In number theory, an average order of an arithmetic function is some simpler or better-understood function which takes the same values "on average".
Contents
- Examples
- Calculating mean values using Dirichlet series
- The density of the k th power free integers in N
- Visibility of lattice points
- Divisor functions
- Better average order
- Definition
- Zeta function and Dirichlet series in FqX
- The density of the k th power free polynomials in FqX
- Polynomial Divisor functions
- Number of divisors
- Polynomial von Mangoldt function
- Polynomial Euler totient function
- References
Let
as
It is conventional to choose an approximating function
In cases where the limit
exists, it is said that
Examples
Calculating mean values using Dirichlet series
In case
for some arithmetic function
This identity often provides a practical way to calculate the mean value in terms of the Riemann zeta function. This is illustrated in the following example.
The density of the k-th power free integers in N
For an integer
We calculate the natural density of these numbers in N, that is, the average value of
The function
By the Möbius inversion formula, we get
where
where
and hence,
By comparing the coefficients, we get
Using (1), we get
We conclude that,
where for this we used the relation
which follows from the Möbius inversion formula.
In particular, the density of the square-free integers is
Visibility of lattice points
We say that two lattice points are visible from one another if there is no lattice point on the open line segment joining them.
Now, if gcd(a, b) = d > 1, then writing a = da2, b = db2 one observes that the point (a2, b2) is on the line segment which joins (0,0) to (a, b) and hence (a, b) is not visible from the origin. Thus (a, b) is visible from the origin implies that (a, b) = 1. Conversely, it is also easy to see that gcd(a, b) = 1 implies that there is no other integer lattice point in the segment joining (0,0) to (a,b). Thus, (a, b) is visible from (0,0) if and only if gcd(a, b) = 1.
Notice that
Thus, one can show that the natural density of the points which are visible from the origin is given by the average,
interestingly,
Divisor functions
Consider the generalization of
The following are true:
where
Better average order
This notion is best discussed through an example. From
(
we have the asymptotic relation
which suggests that the function
Definition
Let h(x) be a function on the set of monic polynomials over Fq. For
This is the mean value (average value) of h on the set of monic polynomials of degree n. We say that g(n) is an average order of h if
as n tends to infinity.
In cases where the limit,
exists, it is said that h has a mean value (average value) c.
Zeta function and Dirichlet series in Fq[X]
Let Fq[X]=A be the ring of polynomials over the finite field Fq.
Let h be a polynomial arithmetic function (i.e. a function on set of monic polynomials over A). Its corresponding Dirichlet series define to be
where for
The polynomial zeta function is then
Similar to the situation in N, every Dirichlet series of a multiplicative function h has a product representation (Euler product):
Where the product runs over all monic irreducible polynomials P.
For example, the product representation of the zeta function is as for the integers:
Unlike the classical zeta function,
In a similar way, If ƒ and g are two polynomial arithmetic functions, one defines ƒ * g, the Dirichlet convolution of ƒ and g, by
where the sum extends over all monic divisors d of m, or equivalently over all pairs (a, b) of monic polynomials whose product is m. The identity
The density of the k-th power free polynomials in Fq[X]
Define
We calculate the average value of
By multiplicativity of
Denote
Making the substitution
Finally, expand the left-hand side in a geometric series and compare the coefficients on
Hence,
And since it doesn't depend on n this is also the mean value of
Polynomial Divisor functions
In Fq[X], we define
We will compute
First, notice that
where
Therefore,
Substitute
Finally we get that,
Notice that
Thus, if we set
which resembles the analogous result for the integers:
Number of divisors
Let
where
Expanding the right-hand side into power series we get,
Substitute
It is interesting to note that not a lot is known about the error term for the integers, while in the polynomials case, there is no error term! This is because of the very simple nature of the zeta function
Polynomial von Mangoldt function
The Polynomial von Mangoldt function is defined by:
Where the logarithm is taken on the basis of q.
Proposition. The mean value of
Proof. Let m be a monic polynomial, and let
We have,
Hence,
and we get that,
Now,
Thus,
We got that:
Now,
Hence,
and by dividing by
Polynomial Euler totient function
Define Euler totient function polynomial analogue,