Generalized Riemann hypothesis


Generalized Riemann hypothesis

The Riemann hypothesis is one of the most important conjectures in mathematics. It is a statement about the zeros of the Riemann zeta function. Various geometrical and arithmetical objects can be described by so-called global L-functions, which are formally similar to the Riemann zeta-function. One can then ask the same question about the zeros of these "L"-functions, yielding various generalizations of the Riemann hypothesis. Many mathematicians believe these generalizations of the Riemann hypothesis to be true. The only cases of these conjectures which have been proven occur in the function field case (not the number field case).

Global "L"-functions can be associated to elliptic curves, number fields (in which case they are called Dedekind zeta-functions), Maass forms, and Dirichlet characters (in which case they are called Dirichlet L-functions). When the Riemann hypothesis is formulated for Dedekind zeta-functions, it is known as the extended Riemann hypothesis (ERH) and when it is formulated for Dirichlet "L"-functions, it is known as the generalized Riemann hypothesis (GRH). These two statements will be discussed in more detail below. (Many mathematicians use the label "generalized Riemann hypothesis" to cover the extension of the Riemann hypothesis to all global "L"-functions, not just the special case of Dirichlet "L"-functions.)

Generalized Riemann hypothesis (GRH)

The generalized Riemann hypothesis (for Dirichlet "L"-functions) was probably formulated for the first time by Piltz in 1884. Like the original Riemann hypothesis, it has far reaching consequences about the distribution of prime numbers.

The formal statement of the hypothesis follows. A Dirichlet character is a completely multiplicative arithmetic function χ such that there exists a positive integer "k" with χ("n" + "k") = χ("n") for all "n" and χ("n") = 0 whenever gcd("n", "k") > 1. If such a character is given, we define the corresponding Dirichlet L-function by:L(chi,s) = sum_{n=1}^infty frac{chi(n)}{n^s}

for every complex number "s" with real part > 1. By analytic continuation, this function can be extended to a meromorphic function defined on the whole complex plane. The generalized Riemann hypothesis asserts that for every Dirichlet character χ and every complex number "s" with L(χ,"s") = 0: if the real part of "s" is between 0 and 1, then it is actually 1/2.

The case χ("n") = 1 for all "n" yields the ordinary Riemann hypothesis.

Consequences of GRH

An "arithmetic progression" in the natural numbers is a set of numbers of the form "a", "a"+"d", "a"+2"d", "a"+3"d", ... where "a" and "d" are natural numbers and "d" is non-zero.
Dirichlet's theorem states that if "a" and "d" are coprime, then such an arithmetic progression contains infinitely many prime numbers.Let π("x","a","d") denote the number of prime numbers in this progression which are less than or equal to "x".If the generalized Riemann hypothesis is true, then for every coprime "a" and "d" and for every ε > 0

:pi(x,a,d) = frac{1}{varphi(d)} int_2^x frac{1}{ln t},dt + O(x^{1/2+epsilon})quadmbox{ as } x oinfty

where φ("d") denotes Euler's phi function and O is the Landau symbol. This is a considerable strengthening of the prime number theorem.

If GRH is true, then for every prime "p" there exists a primitive root modulo "p" (a generator of the multiplicative group of integers modulo "p") which is less than 70 (ln("p"))2; this is often used in proofs.

Goldbach's weak conjecture also follows from the generalized Riemann hypothesis.

If GRH is true, then the Miller-Rabin primality test is guaranteed to run in polynomial time. (A polynomial-time primality test which does not require GRH, the AKS primality test, was published in 2002.)

If GRH is true, then Shanks-Tonelli algorithm is guaranteed to run in polynomial time. Shanks-Tonelli algorithm isuseful for finding solutions to: x^2 equiv n mod pwhere "n" is a quadratic residue mod p,"p" is prime and x is the unknown variable. This algorithm is an important step inthe Quadratic Sieve (Carl Pomerance) factoring algorithm.

Assuming the truth of the GRH, the estimate of the character sum in the Pólya-Vinogradov inequality can be improved to Oleft(sqrt{q}loglog q ight), "q" being the modulus of the character.

Extended Riemann hypothesis (ERH)

Suppose "K" is a number field (a finite-dimensional field extension of the rationals Q) with ring of integers O"K" (this ring is the integral closure of the integers Z in "K"). If "a" is an ideal of O"K", other than the zero ideal we denote its norm by "Na". The Dedekind zeta-function of "K" is then defined by:zeta_K(s) = sum_a frac{1}{(Na)^s}for every complex number "s" with real part > 1. The sum extends over all non-zero ideals "a" of O"K".

The Dedekind zeta-function satisfies a functional equation and can be extended by analytic continuation to the whole complex plane. The resulting function encodes important information about the number field "K". The extended Riemann hypothesis asserts that for every number field "K" and every complex number "s" with ζ"K"("s") = 0: if the real part of "s" is between 0 and 1, then it is in fact 1/2.

The ordinary Riemann hypothesis follows from the extended one if one takes the number field to be Q, with ring of integers Z.

See also

* Artin's conjecture
* Selberg class


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