2 (number)
2 (
two) is the
natural number following
1 and preceding
3.
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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, 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.
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. Despite being a prime, two is also a
highly composite number, because it has more divisors than one. The next highly composite number is
four.
Two is a factor of
ten, so
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 system widely used in
computers.
For any number
x:
:
x+
x = 2·
x Addition to
Multiplication:
x·
x =
x2 Multiplication to
Exponentiation:
xx =
x↑↑2
Exponentiation to
TetrationPowers 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 has two elements.
In the
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.
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.
Two is the number of
n-Queens Problem solutions for n = 4. With one exception, all known solutions to
Znám's problem start with 2.
Two is:
*The number of
polynucleotide strands in a
DNA double helix.
*The first
Magic number (physics).
*The
atomic number of
helium.
In astronomy,
:
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
-2861 May 4 and ended on
-1563 June 21 . 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
-2541 February 21 and ended on
-1225 April 22. The duration of Saros series 2 was 1316.2 years, and it contained 74 lunar eclipses.
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.
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 number 2 is sometimes used as a shorthand for "to". One common example is
computer software which translates data from one format into another, such as
dvi2ps and
texi2roff.
Words that can be used as synonyms for two include
brace,
couple, deuce,
duet,
pair,
twain, and
twins.
In
rhetorics,
hendiadys is a
figure of speech where two words with similar or identical meanings are used where one would be sufficient.
Groups of two:
*
Lists of pairs**
list of famous pairs**
list of twinsTwo is:
*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 demonination of the
$2 New Zealand coin, featuring the white heron.
*The name of several fictional characters:
Number Two.
*In the phrase "Number two", a euphemism for
defecation.
*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.
*The
DVD region in
Europe,
South Africa, the
Middle East and
Japan.
*In
rugby union, the number of the
hooker.
*In
baseball, two represents the
catcher's position.
*In
Astrology,
Taurus is the 2nd
astrological sign of the
Zodiac.
*The years: A.D.
2,
2 B.C.,
1902 or
2002. (The latter pair are more likely to be referred to as "oh-two".)
*The first digit of international
telephone dialing codes primarily for countries in
Africa.
*In
music, the roman numeral ii is the
supertonic scale degree,
chord, or
diatonic function, when distinguished II = major and ii = minor.
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