Chebyshev function


In mathematics, the Chebyshev function is either of two related functions. The first Chebyshev function or is given by
with the sum extending over all prime numbers that are less than or equal to.
The second Chebyshev function is defined similarly, with the sum extending over all prime powers not exceeding
where is the von Mangoldt function. The Chebyshev functions, especially the second one, are often used in proofs related to prime numbers, because it is typically simpler to work with them than with the prime-counting function, Both Chebyshev functions are asymptotic to , a statement equivalent to the prime number theorem.
Both functions are named in honour of Pafnuty Chebyshev.

Relationships

The second Chebyshev function can be seen to be related to the first by writing it as
where is the unique integer such that and. The values of are given in. A more direct relationship is given by
Note that this last sum has only a finite number of non-vanishing terms, as
The second Chebyshev function is the logarithm of the least common multiple of the integers from 1 to .
Values of for the integer variable is given at.

Asymptotics and bounds

The following bounds are known for the Chebyshev functions:
Furthermore, under the Riemann hypothesis,
for any.
Upper bounds exist for both and such that,
for any.
An explanation of the constant 1.03883 is given at.

The exact formula

In 1895, Hans Carl Friedrich von Mangoldt proved an explicit expression for as a sum over the nontrivial zeros of the Riemann zeta function:
Here runs over the nontrivial zeros of the zeta function, and is the same as, except that at its jump discontinuities it takes the value halfway between the values to the left and the right:
From the Taylor series for the logarithm, the last term in the explicit formula can be understood as a summation of over the trivial zeros of the zeta function,, i.e.
Similarly, the first term,, corresponds to the simple pole of the zeta function at 1. It being a pole rather than a zero accounts for the opposite sign of the term.

Properties

A theorem due to Erhard Schmidt states that, for some explicit positive constant, there are infinitely many natural numbers such that
and infinitely many natural numbers such that
In little- notation, one may write the above as
Hardy and Littlewood prove the stronger result, that

Relation to primorials

The first Chebyshev function is the logarithm of the primorial of, denoted :
This proves that the primorial is asymptotically equal to, where "" is the little- notation and together with the prime number theorem establishes the asymptotic behavior of.

Relation to the prime-counting function

The Chebyshev function can be related to the prime-counting function as follows. Define
Then
The transition from to the prime-counting function,, is made through the equation
Certainly, so for the sake of approximation, this last relation can be recast in the form

The Riemann hypothesis

The Riemann hypothesis states that all nontrivial zeros of the zeta function have real part. In this case,, and it can be shown that
By the above, this implies
Good evidence that the hypothesis could be true comes from the fact proposed by Alain Connes and others, that if we differentiate the von Mangoldt formula with respect to we get. Manipulating, we have the "Trace formula" for the exponential of the Hamiltonian operator satisfying
and
where the "trigonometric sum" can be considered to be the trace of the operator , which is only true if.
Using the semiclassical approach the potential of satisfies:
with as .
solution to this nonlinear integral equation can be obtained by
in order to obtain the inverse of the potential:

Smoothing function

The smoothing function is defined as
It can be shown that

Variational formulation

The Chebyshev function evaluated at minimizes the functional
so