Riemann hypothesis

In mathematics, the Riemann hypothesis, first formulated by Bernhard Riemann in 1859, is one of the most famous of all unsolved problems. It has been an open question for well over a century, despite attracting concentrated efforts from many outstanding mathematicians. Unlike some other celebrated problems, it is more attractive to professionals in the field than to amateurs.

The Riemann hypothesis is a conjecture about the distribution of the zeross of the Riemann zeta function ζ(s). The Riemann zeta function is defined for all complex numbers s ≠ 1. It has certain so-called "trivial" zeros for s = −2, s = −4, s = −6, ... The Riemann hypothesis is concerned with the non-trivial zeros, and states that:

The real part of any non-trivial zero of the Riemann zeta function is 1/2.

The Riemann hypothesis is one of the most important open problems of contemporary mathematics; a $1,000,000 prize has been offered by the Clay Mathematics Institute for a proof. Most mathematicians believe the Riemann hypothesis to be true. (J. E. Littlewood and Atle Selberg have been reported as skeptical.) In 2004, Xavier Gourdon verified the Riemann hypothesis through the first ten trillion non-trivial zeros using the Odlyzko-Schönhage algorithm.

History

Riemann mentioned the conjecture that became known as the Riemann hypothesis in his 1859 paper On the Number of Primes Less Than a Given Magnitude, but as it was not essential to his central purpose in that paper, he did not attempt a proof. Riemann knew that the non-trivial zeros of the zeta function were symmetrically distributed about the line z = 1/2 + it, and he knew that all of its non-trivial zeros must lie in the range 0 ≤ Re(z) ≤ 1.

In 1896 Hadamard and de la Vallée-Poussin independently proved that no zeros could lie on the line Re(z) = 1, so all non-trivial zeros must lie in the interior of the critical strip 0 < Re(z) < 1. This was a key step in the first complete proofs of the prime number theorem.

In 1900 Hilbert included the Riemann hypothesis in his famous list of 23 unsolved problems - it is part of Problem 8 in Hilbert's list. He said of the problem: "If I were to awaken after having slept for a thousand years, my first question would be: Has the Riemann hypothesis been proven?".

In 1914 Hardy proved that an infinite number of zeros lie on the critical line Re(z) = 1/2. However, it was still possible that an infinite number (and possibly the majority) of non-trivial zeros could lie elsewhere in the critical strip. Later work by Hardy and Littlewood in 1921 and by Selberg in 1942 gave estimates for the average density of zeros on the critical line.

Recent work has focused on the explicit calculation of the locations of large numbers of zeros (in the hope of finding a counterexample) and placing upper bounds on the proportion of zeros that can lie away from the critical line (in the hope of reducing this to zero).

The Riemann hypothesis and primes

The traditional formulation of the Riemann hypothesis obscures somewhat the true importance of the conjecture. The zeta function has a deep connection to the distribution of prime numbers and Helge von Koch proved in 1901 that the Riemann hypothesis is equivalent to the following considerable strengthening of the prime number theorem:

where, π(x) is the prime-counting function, ln(x) is the natural logarithm of x, and the O-notation is the Landau symbol.

The Riemann hypothesis can be generalized in various ways by replacing the Riemann zeta function by the formally similar global L-functions. None of these generalizations has been proven or disproven. See generalized Riemann hypothesis.

Other consequences of the Riemann hypothesis

The statement that the equation
:
is valid for every s with real part greater than 1/2, with the sum on the right hand side converging, is equivalent to the Riemann hypothesis. From this we can also conclude that if the Mertens function is defined by

e > 1/2

The Riemann hypothesis is equivalent to certain conjectures about the rate of growth of other multiplicative functions aside from μ(n). For instance, if σ(n) is the sum of divisors function, given by

Two other equivalent statements to the Riemann hypothesis involve the Farey sequence. If F_n is the Farey sequence of order n, beginning with 1/n and up to 1/1, then the claim that
:
is equivalent to the Riemann hypothesis. Here is the number of terms in the Farey sequence of order n, and e>1/2. Similarly, equivalent to the Riemann hypothesis is
:
where now

e > −1.

The Riemann hypothesis is equivalent to the statement that has no zeros in the strip

.

Stronger conjectures than the Riemann hypothesis have also been formulated, but they have a tendency to be disproven. Paul Turan showed that if the sums

The Riemann hypothesis has various weaker consequences as well; one is the Lindelöf hypothesis on the rate of growth of the zeta function on the critical line, which says

where

e > 0;

Another conjecture is the large prime gap conjecture; Cramér proved that
on the Riemann hypothesis we have that the largest gaps between successive
prime numbers is . On average, the gap is
merely and numerical evidence does not suggest it
can grow nearly as fast as the Riemann hypothesis seems to allow, much less
as fast as the best that can at present be shown without it.

Possible connection with operator theory

It has long been speculated that the correct way to derive the Riemann hypothesis has been to find a self-adjoint operator, from the existence of which the statement on the real parts of the zeroes of ζ(s) would follow when one applies the criterion on real eigenvalues. This has led to many investigations; but has not yet proven fruitful.

External links

• The Riemann hypothesis
• Computation of zeros of the Zeta function
• The million dollar prize for its solution
• Prime Obsession: Bernhard Riemann and the Greatest Unsolved Problem in Mathematics by John Derbyshire. National Academies Press, 2003. 448 page book at a non-specialist level, can be read online for free.
• Press release regarding the proposed proof
• Math Games: Ten Trillion Zeta Zeros A discussion of Xavier Gourdon's calculation of the first ten trillion non-trivial zeros.
• Graphs of the Riemann Zeta function (animation)

  References

• Edwards, H. M., Riemann's Zeta Function, Academic Press, 1974
• du Sautoy, Marcus, The Music of the Primes, HarperCollins, 2003
• Sabbagh, Karl, The Riemann Hypothesis, first American paperback edition, Farrar, Straus and Giroux, 2004
• Titchmarsh, E. C, The Theory of the Riemann Zeta Function, second revised (Heath-Brown) edition, Oxford University Press, 1986