2

An integer is called even if it is divisible by 2. For integers written in a numeral system based on an even number, such as decimal, hexadecimal, or in any other base that is even, divisibility by 2 is easily tested by merely looking at the last digit. If it is even, then the whole number is even. In particular, when written in the decimal system, all multiples of 2 will end in 0, 2, 4, 6, or 8.

Two is the smallest prime number, and the only even prime number (for this reason it is sometimes called "the oddest prime").[3] The next prime is three. Two and three are the only two consecutive prime numbers. 2 is the first Sophie Germain prime, the first factorial prime, the first Lucas prime, and the first Ramanujan prime.[4]

Two is a pronic number and the only pronic prime.[5]

Two is the third (or fourth) Fibonacci number.

Two is the base of the binary system, the numeral system with the fewest tokens that allows denoting a natural number n substantially more concisely (with log₂ n tokens) than a direct representation by the corresponding count of a single token (with n tokens). This binary number system is used extensively in computing.

For any number x:

x + x = 2 · x addition to multiplication

x · x = x² multiplication to exponentiation

x^x = x↑↑2 exponentiation to tetration

Extending this sequence of operations by introducing the notion of hyperoperations, here denoted by "hyper(a,b,c)" with a and c being the first and second operand, and b being the level in the above sketched sequence of operations, the following holds in general:

hyper(x,n,x) = hyper(x,(n + 1),2).

Two has therefore the unique property that 2 + 2 = 2 · 2 = 2² = 2↑↑2 = 2↑↑↑2 = ..., disregarding the level of the hyperoperation, here denoted by Knuth's up-arrow notation. The number of up-arrows refers to the level of the hyperoperation.

Two is the only number x such that the sum of the reciprocals of the natural powers of x equals itself. In symbols

${\displaystyle \sum _{k=0}^{\infty }{\frac {1}{2^{k}}}=1+{\frac {1}{2}}+{\frac {1}{4}}+{\frac {1}{8}}+{\frac {1}{16}}+\cdots =2.}$

This comes from the fact that:

${\displaystyle \sum _{k=0}^{\infty }{\frac {1}{n^{k}}}=1+{\frac {1}{n-1}}\quad {\mbox{for all}}\quad n\in \mathbb {R} >1.}$

A Cantor space is a topological space ${\displaystyle 2^{\mathbb {N} }}$ homeomorphic to the Cantor set. The countably infinite product topology of the simplest discrete two-point space, {0, 1}, is the traditional elementary example.

Powers of two are central to the concept of Mersenne primes, and important to computer science. Two is the first Mersenne prime exponent.

Taking the square root of a number is such a common mathematical operation, that the spot on the root sign where the index would normally be written for cubic and other roots, may simply be left blank for square roots, as it is tacitly understood.

The square root of 2 was the first known irrational number.

The smallest field has two elements.

In a set-theoretical construction of the natural numbers, 2 is identified with the set {{∅},∅}. This latter set is important in category theory: it is a subobject classifier in the category of sets.

Two consecutive twos (as in "22" for "two twos"), or equivalently "2-2", is the only fixed point of John Conway's look-and-say function.[6] This in contrast, for example, with "1211", which would read as "one 1, one 2, and two 1s" or "111221".

There are no 2 x 2 magic squares; they also can be defined as the only null ${\displaystyle n}$ by ${\displaystyle n}$ magic square set.[7]

Two also has the unique property such that

${\displaystyle \sum _{k=0}^{n-1}2^{k}=2^{n}-1}$

and also

${\displaystyle \sum _{k=a}^{n-1}2^{k}=2^{n}-\sum _{k=0}^{a-1}2^{k}-1}$

for a not equal to zero

In any n-dimensional, euclidean space two distinct points determine a line.

In two dimensions, a digon is a polygon with two sides (or edges) and two vertices. On a circle, it is a tessellation with two antipodal points and 180° arc edges.

The simplest tessellation in two-dimensional space, though an improper tessellation, is that of two ${\displaystyle \infty }$-sided apeirogons joined along all their edges, coincident about a line that divides the plane in two. This order-2 apeirogonal tiling is the arithmetic limit of the family of dihedra {p, 2}.

For any polyhedron homeomorphic to a sphere, the Euler characteristic is χ = V − E + F = 2, where V is the number of vertices, E is the number of edges, and F is the number of faces.

There are two known sublime numbers, which are numbers with a perfect number of factors, whose sum itself yields a perfect number. 12 is one of the two sublime numbers, with the other being 76 digits long.[8]

• The number of polynucleotide strands in a DNA double helix.[9]
• The first magic number.[10]
• The atomic number of helium.[11]
• The ASCII code of "Start of Text".
• 2 Pallas, a large asteroid in the main belt and the second asteroid ever to be discovered.[12]
• The Roman numeral II (usually) stands for the second-discovered satellite of a planet or minor planet (e.g. Pluto II or (87) Sylvia II Remus).
• A binary star is a stellar system consisting of two stars orbiting around their center of mass.[13]
• The number of brain and cerebellar hemispheres.[14]

• The number of points scored on a safety in American football
• A field goal inside the three-point line is worth two points in basketball.
• The two in basketball is called the Shooting Guard
• 2 represents the catcher position in baseball.

International maritime pennant for 2

International maritime signal flag for 2

In pre-1972 Indonesian and Malay orthography, 2 was shorthand for the reduplication that forms plurals: orang (person), orang-orang or orang2 (people). In Astrology, Taurus is the second sign of the Zodiac. For Pythagorean numerology (a pseudoscience) the number 2 represents duality, the positive and negative poles that come into balance and seek harmony.

• List of highways numbered 2
• Binary number

Wikimedia Commons has media related to:
2 (number) (category)

• Prime curiosities: 2

Look up two or both in Wiktionary, the free dictionary.