Lindelöf hypothesis

In mathematics, the Lindelöf hypothesis is a conjecture by Finnish mathematician Ernst Leonard Lindelöf (see Lindelöf (1908)) about the rate of growth of the Riemann zeta function on the critical line that is implied by the Riemann hypothesis.

It says that, for any ε > 0,

$\zeta\left(\frac12 + it\right) =O(t^\varepsilon),$

as t tends to infinity (see O notation). Since ε can be replaced by a smaller value, we can also write the conjecture as, for any positive ε,

$\zeta\left(\frac12 + it\right) =o(t^\varepsilon).$

The μ function

If σ is real, then μ(σ) is defined to be the infimum of all real numbers a such that ζ(σ + iT) = O(T ^a). It is trivial to check that μ(σ) = 0 for σ > 1, and the functional equation of the zeta function implies that μ(σ) = μ(1 − σ) − σ + 1/2. The Phragmen–Lindelöf theorem implies that μ is a convex function. The Lindelöf hypothesis states μ(1/2) = 0, which together with the above properties of μ implies that μ(σ) is 0 for σ ≥ 1/2 and 1/2 − σ for σ ≤ 1/2.

Lindelöf's convexity result together with μ(1) = 0 and μ(0) = 1/2 implies that 0 ≤ μ(1/2) ≤ 1/4. The upper bound of 1/4 was lowered by Hardy and Littlewood to 1/6 by applying Weyl's method of estimating exponential sums to the approximate functional equation. It has since been lowered to slightly less than 1/6 by several authors using long and technical proofs, as in the following table:

μ(1/2) ≤	μ(1/2) ≤	Author
1/4	0.25	Lindelöf (1908)	Convexity bound
1/6	0.1667	Hardy & Littlewood (?)
163/988	0.1650	Walfisz (1924)
27/164	0.1647	Titchmarsh (1932)
229/1392	0.164512	Phillips (1933)
	0.164511	Rankin (1955)
19/116	0.1638	Titchmarsh (1942)
15/92	0.1631	Min (1949)
6/37	0.16217	Haneke (1962)
173/1067	0.16214	Kolesnik (1973)
35/216	0.16204	Kolesnik (1982)
139/858	0.16201	Kolesnik (1985)
32/205	0.1561	Huxley (2002, 2005)
53/342	0.1549	Bourgain (2014)

Relation to the Riemann hypothesis

Backlund (1918–1919) showed that the Lindelöf hypothesis is equivalent to the following statement about the zeros of the zeta function: for every ε > 0, the number of zeros with real part at least 1/2 + ε and imaginary part between T and T + 1 is o(log(T)) as T tends to infinity. The Riemann hypothesis implies that there are no zeros at all in this region and so implies the Lindelöf hypothesis. The number of zeros with imaginary part between T and T + 1 is known to be O(log(T)), so the Lindelöf hypothesis seems only slightly stronger than what has already been proved, but in spite of this it has resisted all attempts to prove it and is very hard.

Means of powers (or moments) of the zeta function

The Lindelöf hypothesis is equivalent to the statement that

$\frac{1}{T} \int_0^T|\zeta(1/2+it)|^{2k}\,dt = O(T^{\varepsilon})$

for all positive integers k and all positive real numbers ε. This has been proved for k = 1 or 2, but the case k = 3 seems much harder and is still an open problem.

There is a much more precise conjecture about the asymptotic behavior of the integral: it is believed that

$\int_0^T|\zeta(1/2+it)|^{2k} \, dt = T\sum_{j=0}^{k^2}c_{k,j}\log(T)^{k^2-j} + o(T)$

for some constants c_k,j. This has been proved by Littlewood for k = 1 and by Heath-Brown (1979) for k = 2 (extending a result of Ingham (1926) who found the leading term).

Conrey & Ghosh (1998) suggested the value $(42/9!)\prod_ p \left((1-p^{-1})^4(1+4p^{-1}+p^{-2})\right)$ for the leading coefficient when k is 6, and Keating & Snaith (2000) used random matrix theory to suggest some conjectures for the values of the coefficients for higher k. The leading coefficients are conjectured to be the product of an elementary factor, a certain product over primes, and the number of n by n Young tableaux given by the following sequence:

• 1, 1, 2, 42, 24024, 701149020, (sequence A039622 in OEIS).

Other consequences

Denoting by p_n the n-th prime number, a result by Albert Ingham, shows that the Lindelöf hypothesis implies that, for any ε > 0,

$p_{n+1}-p_n\ll p_n^{1/2+\varepsilon}\,$

if n is sufficiently large. However, this result is much worse than that of the large prime gap conjecture.