnth.1997.2137. MR 1462847.

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Category: Zeta and L-functions

Li's criterion

From Wikipedia, the free encyclopedia

[edit] Definition

[edit] A generalization

[edit] References

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