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{{Short description|Integer number 2}}
{{pp-semi|small=yes}}
{{Hatnote|This article is about the number. For the years, see 2 BC and AD 2. For other uses, see II (disambiguation) and Number Two (disambiguation).}}
{{Infobox number
|number=2
|ordinal=2nd (second / twoth)
|numeral=binary
|gaussian integer factorization = <math>(1 + i)(1 - i)</math>
|prime=1st
|divisor=1, 2
|roman =II, ii
|greek prefix=di-
|latin prefix=duo-/bi-
|old english prefix=twi-
|lang1=Greek numeral
|lang1 symbol=β'
|lang2=[[Eastern Arabic numerals|Arabic]], Kurdish, [[Social:Persian language|Persian]], [[Social:Sindhi language|Sindhi]], Urdu
|lang2 symbol={{resize|150%|٢}}
|lang3=Ge'ez
|lang3 symbol=፪
|lang4=[[Bengali numerals|Bengali]]
|lang4 symbol={{resize|150%|২}}
|lang5=Chinese numeral
|lang5 symbol=二，弍，貳
|lang6=Devanāgarī
|lang6 symbol={{resize|150%|२}}
|lang7=[[Indian numerals#Telugu numerals and their names|Telugu]]
|lang7 symbol={{resize|150%|౨}}
|lang8=[[Tamil numerals|Tamil]]
|lang8 symbol={{resize|150%|௨}}
|lang9=Kannada
|lang9 symbol={{resize|150%|೨}}
|lang10=[[Hebrew numerals|Hebrew]]
|lang10 symbol={{resize|150%|ב}}
|lang12=[[Khmer numerals|Khmer]]
|lang12 symbol=២
|lang14=[[Thai numerals|Thai]]
|lang14 symbol=๒
|lang15=[[Georgian numerals|Georgian]]	
|lang15 symbol={{resize|130%| Ⴁ/ⴁ/ბ}}(Bani)
|lang16=[[Malayalam numerals|Malayalam]]
|lang16 symbol=൨
}}

'''2''' ('''two''') is a [[Number|number]], [[Philosophy:Numeral (linguistics)|numeral]] and [[Numerical digit|digit]]. It is the [[Natural number|natural number]] following [[1|1]] and preceding [[3|3]]. It is the smallest and only even [[Prime number|prime number]]. Because it forms the basis of a [[Astronomy:Dualistic cosmology|duality]], it has [[Religion:Religion|religious]] and [[Philosophy:Spirituality|spiritual]] significance in many [[Social:Culture|cultures]].

{{anchor|glyph}}

==Evolution==
===Arabic digit===
[[Image:Evolution2glyph.png|x50px|left]]
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 and Japanese languages (and Korean [[Social:Hanja|Hanja]]) still use this method. The Gupta script rotated the two lines 45&nbsp;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.<ref>Georges Ifrah, ''The Universal History of Numbers: From Prehistory to the Invention of the Computer'' transl. David Bellos et al. London: The Harvill Press (1998): 393, Fig. 24.62</ref>

In fonts with [[Text figures|text figures]], digit 2 usually is of x-height, for example, [[Image:Text figures 256.svg]].{{citation needed|date=January 2024}}

== As a word ==
''Two'' is most commonly a determiner used with plural countable nouns, as in ''two days'' or ''I'll take these two''.<ref>{{Cite book |last1=Huddleston |first1=Rodney D. |url=https://www.worldcat.org/oclc/1255524478 |title=A student's introduction to English grammar |last2=Pullum |first2=Geoffrey K. |last3=Reynolds |first3=Brett |publisher={{wipe|Cambridge University Press}} |year=2022 |isbn=978-1-316-51464-1 |edition=2nd |location=Cambridge, United Kingdom |pages=117 |oclc= 1255524478|author-link2=Geoffrey K. Pullum}}</ref> ''Two'' is a noun when it refers to the number two as in ''two plus two is four.''

=== Etymology of ''two'' ===
The word ''two'' is derived from the [[Social:Old English|Old English]] words {{lang|ang|twā}} (feminine), {{lang|ang|tū}} (neuter), and {{lang|ang|twēġen}} (masculine, which survives today in the form twain).<ref name=OED>{{Cite OED|two, adj., n., and adv.}}</ref>

The pronunciation {{IPA|/tuː/}}, like that of ''who'' is due to the labialization of the vowel by the ''w'', which then disappeared before the related sound. The successive stages of pronunciation for the Old English {{lang|ang|twā}} would thus be {{IPA|/twɑː/}}, {{IPA|/twɔː/}}, {{IPA|/twoː/}}, {{IPA|/twuː/}}, and finally {{IPA|/tuː/}}.<ref name=OED/>

== Mathematics ==

=== Characterizations ===
The number two is the smallest, and only [[Parity (mathematics)|even]], [[Prime number|prime number]]. As the smallest prime number, two is also the smallest non-zero [[Pronic number|pronic number]], and the only pronic prime.<ref>{{Cite web|url=https://oeis.org/A002378|title=Sloane's A002378: Pronic numbers|website=The On-Line Encyclopedia of Integer Sequences|publisher=OEIS Foundation|access-date=2020-11-30|archive-date=2016-06-09|archive-url=https://web.archive.org/web/20160609230431/https://oeis.org/A002378|url-status=live}}</ref> An [[Integer|integer]] is determined to be ''[[Parity (mathematics)|even]]'' if it is divisible by 2. For integers written in a numeral system based on an even number such as [[Decimal|decimal]], divisibility by 2 is easily tested by merely looking at the last digit. If it is even, then the whole number is even. When written in the decimal system, all multiples of 2 will end in [[0|0]], 2, 4, 6, or&nbsp;[[8|8]].<ref>{{Cite OEIS |A005843 |The nonnegative even numbers |access-date=2022-12-15 }}</ref>

Every integer greater than [[1|1]] will have at least two distinct factors; by definition, a prime number ''only'' has two distinct factors (itself and 1). Therefore, the [[Divisor function|number-of-divisors function]] <math>d(n)</math> of positive integers <math>n</math> satisfies,
<math display="block">\liminf_{n\to\infty} d(n)=2,</math>
where <math>\liminf</math> represents the limit inferior (since there will always exist a larger prime number with a maximum of two divisors).<ref>{{Citation |last1=Hardy |first1=G. H. |last2=Wright |first2=E. M. |edition=6th |others=Revised by [[Biography:Roger Heath-Brown|D. R. Heath-Brown]] and [[Biography:Joseph H. Silverman|J. H. Silverman]]. Foreword by [[Biography:Andrew Wiles|Andrew Wiles]]. |title=An Introduction to the Theory of Numbers |publisher=[[Organization:Oxford University Press|Oxford University Press]] |location=Oxford |year=2008  |pages=342–347, §18.1 |isbn=978-0-19-921986-5 |mr=2445243 |zbl=1159.11001 }}
:Also, <math>\limsup_{n\to\infty}\frac{\log d(n)}{\log n/\log\log n} = \log 2</math>.</ref>

Specifically,

{{Bullet list
|A number is ''[[Deficient number|deficient]]'' when the [[Divisor function|sum of its divisors]] is less than twice the number, whereas an [[Abundant number|abundant]] number has a sum of its [[Divisor#Further notions and facts|proper divisor]]s that is larger than the number itself. [[Primitive abundant number|Primitive abundant]] numbers are abundant numbers whose proper divisors are all deficient.
|A number is ''[[Perfect number|perfect]]'' if it is equal to its [[Aliquot sum|aliquot sum]], or the sum of all of its positive divisors excluding the number itself. This is equivalent to describing a perfect number <math>n</math> as having a [[Divisor function|sum of divisors]] <math>\sigma(n)</math> equal to <math>2n</math>.
}}

In a [[Set theory|set-theoretical]] construction of the natural numbers, <math>2</math> is identified with the set <math>\{\{\varnothing\},\varnothing\}</math>, where <math>\varnothing</math> denotes the [[Empty set|empty set]]. This latter set is important in [[Category theory|category theory]]: it is a [[Subobject classifier|subobject classifier]] in the category of sets. A [[Cantor space]] is a [[Topological space|topological space]] <math>2^\mathbb{N}</math> homeomorphic to the [[Cantor set]], whose general set is a [[Closed set|closed set]] consisting purely of [[Boundary (topology)|boundary points]]. The countably [[Infinity|infinite]] [[Product topology|product topology]] of the simplest [[Discrete two-point space|discrete two-point space]], <math>\{0, 1\}</math>, is the traditional elementary example of a Cantor space. More broadly, a set that is a [[Field (mathematics)|field]] has a minimum of two [[Element (mathematics)|elements]].

The [[Binary number|binary system]] has a [[Radix|radix]] of two, and it is the [[Numeral system|numeral system]] with the fewest tokens that allows denoting a natural number substantially more concisely (with <math>\log_{2}</math> <math>n</math> tokens) than a direct representation by the corresponding count of a single token (with <math>n</math> tokens). This number system is used extensively in [[Computer|computing]].{{citation needed|date=January 2024}}

In a [[Euclidean space]] of any dimension greater than zero, two distinct [[Point (geometry)|points]] in a [[Plane (geometry)|plane]] are always [[Necessity and sufficiency|sufficient]] to define a unique [[Line (geometry)|line]].{{citation needed|date=January 2024}}

==== Powers of 2 ====

Two is the first [[Mersenne prime]] exponent, and it is the difference between the first two Fermat primes ([[3|3]] and [[5|5]]). [[Power of two|Powers of two]] are essential in [[Computer science|computer science]], and important in the [[Constructible polygon|constructability]] of [[Regular polygon|regular polygon]]s using basic tools (e.g., through the use of Fermat or [[Pierpont prime|Pierpont]] primes).

<math>2</math> is the only number such that the sum of the reciprocals of its natural powers equals itself. In symbols,
:<math>\sum_{n=0}^{\infin}\frac {1}{2^n}=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+\cdots=2.</math>

Two also has the unique property that <math>2+2=2\times2=2^{2}=2\uparrow\uparrow 2= 2\uparrow\uparrow\uparrow2=\text{ }...</math> up through any level of [[Hyperoperation|hyperoperation]], here denoted in [[Knuth's up-arrow notation]], all equivalent to <math>4.</math>

Notably, row sums in [[Pascal's triangle]] are in equivalence with successive [[Exponentiation|powers]] of two, <math>2^{n}.</math><ref>{{Cite journal |last=Smith |first=Karl J. |title=Pascal's Triangle |url=https://www.jstor.org/stable/2698949 |journal=The Two-Year College Mathematics Journal |volume=4 |number=1 |publisher=Mathematical Association of America |location=Washington, D.C. |year=1973 |page=4 |doi=10.1080/00494925.1973.11974228 |jstor=2698949 |s2cid=265738469 }}</ref><ref>{{Cite OEIS |A000079 |Powers of 2: a(n) equal to 2^n. |access-date=2023-01-06 }}</ref>

=== Properties ===

The numbers two and [[3|three]] are the only two consecutive prime numbers. Two is the first prime number that does not have a proper [[Twin prime|twin prime]] with a difference two, while three is the first such prime number to have a twin prime, [[5|five]].<ref>{{Cite OEIS |A007510 |Single (or isolated or non-twin) primes: Primes p such that neither p-2 nor p+2 is prime. |access-date=2022-12-05 }}</ref><ref>{{Cite OEIS |A001359 |Lesser of twin primes. |access-date=2022-12-05 }}</ref> In consequence, three and five encase [[4|four]] in-between, which is the [[Square number|square]] of two, <math>2^2</math>. These are also the two odd prime numbers that lie amongst the only [[Harshad number|all-Harshad]] numbers ([[1|1]], '''2''', 4, and [[6|6]])<ref>{{Cite web |author=PrimeFan |title=Harshad number |url=https://planetmath.org/harshadnumber |website=[[PlanetMath]] |date=2013-03-22 |access-date=2023-12-18 }}</ref> that are also the first four [[Highly composite number|highly composite number]]s,<ref>{{Cite OEIS |A002182 |Highly composite numbers, definition (1): numbers n where d(n), the number of divisors of n (A000005), increases to a record. |access-date=2023-12-18 }}</ref> with 2 the only number that is both a prime number and a highly composite number. 

Furthermore, <math>(3,5)</math> are the unique pair of [[Twin prime|twin prime]]s <math>(q, q+2)</math> that yield the second and only [[Prime quadruplet|prime quadruplet]] <math>(11, 13, 17, 19)</math> that is of the form <math>(d-4, d-2, d+2, d+4)</math>, where <math>d</math> is the product of said twin primes.<ref>{{Cite OEIS|A136162|List of prime quadruplets {p, p+2, p+6, p+8}.|access-date=2023-06-09 }}
:"{11, 13, 17, 19} is the only prime quadruplet {p, p+2, p+6, p+8} of the form {Q-4, Q-2, Q+2, Q+4} where Q is a product of a pair of twin primes {q, q+2} (for prime q = 3) because numbers Q-2 and Q+4 are for q>3 composites of the form 3*(12*k^2-1) and 3*(12*k^2+1) respectively (k is an integer)."</ref> 

In particular, the sum of the [[Multiplicative inverse|reciprocals]] of all non-zero [[Triangular number|triangular number]]s converges to 2.<ref>{{Cite journal |last=Grabowski |first=Adam |title=Polygonal numbers |journal=Formalized Mathematics |year=2013 |volume=21 |number=2 |pages=103–113 |publisher=Sciendo ([[Company:De Gruyter|De Gruyter]])|doi=10.2478/forma-2013-0012 |doi-access=free |s2cid=15643540 |zbl=1298.11029 }}</ref>

==== Integer sequences ====

The first number to return zero for the [[Mertens function]] is 2,<ref>{{Cite OEIS |A028442 |Numbers k such that Mertens's function M(k) (A002321) is zero. |access-date=2023-09-02 }}</ref> and the [[Harmonic mean|harmonic mean]] of the divisors of [[6|6]], the smallest [[Harmonic divisor number|Ore number]] greater than [[1|1]], is also '''2'''.

Inside other important [[Integer sequence|integer sequence]]s, 
{{Bullet list
|2 is the first Sophie Germain prime,<ref>{{Cite OEIS |A005384 |Sophie Germain primes p: 2p+1 is also prime. |access-date=2022-12-15 }}</ref> the first [[Factorial prime|factorial prime]],<ref>{{Cite OEIS |A088054 |Factorial primes: primes which are within 1 of a factorial number. |access-date=2022-12-15 }}</ref> the first [[Lucas prime]],<ref>{{Cite OEIS |A005479 |Prime Lucas numbers |access-date=2022-12-15 }}</ref> and the first [[Ramanujan prime]].<ref>{{Cite web |url=https://oeis.org/A104272 |title=Sloane's A104272 : Ramanujan primes |website=The On-Line Encyclopedia of Integer Sequences |publisher=OEIS Foundation |access-date=2016-06-01 |archive-date=2011-04-28 |archive-url=https://web.archive.org/web/20110428165633/https://oeis.org/A104272 |url-status=dead }}</ref> 
|2 is also a [[Motzkin number]],<ref>{{Cite OEIS |A001006 |Motzkin numbers: number of ways of drawing any number of nonintersecting chords joining n (labeled) points on a circle. |access-date=2022-12-15 }}</ref>  a [[Bell number]],<ref>{{Cite OEIS |A000110 |Bell or exponential numbers: number of ways to partition a set of n labeled elements. |access-date=2022-12-15 }}</ref> and the third (or fourth) [[Fibonacci number]].<ref>{{Cite OEIS |A000045 |Fibonacci numbers |access-date=2022-12-15 }}</ref>
|2 is a meandric number,<ref>{{Cite OEIS |A005315 |Closed meandric numbers (or meanders): number of ways a loop can cross a road 2n times. |access-date=2022-12-15 }}</ref> a semi-meandric number,<ref>{{Cite OEIS |A000682 |Semi-meanders: number of ways a semi-infinite directed curve can cross a straight line n times. |access-date=2022-12-15 }}</ref> and an [[Meander (mathematics)|open meandric number]].<ref>{{Cite OEIS |A005316 |Meandric numbers: number of ways a river can cross a road n times. |access-date=2022-12-15 }}</ref>
}}

Two consecutive twos (as in "22" for "two twos"), or equivalently "2-2", is the only [[Fixed point (mathematics)|fixed point]] of [[Biography:John Horton Conway|John Conway's]] [[Look-and-say sequence|look-and-say function]].<ref>
{{cite journal |title=Look-and-Say Biochemistry: Exponential RNA and Multistranded DNA |first=Oscar |last=Martin |journal=American Mathematical Monthly |year=2006 |volume=113 |issue=4 |pages=289&ndash;307 |publisher=Mathematical association of America |issn=0002-9890 |url=http://www.uam.es/personal_pdi/ciencias/omartin/Biochem.PDF |archive-url=https://web.archive.org/web/20061224154744/http://www.uam.es/personal_pdi/ciencias/omartin/Biochem.PDF |archive-date=2006-12-24 |access-date=2022-07-21 |doi=10.2307/27641915 |jstor=27641915 }}</ref> On the other hand, there are no <math>2\times2</math> [[Magic square|magic square]]s, and as such they are the only [[Null set|null]] <math>n</math> by <math>n</math> magic square set.<ref>{{Cite OEIS |A006052|Number of magic squares of order n composed of the numbers from 1 to n^2, counted up to rotations and reflections. |access-date=2022-07-21 }}</ref>{{efn|1=Meanwhile, the [[Magic constant|magic constant]] of an <math>n</math>-pointed normal [[Magic star|magic star]] is <math>M = 4n + 2</math>. }}

There are only two known [[Sublime number|sublime number]]s, which are numbers with a [[Perfect number|perfect]] number of factors, whose sum itself yields a [[Perfect number|perfect number]]. [[12 (number)|12]] is one of the two sublime numbers, with the other being 76 digits long.<ref>{{cite OEIS|A081357 |Sublime numbers |access-date=2022-07-13 }}</ref>

==== Euler's number ====

<math>e</math> can be simplified to equal,

<math display ="block">e =  \sum\limits_{n = 0}^{\infty} \frac{1}{n!} = 2 + \frac{1}{1\cdot 2} + \frac{1}{1\cdot 2\cdot 3} + \cdots</math>

A [[Continued fraction|continued fraction]] for <math>e = [2; 1, 2, 1, 1, 4, 1, 1, 8, ...]</math> repeats a <math>\{1, 2n, 1\}</math> pattern from the second term onward.<ref>{{Cite journal |last=Cohn |first=Henry |title=A Short Proof of the Simple Continued Fraction Expansion of ''e'' |journal=[[The American Mathematical Monthly]] |publisher=Taylor & Francis, Ltd. |volume=113 |issue=1 |year=2006 |pages=57–62 |url=https://www.jstor.org/stable/27641837 |doi=10.1080/00029890.2006.11920278 |jstor=27641837 |mr=2202921 |s2cid=43879696 |zbl=1145.11012 |access-date=2023-04-30 |archive-date=2023-04-30 |archive-url=https://web.archive.org/web/20230430234614/https://www.jstor.org/stable/27641837 |url-status=live }}</ref><ref>{{Cite OEIS |A005131 |A generalized continued fraction for Euler's number e. |access-date=2023-04-30 }}
:"Only a(1) = 0 prevents this from being a simple continued fraction. The motivation for this alternate representation is that the simple pattern {1, 2*n, 1} (from n=0) may be more mathematically appealing than the pattern in the corresponding simple continued fraction (at A003417)."</ref>

==== Geometry ====
Regarding [[Regular polygon|regular polygon]]s in two dimensions:

*The [[Equilateral triangle|equilateral triangle]] has the smallest ratio of the circumradius <math>R</math> to the inradius <math>r</math> of any triangle by [[Euler's theorem in geometry|Euler's inequality]], with <math>\tfrac{R}{r}=2.</math><ref>{{cite journal |last1=Svrtan |first1=Dragutin |last2=Veljan |first2=Darko |title=Non-Euclidean versions of some classical triangle inequalities |journal=[[Organization:Forum Geometricorum|Forum Geometricorum]] |publisher=Department of Mathematical Sciences, [[Organization:Florida Atlantic University|Florida Atlantic University]] |date=2012 |volume=12 |pages=198 |location=Boca Raton, FL |url=http://forumgeom.fau.edu/FG2012volume12/FG201217.pdf |issn=1534-1178 |mr=2955631 |s2cid=29722079 |zbl=1247.51012 |access-date=2023-04-30 |archive-date=2023-05-03 |archive-url=https://web.archive.org/web/20230503213319/https://forumgeom.fau.edu/FG2012volume12/FG201217.pdf |url-status=live }}</ref>

*The [[Diagonal#Regular polygons|long diagonal]] of a regular [[Hexagon|hexagon]] is of length 2 when its sides are of [[Unit vector|unit length]].{{citation needed|date=January 2024}}

*The ''span'' of an [[Octagon|octagon]] is in [[Silver ratio|silver ratio]] <math>\delta_s</math> with its sides, which can be computed with the continued fraction <math>[2;2,2,...] = 2.414\;235\dots</math><ref>{{Cite journal |url=http://www.mi.sanu.ac.rs/vismath/spinadel/index.html |author=Vera W. de Spinadel |title=The Family of Metallic Means |journal=Visual Mathematics |publisher=Mathematical Institute of the Serbian Academy of Sciences |volume=1 |number=3 |location=Belgrade |date=1999 |eissn=1821-1437 |s2cid=125705375 |zbl=1016.11005 |access-date=2023-02-25 |archive-date=2023-03-26 |archive-url=https://web.archive.org/web/20230326024031/http://www.mi.sanu.ac.rs/vismath/spinadel/index.html |url-status=live }}</ref>

Whereas a [[Square|square]] of unit side length has a diagonal equal to <math>\sqrt{2}</math>, a [[Space diagonal|space diagonal]] inside a [[Tesseract|tesseract]] measures 2 when its side lengths are of unit length.{{citation needed|date=January 2024}}

A [[Digon|digon]] is a polygon with two sides (or [[Edge (geometry)|edges]]) and two [[Vertex (geometry)|vertices]]. On a [[Circle|circle]], it is a [[Tessellation|tessellation]] with two antipodal points and 180° arc edges.{{citation needed|date=January 2024}}

For any [[Polyhedron|polyhedron]] homeomorphic to a [[Sphere|sphere]], the [[Euler characteristic]] is <math>\chi=V-E+F=2 </math>, where <math>V</math> is the number of [[Vertex (geometry)|vertices]], <math>E</math> is the number of edges, and <math>F</math> is the number of [[Face (geometry)|face]]s. A double torus has an Euler characteristic of <math>-2</math>, on the other hand, and a non-orientable surface of like [[Genus (mathematics)|genus]] <math>k</math> has a characteristic <math>\chi=2-k</math>.{{citation needed|date=January 2024}}

The simplest tessellation in [[Plane (mathematics)|two-dimensional space]], though an improper tessellation, is that of two [[Infinity|<math>\infty</math>]]-sided [[Apeirogon|apeirogon]]s joined along all their [[Edge (geometry)|edges]], coincident about a [[Line (geometry)|line]] that divides the [[Plane (geometry)|plane]] in two. This [[Order-2 apeirogonal tiling|order-2 apeirogonal tiling]] is the arithmetic limit of the family of dihedra <math>\{p, 2\}</math>.{{citation needed|date=January 2024}}

=== List of basic calculations ===
{|class="wikitable" style="text-align: center; background: white"
|+
![[Multiplication]]
!1
!2
!3
!4
!5
!6
!7
!8
!9
!10
!11
!12
!13
!14
!15
!16
!17
!18
!19
!20
!21
!22
!23
!24
!25
!50
!100
|-
|'''2 × ''x'''''
|'''2'''
|[[4|4]]
|[[6|6]]
|[[8|8]]
|[[10|10]]
|[[12 (number)|12]]
|[[14 (number)|14]]
|[[16 (number)|16]]
|[[18 (number)|18]]
|[[20 (number)|20]]
|[[22 (number)|22]]
|[[24 (number)|24]]
|[[26 (number)|26]]
|[[28 (number)|28]]
|[[30 (number)|30]]
|[[32 (number)|32]]
|[[34 (number)|34]]
|[[36 (number)|36]]
|[[38 (number)|38]]
|[[40 (number)|40]]
|[[42 (number)|42]]
|[[44 (number)|44]]
|[[46 (number)|46]]
|[[48 (number)|48]]
|[[50 (number)|50]]
|[[100|100]]
|[[200 (number)|200]]
|}

{|class="wikitable" style="text-align: center; background: white"
|-
!width="105px"|[[Division (mathematics)|Division]]
!1
!2
!3
!4
!5
!6
!7
!8
!9
!10
!width="5px"|
!11
!12
!13
!14
!15
!16
!17
!18
!19
!20
|-
|'''2 ÷ ''x'''''
|'''2'''
| rowspan="2"|[[1|1]]
|0.{{overline|6}}
|0.5
|0.4
|0.{{overline|3}}
|0.{{overline|285714}}
|0.25
|0.{{overline|2}}
|0.2
!
|0.{{overline|18}}
|0.1{{overline|6}}
|0.{{overline|153846}}
|0.{{overline|142857}}
|0.1{{overline|3}}
|0.125
|0.{{overline|1176470588235294}}
|0.{{overline|1}}
|0.{{overline|105263157894736842}}
|0.1
|-
|'''''x'' ÷ 2'''
|0.5
|1.5
|'''2'''
|2.5
|[[3|3]]
|3.5
|[[4|4]]
|4.5
|[[5|5]]
!
|5.5
|[[6|6]]
|6.5
|[[7|7]]
|7.5
|[[8|8]]
|8.5
|[[9|9]]
|9.5
|[[10|10]]
|}

{|class="wikitable" style="text-align: center; background: white"
|-
!width="105px"|[[Exponentiation]]
!1
!2
!3
!4
!5
!6
!7
!8
!9
!10
!width="5px"|
!11
!12
!13
!14
!15
!16
!17
!18
!19
!20
|-
|'''2{{sup|''x''}}'''
|'''2'''
| rowspan="2" |[[4|4]]
|[[8|8]]
| rowspan="2" |[[16 (number)|16]]
|[[32 (number)|32]]
|[[64 (number)|64]]
|[[128 (number)|128]]
|[[256 (number)|256]]
|[[512 (number)|512]]
|[[1024 (number)|1024]]
!
|2048
|4096
|[[8192 (number)|8192]]
|16384
|32768
|[[65,536|65536]]
|131072
|262144
|524288
|1048576
|-
|'''''x''{{sup|2}}'''
|1
|[[9|9]]
|[[25 (number)|25]]
|[[36 (number)|36]]
|[[49 (number)|49]]
|[[64 (number)|64]]
|[[81 (number)|81]]
|[[100|100]]
!
|[[121 (number)|121]]
|[[144 (number)|144]]
|[[169 (number)|169]]
|[[196 (number)|196]]
|[[225 (number)|225]]
|[[256 (number)|256]]
|[[289 (number)|289]]
|324
|361
|[[400 (number)|400]]
|}

== In science ==
*The number of [[Biology:Polynucleotide|polynucleotide]] strands in a [[Biology:DNA|DNA]] double helix.<ref>{{Cite web|url=https://www.nature.com/scitable/content/double-stranded-dna-6834149/|title=Double-stranded DNA|work=Scitable|publisher=Nature Education|access-date=2019-12-22|archive-date=2020-07-24|archive-url=https://web.archive.org/web/20200724191225/https://www.nature.com/scitable/content/double-stranded-dna-6834149/|url-status=live}}</ref>
*The first [[Physics:Magic number|magic number]].<ref>{{Cite web|url=https://www.sjsu.edu/faculty/watkins/magicnumbers2.htm|title=The Complete Explanation of the Nuclear Magic Numbers Which Indicate the Filling of Nucleonic Shells and the Revelation of Special Numbers Indicating the Filling of Subshells Within Those Shells|website=www.sjsu.edu|access-date=2019-12-22|archive-date=2019-12-02|archive-url=https://web.archive.org/web/20191202130317/http://www.sjsu.edu/faculty/watkins/magicnumbers2.htm|url-status=dead}}</ref>
*The [[Physics:Atomic number|atomic number]] of [[Chemistry:Helium|helium]].<ref>{{Cite journal|last=Bezdenezhnyi|first=V. P.|date=2004|title=Nuclear Isotopes and Magic Numbers|bibcode=2004OAP....17...11B|journal=Odessa Astronomical Publications|volume=17|pages=11}}</ref>

== See also ==
*List of highways numbered 2
*[[Binary number]]

== Notes ==
{{Notelist}}

== References ==
{{Reflist}}

==External links==
{{Commons and category|2 (number)|2 (number)}}
*[http://primes.utm.edu/curios/page.php/2.html Prime curiosities: 2]



{{DEFAULTSORT:2 (Number)}}
[[Category:2 (number)]]
[[Category:Integers]]

{{Sourceattribution|2}}