Explicit formulae for L-functions
In mathematics, the explicit formulae for L-functions are relations between sums over the complex number zeroes of an L-function and sums over prime powers, introduced by Riemann (1859) for the Riemann zeta function. Such explicit formulae have been applied also to questions on bounding the discriminant of an algebraic number field, and the conductor of a number field.
is a Fourier transform of F:
, where is the Γ′/Γ.
digamma function
There are several slightly different ways to state the explicit formula.[5] André Weil's form of the explicit formula states
where
Roughly speaking, the explicit formula says the Fourier transform of the zeros of the zeta function is the set of prime powers plus some elementary factors. Once this is said, the formula comes from the fact that the Fourier transform is a unitary operator, so that a scalar product in time domain is equal to the scalar product of the Fourier transforms in the frequency domain.
The terms in the formula arise in the following way.
Weil's explicit formula can be understood like this. The target is to be able to write that:
where Λ is the von Mangoldt function.
So that the Fourier transform of the non trivial zeros is equal to the primes power symmetrized plus a minor term. Of course, the sum involved are not convergent, but the trick is to use the unitary property of Fourier transform which is that it preserves scalar product:
where are the Fourier transforms of .
At a first look, it seems to be a formula for functions only, but in fact in many cases it also works when is a distribution. Hence, by setting
Generalizations[edit]
The Riemann zeta function can be replaced by a Dirichlet L-function of a Dirichlet character χ. The sum over prime powers then gets extra
factors of χ(p m), and the terms Φ(1) and Φ(0) disappear because the L-series has no poles.
More generally, the Riemann zeta function and the L-series can be replaced by the Dedekind zeta function of an algebraic number field or a Hecke L-series. The sum over primes then gets replaced by a sum over prime ideals.
Applications[edit]
Riemann's original use of the explicit formula was to give an exact formula for the number of primes less than a given number. To do this, take F(log(y)) to be y1/2/log(y) for 0 ≤ y ≤ x and 0 elsewhere. Then the main term of the sum on the right is the number of primes less than x. The main term on the left is Φ(1); which turns out to be the dominant terms of the prime number theorem, and the main correction is the sum over non-trivial zeros of the zeta function. (There is a minor technical problem in using this case, in that the function F does not satisfy the smoothness condition.)