View source for One half

{{for|the computer virus|OneHalf}}
{{redirect|Half|other uses|Half (disambiguation)}}
{{short description|Reverse fraction}}
{{more references|date=January 2014}}

{| class="wikitable" style="float: right; border-collapse: collapse;" border="1"
| colspan="2" align="center" | One half<br/>[[File:One-half.svg|50px]]
|-
| [[numerical prefix|prefix]]es
| [[Wiktionary:hemi-|hemi-]] (from [[Greek language|Greek]])
[[Wiktionary:semi-|semi-]]/[[Wiktionary:demi-|demi-]] (from [[Latin]])
|-
|[[Binary number|Binary]]
| 0.1 or 0.011111111111...
|-
|[[Ternary numeral system|Ternary]]
| 0.11111111111...
|-
| [[Decimal]]
| 0.5 or 0.499999999999...
|-
| [[Duodecimal]]
| 0.6 or 0.5BBBBBBBBBBBB...
|-
| [[Hexadecimal]]
| 0.8 or 0.7FFFFFFFFFFF...
|-
| [[Continued fraction]]
| [0; 1, 1] or [0; 2]
|-
| Single-precision 
[[floating point]]
| 3F000000 (hex) =
00111111000000000000000000000000<br/>(binary)
|}
[[File:Eye of Horus square.png|thumb|125px|[[Eye of Horus]]]]

'''One half''' is the [[irreducible fraction]] resulting from dividing [[1 (number)|one]] by [[2 (number)|two]] {{nowrap|(2)}} or the fraction resulting from dividing any number by its double. Multiplication by one half is equivalent to [[division by two]], or "halving"; conversely, division by one half is equivalent to multiplication by two, or "doubling". One half appears often in mathematical equations, recipes, measurements, etc. Half can also be said to be one part of something divided into two equal parts.

For instance, the area ''S'' of a triangle is computed

:''S'' = {{sfrac|2}} &times; base &times; perpendicular height.

One half also figures in the formula for calculating [[figurate numbers]], such as [[triangular number]]s and [[pentagonal number]]s:

<math>
\frac{1}{2} {n\left[\left(s - 2\right)n - (4-s) \right]}
</math>

and in the formula for computing magic constants for [[magic square]]s

<math>
M_2(n) = \frac{1}{2} n\left(n^{2} + 1\right)
</math>

The [[Riemann hypothesis]] states that every nontrivial [[complex number|complex root]] of the [[Riemann zeta function]] has a real part equal to {{sfrac|2}}.

One half has two different [[decimal]] expansions, the familiar 0.5 and the [[recurring decimal|recurring]] 0.49999999... It has a similar pair of expansions in any even base. It is a common trap to believe these expressions represent distinct numbers: see the [[proof that 0.999... equals 1]] for detailed discussion of a related case. In odd bases, one half has no terminating representation, only a single representation with a repeating fractional component, such as 0.11111111... in [[Ternary numeral system|ternary]].

==Particularities in writing and language==
[[File:Ireland-Stamp-1940-HalfCentPostageDue.jpg|thumb|160px|right|Postal stamp, Ireland, 1940: one halfpenny postage due.]]

{{frac|2}} is also one of the few fractions to usually have a key of its own on [[typewriter]]s (see [[Fractions#Typographical variations|fractions]]). It also has its own [[code point]] in some early [[Code page 437|extensions of ASCII]] at 171. In [[Unicode]], it has its own code unit at U+00BD (decimal 189) in the [[C1 Controls and Latin-1 Supplement]] block, and a cross-reference in the [[Number Forms]] block, rendering as "½".
The [[HTML]] entity is <code>&amp;frac12;</code>.

One half is also one of the few fractions which is commonly expressed in natural [[language]]s by [[suppletion]] rather than regular derivation: in [[English language|English]], for example, compare ''"one half"'' against other regular formations like ''"one sixth"''.

It is acceptable to write one half as a hyphenated word, "one-half" or non-hyphenated, "one half".

==Code Points==
In the Unicode standard, the symbol ½ has the code point
U+00BD ½ Vulgar Fraction One Half (HTML #0189· inherited from Latin-1)<ref>{{cite web |title=Unicode® Character Table |url=https://unicode-table.com/en/ |website=UT Unicode® Character Table |publisher=idea.informer.com |accessdate=1 October 2019}}</ref>

=== PC Entry ===
* {{keypress|Alt}}+{{keypress|0}}{{keypress|1}}{{keypress|8}}{{keypress|9}} (keeping Alt pressed until all 4 digits have been typed on the numeric keypad only)

==See also==
*[[List of numbers]]
*[[Division by two]]

==References==
{{Reflist}}

{{Fractions and ratios}}

[[Category:Fractions (mathematics)|Half]]
[[Category:Rational numbers]]