# Portal:Number theory

## Number theory

**Number theory** is the branch of pure mathematics concerned with the properties of numbers in general, and integers in particular, as well as the wider classes of problems that arise from their study. Number theory may be subdivided into several fields, according to the methods used and the type of questions investigated. (*See the list of number theory topics*.)

The term "arithmetic" is also used to refer to number theory. This is a somewhat older term, which is no longer as popular as it once was. Number theory used to be called *the higher arithmetic*, but this too is dropping out of use. Nevertheless, it still shows up in the names of mathematical fields (arithmetic functions, arithmetic of elliptic curves, arithmetic geometry). This sense of the term *arithmetic* should not be confused either with elementary arithmetic, or with the branch of logic which studies Peano arithmetic as a formal system. Mathematicians working in the field of number theory are called **number theorists**.

## Selected article

The **Riemann hypothesis**, first formulated by Bernhard Riemann in 1859, is one of the most famous and important unsolved problems in mathematics. It has been an open question for almost 150 years, despite attracting concentrated efforts from many outstanding mathematicians. Unlike some other celebrated problems, it is more attractive to professionals in the field than to amateurs.

The Riemann hypothesis (RH) is a conjecture about the distribution of the zeros of the Riemann zeta-function ζ(*s*). The Riemann zeta-function is defined for all complex numbers *s* ≠ 1. It has zeros at the negative even integers. These are called the trivial zeros. The Riemann hypothesis is concerned with the non-trivial zeros, and states that the real part of any non-trivial zero of the Riemann zeta function is ½.

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## Selected picture

Time-keeping on a clock gives an example of modular arithmetic, the "clock group" is represented by the group **Z/12Z** for a 12-hour clock and **Z/24Z** for a 24-hour clock.

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## Did you know?

- ...that every positive integer can be expressed as the sum of four squares of integers?
- ...that it is impossible to separate any power higher than the second into two like powers?
- ...that only 35 even numbers have been identified which are not the sum of a pair of Twin primes?
- ...that 16 is the only integer that satisfies x^y=y^x for distinct positive integers x and y, being 4^2 = 2^4

## Categories

*Topics in Number theory*

Types of number theory | Numbers | Equations | Arithmetic |
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