|Unicode representation of Roman numeral||Ⅱ, ⅱ|
|prefixes||di- (from Greek)|
Two has many properties in mathematics. 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 and hexadecimal, divisibility by 2 is easily tested by merely looking at the one's place digit. If it's even, then the whole number is even.
Two is the smallest and the first prime number, and the only even one. The next prime is three. 2 is the first Sophie Germain prime, the first factorial prime, the first Lucas prime, and the first Smarandache-Wellin prime. It is an Eisenstein prime with no imaginary part and real part of the form . It is also a Stern prime, a Pell number, and a Markov number, appearing in infinitely many solutions to the Markov Diophantine equation involving odd-indexed Pell numbers.
Two is the third Fibonacci number, being the sum of the first two, 1 and 1.
For any number x:
- x+x = 2·x addition to multiplication
- x·x = x2 multiplication to exponentiation
- xx = x↑↑2 exponentiation to tetration
Two also has the unique property that 2+2 = 2·2 = 22.
Taking the square root of a number is such a common mathematical operation, that the spot on the root sign where the exponent would normally be written for cubic roots and other such roots, is left blank for square roots, as it is considered tacit.
The smallest field has two elements.
Two is a primorial, as well as its own factorial. Two often occurs in numerical sequences, such as the Fibonacci number sequence, but not quite as often as one does. Two is also a Motzkin number, a Bell number, an all-Harshad number, a meandric number, a semi-meandric number, and an open meandric number.
The Arabic glyph
The glyph we use today in the Western world to represent the number 2 traces its roots back to the Brahmin Indians, who wrote 2 as two horizontal lines (it is still written that way in modern Chinese, and is analogous to the Roman numeral II). The Gupta rotated the two lines 45 degrees, making them diagonal, and sometimes also made the top line shorter and made its bottom end curve towards the center of the bottom line. Apparently for speed, the Nagari started making the top line more like a curve and connecting to the bottom line. The Ghubar Arabs made the bottom line completely vertical, and now the glyph 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 glyph.
- The number of polynucleotide strands in a DNA double helix.
- The first Magic number (physics).
- The atomic number of helium.
- The number of stars in a binary star system.
- Messier object M2, a magnitude 6.5 globular cluster in the constellation Aquarius.
- The New General Catalogue object NGC 2, a magnitude 14.2 spiral galaxy in the constellation Pegasus
- The Saros number of the solar eclipse series which began on May 4 2861 BC and ended on June 21 1563 BC . The duration of Saros series 2 was 1298.1 years, and it contained 73 solar eclipses.
- The Saros number of the lunar eclipse series which began on February 21 2541 BC and ended on April 22 1225 BC. The duration of Saros series 2 was 1316.2 years, and it contained 74 lunar eclipses.
In human culture
The duality of all things is an important notion in most cultures and religions. The most common philosophical dichotomy is perhaps the one of good and evil, but there are many others. See dualism for an overview. In Hegelian dialectic, the process of antithesis creates two perspectives from one.
Two (二, èr) is a good number in Chinese culture. There is a Chinese saying "good things come in pairs". It is common to use double symbols in product brandnames, e.g. double happiness, double coin, double elephants etc. Cantonese people like the number two because it sounds the same as the word "easy" (易) in Cantonese.
Two is the first digit of international telephone dialing codes primarily for countries in Africa. On most phones, the 2 key is associated with the letters A, B, and C, but on the BlackBerry it is the key for T and Y.
In other fields
Groups of two:
- The denomination of U.S. paper currency with Thomas Jefferson's portrait on it; (last issued 1996; as of 2004 rarely seen in general circulation).
- The denomination of the Canadian monetary coin featuring a polar bear, commonly known as the "toonie" or "twonie", after the $1 coin, the loonie.
- The denomination of the $2 New Zealand coin, featuring the white heron.
- The name of several fictional characters: Number Two.
- The designation of the Trans-Canada Highway in most of the province of New Brunswick.
- One of the radio callsign prefixes allocated to the United Kingdom.
- In Astrology, Taurus is the 2nd astrological sign of the Zodiac.
- In music, the Roman numeral ii is the supertonic scale degree, chord, or diatonic function, when distinguished II = major and ii = minor.
- The years: A.D. 2, 2 B.C., 1902 or 2002.
ca:Dos da:2 (tal) de:Zwei es:Dos eo:Du fr:2 (nombre) he:2 (מספר) ia:2 it:Due ja:2 ko:2 la:2 nah:Ome nl:Twee no:2 (tall) nn:Talet 2 pl:Dwa pt:Dois simple:Two sl:2 (število) fi:2 (luku) sv:Två th:2 (จำนวน) zh:2