# Half-integer

In mathematics, a **half-integer** is a number of the form

- ,

where is an integer. For example,

- 4½, 7/2, −13/2, 8.5

are all half-integers. Half-integer is perhaps a misnomer, as the set may be misunderstood to include numbers such as 1 (being half the integer 2). A name such as "integer-plus-half" may be more representative, but "half integer" is the traditional term.^{[citation needed]} Half-integers occur frequently enough in mathematics that a distinct term is convenient.

Note that a halving an integer does not always produce a half-integer; this is only true for odd integers. For this reason, half-integers are also sometimes called **half-odd-integers**. Half-integers are a special case of the dyadic rationals (numbers produced by dividing an integer by a power of two).^{[1]}

## Notation and algebraic structureEdit

The set of all half-integers is often denoted

The integers and half-integers together form a group under the addition operation, which may be denoted^{[2]}

- .

However, these numbers do not form a ring because the product of two half-integers cannot be itself a half-integer.^{[3]}

## UsesEdit

### Sphere packingEdit

The densest lattice packing of unit spheres in four dimensions (called the *D*_{4} lattice) places a sphere at every point whose coordinates are either all integers or all half-integers. This packing is closely related to the Hurwitz integers: quaternions whose real coefficients are either all integers or all half-integers.^{[4]}

### PhysicsEdit

In physics, the Pauli exclusion principle results from definition of fermions as particles which have spins that are half-integers.^{[5]}

The energy levels of the quantum harmonic oscillator occur at half-integers and thus its lowest energy is not zero.^{[6]}

### Sphere volumeEdit

Although the factorial function is defined only for integer arguments, it can be extended to fractional arguments using the gamma function. The gamma function for half-integers is an important part of the formula for the volume of an *n*-dimensional ball of radius *R*,^{[7]}

The values of the gamma function on half-integers are integer multiples of the square root of pi:

where *n*!! denotes the double factorial.

## ReferencesEdit

**^**Sabin, Malcolm (2010),*Analysis and Design of Univariate Subdivision Schemes*, Geometry and Computing,**6**, Springer, p. 51, ISBN 9783642136481.**^**Turaev, Vladimir G. (2010),*Quantum Invariants of Knots and 3-Manifolds*, De Gruyter Studies in Mathematics,**18**(2nd ed.), Walter de Gruyter, p. 390, ISBN 9783110221848.**^**Boolos, George; Burgess, John P.; Jeffrey, Richard C. (2002),*Computability and Logic*, Cambridge University Press, p. 105, ISBN 9780521007580.**^**John, Baez (2005), "*On Quaternions and Octonions: Their Geometry, Arithmetic, and Symmetry*by John H. Conway and Derek A. Smith",*Bulletin of the American Mathematical Society*,**42**: 229–243, doi:10.1090/S0273-0979-05-01043-8.**^**Mészáros, Péter (2010),*The High Energy Universe: Ultra-High Energy Events in Astrophysics and Cosmology*, Cambridge University Press, p. 13, ISBN 9781139490726.**^**Fox, Mark (2006),*Quantum Optics : An Introduction*, Oxford Master Series in Physics,**6**, Oxford University Press, p. 131, ISBN 9780191524257.**^**Equation 5.19.4,*NIST Digital Library of Mathematical Functions.*http://dlmf.nist.gov/, Release 1.0.6 of 2013-05-06.