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2 (number)
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{{otherusesof|2|2 (disambiguation)}}
{{redirect|II}}
{| width="250" border="1" style="float: right; background: #feffff; border-collapse: collapse; margin: 0.5em"
|-
| colspan="2" | {{numbers (digits)}}
|-
|
Cardinal number Cardinal
| 2
two
|-
|
Ordinal number Ordinal
| 2nd
second
|-
|
Numeral system
|
binary numeral system binary
|-
|
Factorization
|
prime number prime
|-
|
Divisors
| 1, 2
|-
|
Roman numeral
| II
|-
| Unicode representation of Roman numeral
| â…¡, â…±
|-
|
numerical prefix prefixes
|
Wiktionary:di- di- (from
Greek language Greek)
Wiktionary:duo- duo- Wiktionary:bi- bi- (from
Latin)
Wiktionary:twi- twi- (
Old English language Old English)
|-
|
Binary numeral system Binary
| 10
|-
|
Octal
| 2
|-
|
Duodecimal
| 2
|-
|
Hexadecimal
| 2
|-
|
Hebrew (language) Hebrew ||
ב (Bet)
|}
'''2''' ('''two''') is a
number,
numeral, and
glyph. It is the
natural number following
1 (number) 1 and preceding
3 (number) 3.
In mathematics
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
3 (number) 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.
Despite being a prime, two is also a
highly composite number, because it has more divisors than one. The next highly composite number is
4 (number) four.
Two is a factor of
10 (number) ten, so
vulgar fractions with 2 in the
denominator do not yield infinite
decimal expansions, as is the case with most primes.
Two is the
base of the simplest
numeral system in which
natural numbers can be written concisely, the
binary numeral system binary system widely used in
computers.
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
Two also has the unique property that 2+2 = 2·2 = 2
2.
Power of two Powers of 2 are central to the concept of
Mersenne primes, and important to
computer science. 2 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 exponent would normally be written for cubic roots and other such roots, is left blank for square roots, as it is considered tacit.
The
square root of 2 was the first known
irrational number.
The smallest
field (mathematics) field has two elements.
In the
set theory set-theoretical construction of the natural numbers, 2 is identified with the set {0,1}. This latter set is important in
category theory: it is a
subobject classifier in the category of sets.
It is possible to have a mathemathical expression that has only the number 2 in it with more than one
mathematical procedure.
eg. (√ 2^2 − 2 + 2 ÷ 2 × 2 = 2)
Two is a
primorials primorial, as well as its own
factorial. Two often occurs in numerical sequences, such as the
Fibonacci numbers Fibonacci number sequence, but not quite as often as one does. Two is also a
Motzkin number, a
Bell number, an
Harshad number all-Harshad number, a
meandric number, a
semi-meandric number, and an
open meandric number.
Two is the number of
Eight queens puzzle n-Queens Problem solutions for n = 4. With one exception, all known solutions to
Znám's problem start with 2.
The Arabic glyph
Image:Evolution2glyph.png
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 written language Chinese and
Japanese writing system Japanese, 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.
In fonts with
text figures, 2 usually has the same height as a lowercase X, for example,
Image:TextFigs256.png.
In science
Two is:
*The number of
polynucleotide strands in a
DNA double helix.
*The first
Magic number (physics) Magic Number.
*The
atomic number of
helium.
'''In
astronomy'''
*The number of stars in a
binary star system.
*
Messier object Globular Cluster M2 M2, a magnitude 6.5
globular cluster in the constellation
Aquarius (constellation) Aquarius.
* The
New General Catalogue [http://www.ngcic.org/ object]
NGC 2, a
visual magnitude magnitude 14.2
spiral galaxy in the
constellation Pegasus (constellation) Pegasus
*The
Saros [http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html 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 [http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html 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
Goodness and value theory good and
evil, but there are many others. See
dualism for an overview. In
Georg Wilhelm Friedrich Hegel 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.
Guangdong Cantonese people like the number two because it sounds the same as the word "easy" (易) in
Cantonese language Cantonese.
In
Finland, two
candles are lit on
Independence Day. Putting them on the windowsill invokes the symbolical meaning of division, and thus independence.
In the philosophy of
Pythagoras, the
dyad is the second thing created (see the article
monad for details).
The numeral 2 is sometimes used as a shorthand for "to", such as
b2b (business-to-business). This form of abbreviation is especially common in the world of
popular music,
Hip hop music hip-hop, rap, and
R&B, where the "too" or "to" in group names, album titles, and song names is replaced with "2". The
Eurodance group
2 Unlimited is one example. In some cases, the
Roman numeral II is substituted, as seen in the name of R&B group
Boyz II Men. The names of
computer software that translate data from one format into another also commonly employ the numeral 2, such as
dvi2ps and
texi2roff.
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".
Words that can be used as synonyms for two include
brace (grouping) brace,
couple,
deuce,
duet,
duo,
pair,
twain,
twins, and both.
In
rhetorics,
hendiadys is a
figure of speech where two words with similar or identical meanings are used where one would be sufficient.
Among children, or when otherwise calling for subtlety, the phrase "number 2" can refer to the act of
defecating (with
urination being "number 1").
In sports
In
rugby union, 2 is the number of the
Rugby union positions#2. Hooker hooker. In
baseball, two represents the
catcher's position. In
basketball terminology, two represents the
shooting guard or "off guard".
2 is the retired number for former baseball great
Charlie Gehringer.
From
1991 until his retirement in
2005, 2 was the number borne on the car of
NASCAR
Nextel Cup driver
Rusty Wallace.
In technology
Two is the
DVD region in
Europe,
South Africa, the
Middle East and
Japan.
Two is the first digit of international
telephone List of country calling codes 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.
On American
computer keyboards, the "2" key is associated with the "
@" symbol. On English
computer keyboards, the "2" key is associated with the
" symbol.
The "
2 Meters two meter band" in
amateur radio includes the frequencies from 144 to 146 MHz. In the Americas, the two meter band extends to 148 MHz.
In other fields
Image:ICS_Two.svg right|thumb|100px|[[International maritime signal flag for 2]]
Groups of two:
*
Lists of pairs
**
list of famous pairs
**
list of twins
Two is:
*The denomination of
United States dollar 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
Canada Canadian Canadian dollar monetary coin featuring a
polar bear, commonly known as the "
toonie" or "twonie", after the $1 coin, the
loonie.
*The denomination of the
New Zealand dollar $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.
*
U.S. Route 2, two separated highways in the northern tier of the United States, the western segment connecting
Everett, Washington to
St. Ignace, Michigan and the eastern route connecting
Rouses Point, New York to
Houlton, Maine.
*The lowest channel of television in the United States, Canada, and Mexico on which television signals are broadcast.
*One of the radio
ITU prefix callsign prefixes allocated to the
United Kingdom.
*In
Astrology,
Taurus (constellation) Taurus is the 2nd
astrological sign of the
Zodiac.
*In
music, the Roman numeral ii is the
supertonic scale degree,
Chord (music) chord, or
diatonic function, when distinguished II = major and ii = minor.
*
Franz Schubert and
Felix Mendelssohn each wrote two trios for piano, violin, and violoncello.
*
Béla Bartók and
Felix Mendelssohn each wrote two concertos for violin.
*The years: A.D. '''
2''',
2 BC 2 B.C.,
1902 or
2002.
*
February 2 is known in the
United States and
Canada as
Groundhog Day.
Category:Famous numbers
Category:Integers 02
gn:2
ca:Dos
da:2 (tal)
de:Zwei
es:Dos
eo:Du (nombro)
fa:Û²
fr:2 (nombre)
ko:2
it:Due
he:2 (מספר)
la:Duo
lt:2 (skaiÄ?ius)
nah:Ome
nl:2 (getal)
ja:2
no:2 (tall)
nn:Talet 2
pl:2 (liczba)
pt:Dois
ru:2 (чиÑ?ло)
simple:Two
sl:2 (Å¡tevilo)
fi:2 (luku)
sv:2 (tal)
th:2
zh:2
*** Shopping-Tip: 2 (number)
