Two F-22 Raptor in flying.jpg
   HOME

TheInfoList



OR:

2 (two) is a number, numeral and
digit Digit may refer to: Mathematics and science * Numerical digit, as used in mathematics or computer science ** Hindu-Arabic numerals, the most common modern representation of numerical digits * Digit (anatomy), the most distal part of a limb, such ...
. It is the natural number following 1 and preceding 3. It is the smallest and only even prime number. Because it forms the basis of a
duality Duality may refer to: Mathematics * Duality (mathematics), a mathematical concept ** Dual (category theory), a formalization of mathematical duality ** Duality (optimization) ** Duality (order theory), a concept regarding binary relations ** Dual ...
, it has religious and spiritual significance in many cultures.


Evolution


Arabic digit

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. The modern
Chinese Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of va ...
and Japanese languages (and Korean Hanja) still use this method. The
Gupta script The Gupta script (sometimes referred to as Gupta Brahmi script or Late Brahmi script)Sharma, Ram. '' 'Brahmi Script' ''. Delhi: BR Publishing Corp, 2002 was used for writing Sanskrit and is associated with the Gupta Empire of the Indian subcon ...
rotated the two lines 45 degrees, making them diagonal. The top line was sometimes also shortened and had its bottom end curve towards the center of the bottom line. In the Nagari script, the top line was written more like a curve connecting to the bottom line. In the Arabic Ghubar writing, the bottom line was completely vertical, and the digit looked like a dotless closing question mark. Restoring the bottom line to its original horizontal position, but keeping the top line as a curve that connects to the bottom line leads to our modern digit. In fonts with text figures, digit 2 usually is of x-height, for example, .


Etymology of ''two''

The word ''two'' is derived from the
Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlement of Britain, Anglo ...
words (
feminine Femininity (also called womanliness) is a set of attributes, behaviors, and roles generally associated with women and girls. Femininity can be understood as socially constructed, and there is also some evidence that some behaviors considered fe ...
), (neuter), and (masculine, which survives today in the form twain). The pronunciation , like that of ''who'' is due to the
labialization Labialization is a secondary articulatory feature of sounds in some languages. Labialized sounds involve the lips while the remainder of the oral cavity produces another sound. The term is normally restricted to consonants. When vowels involve ...
of the vowel by the ''w'' (combare from womb), which then disappeared before the related sound. The successive stages of pronunciation for the Old English would thus be , , , , and finally .


In mathematics

An integer is called ''
even Even may refer to: General * Even (given name), a Norwegian male personal name * Even (surname) * Even (people), an ethnic group from Siberia and Russian Far East ** Even language, a language spoken by the Evens * Odd and Even, a solitaire game w ...
'' if it is divisible by 2. For integers written in a numeral system based on an even number, such as
decimal The decimal numeral system (also called the base-ten positional numeral system and denary or decanary) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic numeral ...
,
hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, hexa ...
, 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, 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. The next prime is three, which makes two and three the only two consecutive prime numbers. Two is the first prime number that does not have a proper twin prime with a difference two, while three is the first such prime number to have a twin prime. In consequence, the first pair of twin primes, three and
five 5 is a number, numeral, and glyph. 5, five or number 5 may also refer to: * AD 5, the fifth year of the AD era * 5 BC, the fifth year before the AD era Literature * ''5'' (visual novel), a 2008 visual novel by Ram * ''5'' (comics), an awa ...
, encase
four 4 (four) is a number, numeral and digit. It is the natural number following 3 and preceding 5. It is the smallest semiprime and composite number, and is considered unlucky in many East Asian cultures. In mathematics Four is the smallest c ...
in-between, which is the square of two, or 2^2. Two is the first Sophie Germain prime, the first
factorial prime A factorial prime is a prime number that is one less or one more than a factorial (all factorials greater than 1 are even). The first 10 factorial primes (for ''n'' = 1, 2, 3, 4, 6, 7, 11, 12, 14) are : : 2 (0! +&n ...
, the first Lucas prime, and the first
Ramanujan prime In mathematics, a Ramanujan prime is a prime number that satisfies a result proven by Srinivasa Ramanujan relating to the prime-counting function. Origins and definition In 1919, Ramanujan published a new proof of Bertrand's postulate which, as ...
. Two is a Motzkin number, a Bell number, an
all-Harshad number In mathematics, a harshad number (or Niven number) in a given number base is an integer that is divisible by the sum of its digits when written in that base. Harshad numbers in base are also known as -harshad (or -Niven) numbers. Harshad number ...
, a meandric number, a semi-meandric number, and an Meander (mathematics), open meandric number. It is also the third (or fourth) Fibonacci number. Two is the radix, base of Binary number, the binary system, the numeral system with the fewest tokens that allows denoting a natural number substantially more concisely (with tokens) than a direct representation by the corresponding count of a single token (with tokens). This binary number system is used extensively in Computer, computing. For any number ''x'': :''x'' + ''x'' = 2 · ''x'' addition to multiplication :''x'' · ''x'' = ''x''2 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 , 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, :\sum_^\frac =1+\frac+\frac+\frac+\frac+\cdots=2. This comes from the fact that: :\sum_^\infin \frac =1+\frac \quad\mbox \quad n\in\mathbb R > 1. A Cantor space is a topological space 2^\mathbb homeomorphic to the Cantor set. The countably Infinity, infinite product topology of the simplest discrete two-point space, , is the traditional elementary example. The sum of the Multiplicative inverse, reciprocals of all non-zero triangular numbers converges to 2. Power of two, 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 (mathematics), field has two elements. In a set theory, 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 (mathematics), fixed point of John Horton Conway, John Conway's look-and-say sequence, look-and-say function. 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 set, null n by n magic square set. Two also has the unique property such that, :\sum_^ 2^k = 2^ - 1 and also, with ''a'' not equal to zero, :\sum_^ 2^k = 2^n - \sum_^ 2^k - 1. In any Dimension (mathematics and physics), ''n''-dimensional, euclidean space two distinct point (geometry), points determine a line (geometry), line. In two dimensions, a digon is a polygon with two sides (or Edge (geometry), edges) and two Vertex (geometry), 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 Infinity, \infty-sided apeirogons joined along all their edges, coincident about a line (geometry), line that divides the Plane (geometry), plane in two. This order-2 apeirogonal tiling is the arithmetic limit of the family of dihedra . For any polyhedron homeomorphic to a sphere, the Euler characteristic is , where ''V'' is the number of Vertex (geometry), vertices, ''E'' is the number of Edge (geometry), edges, and ''F'' is the number of Face (geometry), faces. The Diagonal#Regular polygons, long diagonal of a regular hexagon is of length two when its sides are of unit length. Whereas a square of unit side length has a diagonal equal to the square root of two, and a cube of unit side length has a space diagonal equal to the square root of three, a space diagonal inside a tesseract measures two when its side lengths are of length one. There are two known sublime numbers, which are numbers with a Perfect number, perfect number of factors, whose sum itself yields a perfect number. 12 (number), 12 is one of the two sublime numbers, with the other being 76 digits long.


In science

*The number of polynucleotide strands in a DNA double helix. *The first Magic number (physics), magic number. *The atomic number of helium. *The ASCII code of "Start Of Text, Start of Text". *2 Pallas, a large asteroid in the main belt and the second asteroid ever to be discovered. *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 Star system, stellar system consisting of two stars Planetary orbit, orbiting around their center of mass. *The number of brain and cerebellum, cerebellar cerebral hemispheres, hemispheres.


In sports

*The number of points scored on a Safety (gridiron football score), safety in American football *A field goal (basketball), 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.


Other

In pre-1972 Indonesian language, Indonesian and Malay language, Malay orthography, ''2'' was shorthand for the reduplication that forms plurals: ''orang'' (person), ''orang-orang'' or ''orang2'' (people). In Astrology, Taurus (constellation), Taurus is the second astrological sign, 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.


See also

*List of highways numbered 2 *Binary number


References


External links


Prime curiosities: 2
{{DEFAULTSORT:2 (Number) 2 (number) Integers