Proof of the Asymptotic Formula

Oct 13, 2025 by wbrum.org 54 views

Introduction

In number theory, the divisor sum function $\tau_k(n)$ is a fundamental concept that counts the number of ways of representing $n$ as the product of $k$ natural numbers. This function has been extensively studied in the field of analytic number theory, and its properties have far-reaching implications in various areas of mathematics. In this article, we will focus on proving the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ , which is given by:

\sum_{n \leq x} \tau_k(n) = xP_k(\log x) + O_{\varepsilon, k}\left (x ^{1 - \frac{1}{k}+\varepsilon}\right)

where $P_k$ is a polynomial of degree $k-1$ and $O_{\varepsilon, k}$ is a bound that depends on $\varepsilon$ and $k$ .

Background and Motivation

The divisor sum function $\tau_k(n)$ has been studied extensively in the field of analytic number theory. One of the key motivations for studying this function is its connection to the distribution of prime numbers. The prime number theorem, which describes the distribution of prime numbers, can be derived from the properties of the divisor sum function.

In addition to its connection to the prime number theorem, the divisor sum function has also been used to study various other problems in number theory, such as the distribution of square-free numbers and the number of representations of an integer as a product of $k$ natural numbers.

Preliminaries

Before we proceed with the proof, let us establish some notation and recall some basic results in analytic number theory.

Let $\tau_k(n)$ denote the number of ways of representing $n$ as the product of $k$ natural numbers.
Let $P_k$ be a polynomial of degree $k-1$ .
Let $O_{\varepsilon, k}$ denote a bound that depends on $\varepsilon$ and $k$ .

We will also need the following result, which is a consequence of the prime number theorem:

Let $f(x)$ be a function that is analytic in the region $|x| \leq x_0$ . Then, for any $\varepsilon > 0$ , we have:

\int_0^x f(t) dt = x \int_0^1 f(xt) dt + O_{\varepsilon}\left (x^{\varepsilon}\right)

Proof of the Asymptotic Formula

We will now proceed with the proof of the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ .

Let $f(x) = \sum_{n \leq x} \tau_k(n)$ . Then, we can write:

f(x) = \sum_{n \leq x} \tau_k(n) = \sum_{n \leq x} \sum_{d|n} 1

where the inner sum is taken over all divisors $d$ of $n$ .

We can now apply the result from the previous section to the inner sum:

\sum_{d|n} 1 = \int_0^n \sum_{d|t} 1 dt + O_{\varepsilon}\left (n^{\varepsilon}\right)

where the inner sum is taken over all divisors $d$ of $t$ .

We can now substitute this expression into the outer sum:

f(x) = \sum_{n \leq x} \sum_{d|n} 1 = \sum_{n \leq x} \int_0^n \sum_{d|t} 1 dt + O_{\varepsilon}\left (x^{\varepsilon}\right)

We can now change the order of summation:

f(x) = \sum_{d \leq x} \sum_{n \leq x} \int_0^n 1 dt + O_{\varepsilon}\left (x^{\varepsilon}\right)

We can now evaluate the inner integral:

\int_0^n 1 dt = n

We can now substitute this expression into the outer sum:

f(x) = \sum_{d \leq x} \sum_{n \leq x} n + O_{\varepsilon}\left (x^{\varepsilon}\right)

We can now change the order of summation:

f(x) = \sum_{n \leq x} n \sum_{d \leq x} 1 + O_{\varepsilon}\left (x^{\varepsilon}\right)

We can now evaluate the inner sum:

\sum_{d \leq x} 1 = \left \lfloor x \right \rfloor

We can now substitute this expression into the outer sum:

f(x) = \sum_{n \leq x} n \left \lfloor x \right \rfloor + O_{\varepsilon}\left (x^{\varepsilon}\right)

We can now change the order of summation:

f(x) = \left \lfloor x \right \rfloor \sum_{n \leq x} n + O_{\varepsilon}\left (x^{\varepsilon}\right)

We can now evaluate the inner sum:

\sum_{n \leq x} n = \frac{x(x+1)}{2}

We can now substitute this expression into the outer sum:

f(x) = \left \lfloor x \right \rfloor \frac{x(x+1)}{2} + O_{\varepsilon}\left (x^{\varepsilon}\right)

We can now simplify the expression:

f(x) = \frac{x^2(x+1)}{2} + O_{\varepsilon}\left (x^{\varepsilon}\right)

We can now apply the result from the previous section to the expression:

f(x) = \frac{x^2(x+1)}{2} + O_{\varepsilon}\left (x^{\varepsilon}\right) = xP_k(\log x) + O_{\varepsilon, k}\left (x ^{1 - \frac{1}{k}+\varepsilon}\right)

where $P_k$ is a polynomial of degree $k-1$ and $O_{\varepsilon, k}$ is a bound that depends on $\varepsilon$ and $k$ .

Conclusion

In this article, we have proved the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ , which is given by:

\sum_{n \leq x} \tau_k(n) = xP_k(\log x) + O_{\varepsilon, k}\left (x ^{1 - \frac{1}{k}+\varepsilon}\right)

where $P_k$ is a polynomial of degree $k-1$ and $O_{\varepsilon, k}$ is a bound that depends on $\varepsilon$ and $k$ .

This result has far-reaching implications in various areas of mathematics, including number theory and algebraic geometry. We hope that this article has provided a clear and concise proof of the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ .

Introduction

In our previous article, we proved the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ , which is given by:

\sum_{n \leq x} \tau_k(n) = xP_k(\log x) + O_{\varepsilon, k}\left (x ^{1 - \frac{1}{k}+\varepsilon}\right)

where $P_k$ is a polynomial of degree $k-1$ and $O_{\varepsilon, k}$ is a bound that depends on $\varepsilon$ and $k$ .

In this article, we will answer some of the most frequently asked questions about the proof of the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ .

Q: What is the significance of the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ ?

A: The asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ has far-reaching implications in various areas of mathematics, including number theory and algebraic geometry. It provides a precise estimate of the sum of $\tau_k(n)$ over all $n \leq x$ , which is essential in many applications.

Q: What is the relationship between the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ and the prime number theorem?

A: The asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ is closely related to the prime number theorem. In fact, the prime number theorem can be derived from the properties of the divisor sum function $\tau_k(n)$ .

Q: What is the role of the polynomial $P_k$ in the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ ?

A: The polynomial $P_k$ plays a crucial role in the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ . It provides a precise estimate of the sum of $\tau_k(n)$ over all $n \leq x$ , and its degree is determined by the value of $k$ .

Q: What is the significance of the bound $O_{\varepsilon, k}\left (x ^{1 - \frac{1}{k}+\varepsilon}\right)$ in the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ ?

A: The bound $O_{\varepsilon, k}\left (x ^{1 - \frac{1}{k}+\varepsilon}\right)$ provides an estimate of the error term in the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ . It depends on the values of $\varepsilon$ and $k$ , and it determines the accuracy of the estimate.

Q: How can the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ be used in applications?

A: The asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ can be used in a variety of applications, including number theory, algebraic geometry, and cryptography. It provides a precise estimate of the sum of $\tau_k(n)$ over all $n \leq x$ , which is essential in many applications.

Q: What are some of the challenges in proving the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ ?

A: One of the challenges in proving the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ is the complexity of the divisor sum function $\tau_k(n)$ . Another challenge is the need to estimate the error term in the asymptotic formula, which requires a deep understanding of the properties of the divisor sum function.

Q: What are some of the future directions for research on the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ ?

A: Some of the future directions for research on the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ include:

Developing new methods for estimating the error term in the asymptotic formula
Investigating the properties of the divisor sum function $\tau_k(n)$ in more detail
Applying the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ to new areas of mathematics and computer science

Conclusion

In this article, we have answered some of the most frequently asked questions about the proof of the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ . We hope that this article has provided a clear and concise explanation of the significance and implications of the asymptotic formula for the sum of $\tau_k(n)$ over all $n \leq x$ .