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In mathematics, a semifield is an algebraic structure with two binary operations, addition and multiplication, which is similar to a field, but with some axioms relaxed. There are at least two conflicting conventions of what constitutes a semifield.

In projective geometry and finite geometry (MSC 51A, 51E, 12K10), a semifield is the analogue of a division algebra, but defined over the integers Z rather than over a field. More precisely, it is a Z-algebra whose nonzero elements form a loop under multiplication. In other words, a semifield is a set S with two operations + (addition) and · (multiplication), such that
(S,+) is an abelian group,
multiplication is distributive on both the left and right,
there exists a multiplicative identity element, and
division is always possible: for every a and every nonzero b in S, there exist unique x and y in S for which b·x = a and y·b = a.

Note in particular that the multiplication is not assumed to be commutative or associative. A semifield that is associative is a division ring, and one that is both associative and commutative is a field. A semifield by this definition is a special case of a quasifield. If S is finite, the last axiom in the definition above can be replaced with the assumption that there are no zero divisors, so that a·b = 0 implies that a = 0 or b = 0. Note that due to the lack of associativity, the last axiom is not equivalent to the assumption that every nonzero element has a multiplicative inverse, as is usually found in definitions of fields and division rings.

In ring theory, combinatorics, functional analysis, and theoretical computer science (MSC 16Y60), a semifield is a semiring (S,+,·) in which all elements have a multiplicative inverse. These objects are also called proper semifields. A variation of this definition arises if S contains an absorbing zero that is different from the multiplicative unit e, it is required that the non-zero elements be invertible, and a·0 = 0·a = 0. Since multiplication is associative, the (non-zero) elements of a semifield form a group. However, the pair (S,+) is only a semigroup, i.e. additive inverse need not exist, or, colloquially, 'there is no subtraction'. Sometimes, it is not assumed that the multiplication is associative.

Primitivity of Semifields

A semifield D is called right (resp. left) primitive if it has an element w such that the set of nonzero elements of D* is equal to the set of all right (resp. left) principal powers of w.
Examples

We only give examples of semifields in the second sense, i.e. additive semigroups with distributive multiplication. Moreover, addition is commutative and multiplication is associative in our examples.

Positive real numbers with the usual addition and multiplication form a commutative semifield.
Rational functions of the form f /g, where f and g are polynomials in one variable with positive coefficients form a commutative semifield.
Max-plus algebra, or the tropical semiring, (R, max, +) is a semifield. Here the sum of two elements is defined to be their maximum, and the product to be their ordinary sum.
If (A,≤) is a lattice ordered group then (A,+,·) is an additively idempotent semifield. The semifield sum is defined to be the sup of two elements. Conversely, any additively idempotent semifield (A,+,·) defines a lattice-ordered group (A,≤), where a≤b if and only if a + b = b.

Planar ternary ring (first sense)

References

Donald Knuth, Finite semifields and projective planes. J. Algebra, 2, 1965, 182--217 MR 0175942.
Landquist, E.J., "On Nonassociative Division Rings and Projective Planes", Copyright 2000.
Golan, Jonathan S., Semirings and their applications. Updated and expanded version of The theory of semirings, with applications to mathematics and theoretical computer science (Longman Sci. Tech., Harlow, 1992, MR 1163371. Kluwer Academic Publishers, Dordrecht, 1999. xii+381 pp. ISBN 0-7923-5786-8 MR 1746739.
Hebisch, Udo; Weinert, Hanns Joachim, Semirings and semifields. Handbook of algebra, Vol. 1, 425--462, North-Holland, Amsterdam, 1996. MR 1421808.