On this Wikipedia the language links are at the top of the page across from the article two fast birthday. This article is about the number. For the years, see 2 BC and AD 2.

It is the natural number following 1 and preceding 3. The digit used in the modern Western world to represent the number 2 traces its roots back to the Indic Brahmic script, where “2” was written as two horizontal lines. Two is the smallest prime number, and the only even prime number, and for this reason it is sometimes called “the oddest prime”. As the smallest prime number, it is also the smallest non-zero pronic number, and the only pronic prime. 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, divisibility by 2 is easily tested by merely looking at the last digit. If it is even, then the whole number is even.

This binary number system is used extensively in computing. The square root of 2 was the first known irrational number. Powers of two are central to the concept of Mersenne primes, and important to computer science. Two is the first Mersenne prime exponent. This latter set is important in category theory: it is a subobject classifier in the category of sets.

A set that is a field has a minimum of two elements. A number is deficient when the sum of its divisors is less than twice the number, whereas an abundant number has a sum of its proper divisors that is larger than the number itself. A number is perfect if it is equal to its aliquot sum, or the sum of all of its positive divisors excluding the number itself. 2-2″, is the only fixed point of John Conway’s look-and-say function. 2 is the harmonic mean of the divisors of 6, the smallest Ore number greater than one. In a Euclidean space of any dimension greater than zero, two distinct points determine a line. The long diagonal of a regular hexagon is of length two when its sides are of unit length.

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. The number of polynucleotide strands in a DNA double helix. The ASCII code of “Start of Text”.

2 Pallas, a large asteroid in the main belt and the second asteroid ever to be discovered. A binary star is a stellar system consisting of two stars orbiting around their center of mass. The number of brain and cerebellar hemispheres. A field goal inside the three-point line is worth two points in basketball. 2 represents the catcher position in baseball. Georges Ifrah, The Universal History of Numbers: From Prehistory to the Invention of the Computer transl.

Subscription or participating institution membership required. The On-Line Encyclopedia of Integer Sequences. Factorial primes: primes which are within 1 of a factorial number. Bell or exponential numbers: number of ways to partition a set of n labeled elements. Semi-meanders: number of ways a semi-infinite directed curve can cross a straight line n times. Meandric numbers: number of ways a river can cross a road n times. 2, counted up to rotations and reflections.