One half

One half is the irreducible fraction resulting from dividing one (1) by two (2), or the fraction resulting from dividing any number by its double.

It often appears in mathematical equations, recipes, measurements, etc.

As a word
One half is one of the few fractions which are commonly expressed in natural languages by suppletion rather than regular derivation. In English, for example, compare the compound "one half" with other regular formations like "one-sixth".

A half can also be said to be one part of something divided into two equal parts. It is acceptable to write one half as a hyphenated word, one-half.

Mathematics
One half is a rational number that lies midway between nil $$0$$ and 1|unity $$1$$ (which are the elementary additive and multiplicative identities) as the quotient of the first two non-zero integers, $$\tfrac {1}{2}$$. It has two different decimal representations in base ten, the familiar $$0.5$$ and the recurring $$0.4\overline{9}$$, with a similar pair of expansions in any even base; while in odd bases, one half has no terminating representation, it has only a single representation with a repeating fractional component (such as $$0.\overline{1}$$ in ternary and $$0.\overline{2}$$ in quinary).

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".



A number $$n$$ raised to the power of one half is equal to the square root of $$n$$,
 * $$n^{\tfrac {1}{2}} = \sqrt {n}.$$

Properties
A hemiperfect number is a positive integer with a half-integer abundancy index:


 * $$\frac {\sigma(n)}{n} = \frac {k}{2},$$

where $$k$$ is odd, and $$\sigma (n)$$ is the sum-of-divisors function. The first three hemiperfect numbers are 2, 24, and 4320.

The area $$T$$ of a triangle with base $$b$$ and altitude $$h$$ is computed as
 * $$T = \frac {b}{2} \times h.$$



One half figures in the formula for calculating figurate numbers, such as the $$n$$-th triangular number:



P_2(n) = \frac{n(n+1)}{2}; $$

and in the formula for computing magic constants for magic squares,



M_2(n) = \frac{n}{2} \left(n^{2} + 1\right). $$

Successive natural numbers yield the $$n$$-th metallic mean $$M$$ by the equation,



M_{(n)} = \frac{n+\sqrt{n^2+4}}{2}. $$

In the study of finite groups, alternating groups have order

\frac {n!}{2}. $$

By Euler, a classical formula involving pi, and yielding a simple expression:



\frac{\pi}{2}=\sum_{n=1}^\infty \frac{(-1)^{\varepsilon (n)}}{n}=1+\frac{1}{2}-\frac{1}{3}+\frac{1}{4}+\frac{1}{5}-\frac{1}{6}-\frac{1}{7}+\cdots, \text { } $$

where $$\varepsilon (n)$$ is the number of prime factors of the form $$p\equiv 3\, (\mathrm{mod}\, 4)$$ of $$n$$ (see modular arithmetic).



For the gamma function, a non-integer argument of one half yields,



\Gamma (\tfrac {1}{2})=\sqrt {\pi}; $$

while inside Apéry's constant, which represents the sum of the reciprocals of all positive cubes, there is



\zeta(3) = -\tfrac{1}{2} \Gamma'(1) + \tfrac{3}{2} \Gamma'(1) \Gamma(1) - \big(\Gamma'(1)\big)^3 = -\tfrac{1}{2} \psi^{(2)}(1); \text { } $$

with $$\psi^{(m)}(z)$$ the polygamma function of order $$m$$ on the complex numbers $$\mathbb C$$.

The upper half-plane $$\mathcal{H}$$ is the set of points $$(x,y)$$ in the Cartesian plane with $$y > 0$$. In the context of complex numbers, the upper half-plane is defined as



\mathcal{H} := \{x + iy \mid y > 0;\ x, y \in \mathbb{R} \}. $$

In differential geometry, this is the universal covering space of surfaces with constant negative Gaussian curvature, by the uniformization theorem.

The Bernoulli number $$B_{1}$$ has the value $$\pm \tfrac {1}{2}$$ (its sign depending on competing conventions).

The Riemann hypothesis is the conjecture that every nontrivial complex root of the Riemann zeta function has a real part equal to $$\tfrac {1}{2}$$.

Computer characters
The "one-half" symbol has its own code point as a precomposed character in the Number Forms block of Unicode, rendering as ½.

The reduced size of this symbol may make it illegible to readers with relatively mild visual impairment; consequently the decomposed forms $1/2$ or $1/undefined$ may be more appropriate.