Half-integer
Rational number equal to an integer plus 1/2 From Wikipedia, the free encyclopedia
In mathematics, a half-integer is a number of the form where is an integer. For example, are all half-integers. The name "half-integer" is perhaps misleading, as each integer is itself half of the integer . A name such as "integer-plus-half" may be more accurate, but while not literally true, "half integer" is the conventional term.[citation needed] Half-integers occur frequently enough in mathematics and in quantum mechanics that a distinct term is convenient.
Note that 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 subset of the dyadic rationals (numbers produced by dividing an integer by a power of two).[1]
Notation and algebraic structure
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 is not a half-integer; e.g. [3] The smallest ring containing them is , the ring of dyadic rationals.
Properties
- The sum of half-integers is a half-integer if and only if is odd. This includes since the empty sum 0 is not half-integer.
- The negative of a half-integer is a half-integer.
- The cardinality of the set of half-integers is equal to that of the integers. This is due to the existence of a bijection from the integers to the half-integers: , where is an integer.
Uses
Summarize
Perspective
Sphere packing
The densest lattice packing of unit spheres in four dimensions (called the D4 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]
Physics
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 volume
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 ,[7] The values of the gamma function on half-integers are integer multiples of the square root of pi: where denotes the double factorial.
References
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