Von Mangoldt function

In mathematics, the von Mangoldt function is an arithmetic function named after German mathematician Hans von Mangoldt. It is an example of an important arithmetic function that is neither multiplicative nor additive.

Definition

The von Mangoldt function, denoted by Λ(n), is defined as

$\Lambda(n) = \begin{cases} \log p & \text{if }n=p^k \text{ for some prime } p \text{ and integer } k \ge 1, \\ 0 & \text{otherwise.} \end{cases}$

The values of Λ(n) for the first nine positive numbers are

$0 , \log 2 , \log 3 , \log 2 , \log 5 , 0 , \log 7 , \log 2 , \log 3,$

which is related to (sequence A014963 in OEIS).

The summatory von Mangoldt function, ψ(x), also known as the Chebyshev function, is defined as

$\psi(x) = \sum_{n\le x} \Lambda(n).$

von Mangoldt provided a rigorous proof of an explicit formula for ψ(x) involving a sum over the non-trivial zeros of the Riemann zeta function. This was an important part of the first proof of the prime number theorem.

Properties

The von Mangoldt function satisfies the identity

$\log(n) = \sum_{d \mid n} \Lambda(d).$

The sum is taken over all integers d that divide n. This is proved by the fundamental theorem of arithmetic, since the terms that are not powers of primes are equal to 0. For example, consider the case n = 12 = 2² × 3. Then

$\begin{align} \sum_{d \mid 12} \Lambda(d) &= \Lambda(1) + \Lambda(2) + \Lambda(3) + \Lambda(4) + \Lambda(6) + \Lambda(12) \\ &= \Lambda(1) + \Lambda(2) + \Lambda(3) + \Lambda \left (2^2 \right ) + \Lambda(2 \times 3) + \Lambda \left (2^2 \times 3 \right) \\ &= 0 + \log(2) + \log(3) + \log(2) + 0 + 0 \\ &=\log (2 \times 3 \times 2) \\ &= \log(12). \end{align}$

By Möbius inversion, we have

$\Lambda (n) = - \sum_{d \mid n} \mu(d) \log(d) \ .$

Dirichlet series

The von Mangoldt function plays an important role in the theory of Dirichlet series, and in particular, the Riemann zeta function. In particular, one has

$\log \zeta(s)=\sum_{n=2}^\infty \frac{\Lambda(n)}{\log(n)}\,\frac{1}{n^s}, \qquad \text{Re}(s) > 1.$

The logarithmic derivative is then

$\frac {\zeta^\prime(s)}{\zeta(s)} = -\sum_{n=1}^\infty \frac{\Lambda(n)}{n^s}.$

These are special cases of a more general relation on Dirichlet series. If one has

$F(s) =\sum_{n=1}^\infty \frac{f(n)}{n^s}$

for a completely multiplicative function  f (n), and the series converges for Re(s) > σ₀, then

$\frac {F^\prime(s)}{F(s)} = - \sum_{n=1}^\infty \frac{f(n)\Lambda(n)}{n^s}$

converges for Re(s) > σ₀.

Chebyshev function

The second Chebyshev function ψ(x) is the summatory function of the von Mangoldt function:

$\psi(x) = \sum_{p^k\le x}\log p=\sum_{n \leq x} \Lambda(n) \ .$

The Mellin transform of the Chebyshev function can be found by applying Perron's formula:

$\frac{\zeta^\prime(s)}{\zeta(s)} = - s\int_1^\infty \frac{\psi(x)}{x^{s+1}}\,dx$

which holds for Re(s) > 1.

Exponential series

Hardy and Littlewood examined the series

$F(y)=\sum_{n=2}^\infty \left(\Lambda(n)-1\right) e^{-ny}$

in the limit y → 0⁺. Assuming the Riemann hypothesis, they demonstrate that

$F(y)=O\left(\frac{1}{\sqrt{y}}\right).$

Curiously, they also show that this function is oscillatory as well, with diverging oscillations. In particular, there exists a value K > 0 such that

$F(y)< -\frac{K}{\sqrt{y}}, \quad \text{ and } \quad F(y)> \frac{K}{\sqrt{y}}$

infinitely often. The graphic to the right indicates that this behaviour is not at first numerically obvious: the oscillations are not clearly seen until the series is summed in excess of 100 million terms, and are only readily visible when y < 10⁻⁵.

Riesz mean

The Riesz mean of the von Mangoldt function is given by

$\begin{align} \sum_{n\le \lambda} \left(1-\frac{n}{\lambda}\right)^\delta \Lambda(n) &= -\frac{1}{2\pi i} \int_{c-i\infty}^{c+i\infty} \frac{\Gamma(1+\delta)\Gamma(s)}{\Gamma(1+\delta+s)} \frac{\zeta^\prime(s)}{\zeta(s)} \lambda^s ds \\ &= \frac{\lambda}{1+\delta} + \sum_\rho \frac{\Gamma(1+\delta)\Gamma(\rho)}{\Gamma(1+\delta+\rho)} + \sum_n c_n \lambda^{-n}. \end{align}$

Here, λ and δ are numbers characterizing the Riesz mean. One must take c > 1. The sum over ρ is the sum over the zeroes of the Riemann zeta function, and

$\sum_n c_n \lambda^{-n}\,$

can be shown to be a convergent series for λ > 1.

Approximation by Riemann zeta zeros

File:Real part of n raised to first zeta zero.svg

The first Riemann zeta zero wave in the sum that approximates the von Mangoldt function

The real part of the sum over the zeta zeros:

$-\sum_{i=1}^{\infty} n^{\rho(i)}$, where ρ(i) is the i-th zeta zero, peaks at primes, as can be seen in the adjoining graph, and can also be verified through numerical computation. It does not sum up to the Von Mangoldt function.

File:Von Mangoldt function Fourier transform zeta zero duality.PNG

The Fourier transform of the von Mangoldt function gives a spectrum with imaginary parts of Riemann zeta zeros as spikes at the x-axis ordinates (right), while the von Mangoldt function can be approximated by zeta zero waves (left)

The Fourier transform of the von Mangoldt function gives a spectrum with spikes at ordinates equal to imaginary part of the Riemann zeta function zeros. This is sometimes called a duality.

See also

• Prime-counting function

External links

• Allan Gut, Some remarks on the Riemann zeta distribution (2005)
• Chris King, Primes out of thin air (2010)
• Heike, How plot Riemann zeta zero spectrum in Mathematica? (2012)