# 1

1 is a number, and a numerical digit used to represent that number in numerals. It represents a single entity, the unit of counting or measurement. For example, a line segment of unit length is a line segment of length 1. 1 is the smallest positive integer. It is also sometimes considered the first of the infinite sequence of natural numbers, followed by 2, although by other definitions 1 is the second natural number, following 0.
The fundamental mathematical property of 1 is to be a multiplicative identity, meaning that any number multiplied by 1 returns that number. Most if not all properties of 1 can be deduced from this. In advanced mathematics, a multiplicative identity is often denoted 1, even if it is not a number. 1 is by convention not considered a prime number; although universal today, this was a matter of some controversy until the mid-20th century.

## Etymology

The word one can be used as a noun, an adjective and a pronoun.
It comes from the English word an, which comes from the Proto-Germanic root. The Proto-Germanic root comes from the Proto-Indo-European root *oi-no-.
Compare the Proto-Germanic root to Old Frisian an, Gothic ains, Danish en, Dutch een, German eins and Old Norse einn.
Compare the Proto-Indo-European root *oi-no- to Greek oinos, Latin unus, Old Persian, Old Church Slavonic -inu and ino-, Lithuanian vienas, Old Irish oin and Breton un.

## As a number

One, sometimes referred to as unity, is the first non-zero natural number. It is thus the integer after zero.
Any number multiplied by one remains that number, as one is the identity for multiplication. As a result, 1 is its own factorial, its own square and square root, its own cube and cube root, and so on. One is also the result of the empty product, as any number multiplied by one is itself. It is also the only natural number that is neither composite nor prime with respect to division, but instead considered a unit.

## As a digit

The glyph used today in the Western world to represent the number 1, a vertical line, often with a serif at the top and sometimes a short horizontal line at the bottom, traces its roots back to the Brahmic script of ancient India, where it was a simple vertical line. It was transmitted to Europe via Arabic during the Middle Ages.
In some countries, the serif at the top is sometimes extended into a long upstroke, sometimes as long as the vertical line, which can lead to confusion with the glyph for seven in other countries. Where the 1 is written with a long upstroke, the number 7 has a horizontal stroke through the vertical line.
While the shape of the 1 character has an ascender in most modern typefaces, in typefaces with text figures, the character usually is of x-height, as, for example, in.
, using J as a symbol for 1.
Many older typewriters do not have a separate symbol for 1 and use the lowercase letter l instead. It is possible to find cases when the uppercase J is used, while it may be for decorative purposes.

## Mathematics

### Definitions

Mathematically, 1 is:
• in arithmetic and calculus, the natural number that follows 0 and the multiplicative identity element of the integers, real numbers and complex numbers;
• more generally, in algebra, the multiplicative identity, usually of a group or a ring.
Formalizations of the natural numbers have their own representations of 1. In the Peano axioms, 1 is the successor of 0, in Principia Mathematica it is defined as the set of all singletons, and in the Von Neumann cardinal assignment of natural numbers it is defined as the set.
In a multiplicative group or monoid, the identity element is sometimes denoted 1, but e is also traditional. However, 1 is especially common for the multiplicative identity of a ring, i.e., when an addition and 0 are also present. When such a ring has characteristic n not equal to 0, the element called 1 has the property that . Important examples are finite fields.
By definition, 1 is the magnitude, absolute value, or norm of a unit complex number, unit vector, and a unit matrix. Note that the term unit matrix is sometimes used to mean something quite different.
By definition, 1 is the probability of an event that is almost certain to occur.
In category theory, 1 is sometimes used to denote the terminal object of a category.
In number theory, 1 is the value of Legendre's constant, which was introduced in 1808 by Adrien-Marie Legendre in expressing the asymptotic behavior of the prime-counting function. Legendre's constant was originally conjectured to be approximately 1.08366, but was proven to equal exactly 1 in 1899.

### Properties

is often referred to as "base 1", since only one mark – the tally itself – is needed. This is more formally referred to as a unary numeral system. Unlike base 2 or base 10, this is not a positional notation.
Since the base 1 exponential function always equals 1, its inverse does not exist.
There are two ways to write the real number 1 as a recurring decimal: as 1.000..., and as 0.999....
1 is the first figurate number of every kind, such as triangular number, pentagonal number and centered hexagonal number, to name just a few.
In many mathematical and engineering problems, numeric values are typically normalized to fall within the unit interval from 0 to 1, where 1 usually represents the maximum possible value in the range of parameters. Likewise, vectors are often normalized to give unit vectors, that is vectors of magnitude one, because these often have more desirable properties. Functions, too, are often normalized by the condition that they have integral one, maximum value one, or square integral one, depending on the application.
Because of the multiplicative identity, if f is a multiplicative function, then f must equal 1.
It is also the first and second number in the Fibonacci sequence and is the first number in many other mathematical sequences.
The definition of a field requires that 1 must not be equal to 0. Thus, there are no fields of characteristic 1. Nevertheless, abstract algebra can consider the field with one element, which is not a singleton and is not a set at all.
1 is the most common leading digit in many sets of data, a consequence of Benford's law.
1 is the only known Tamagawa number for a simply connected algebraic group over a number field.
The generating function that has all coefficients 1 is given by
This power series converges and has finite value if and only if,.

### Primality

1 is by convention neither a prime number nor a composite number, but a unit, like −1 and, in the Gaussian integers, i and −i. The fundamental theorem of arithmetic guarantees unique factorization over the integers only up to units.
1 appears to meet the naïve definition of a prime number, being evenly divisible only by 1 and itself. As such, some mathematicians considered it a prime number as late as the middle of the 20th century, but mathematical consensus has generally and since then universally been to exclude it for a variety of reasons such as complicating the fundamental theorem of arithmetic and other theorems related to prime numbers. 1 is the only positive integer divisible by exactly one positive integer, whereas prime numbers are divisible by exactly two positive integers, composite numbers are divisible by more than two positive integers, and zero is divisible by all positive integers.

### Table of basic calculations

 Division 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 ÷ x 1 0.5 0 0.25 0.2 0.1 0 0.125 0 0.1 0 0.08 0 0 0 x ÷ 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

 Exponentiation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 x 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

## In technology

• The resin identification code used in recycling to identify polyethylene terephthalate.
• The ITU country code for the North American Numbering Plan area, which includes the United States, Canada, and parts of the Caribbean
• A binary code is a sequence of 1 and 0 that is used in computers for representing any kind of data.
• In many physical devices, 1 represents the value for "on", which means that electricity is flowing.
• The numerical value of true in many programming languages.
• 1 is the ASCII code of "Start of Header".

## In science

• Dimensionless quantities are also known as quantities of dimension one.
• 1 is the atomic number of hydrogen.
• +1 is the electric charge of positrons and protons.
• Group 1 of the periodic table consists of the alkali metals.
• Period 1 of the periodic table consists of just two elements, hydrogen and helium.
• The dwarf planet Ceres has the minor-planet designation 1 Ceres because it was the first asteroid to be discovered.
• The Roman numeral I often stands for the first-discovered satellite of a planet or minor planet. For some earlier discoveries, the Roman numerals originally reflected the increasing distance from the primary instead.

## In philosophy

In the philosophy of Plotinus and a number of other neoplatonists, The One is the ultimate reality and source of all existence. Philo of Alexandria regarded the number one as God's number, and the basis for all numbers.