Semifield

In mathematics, a semifield is an algebraic structure with two binary operations, addition and multiplication, which is similar to the 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, a semifield is a semiring (MSC 16Y60) (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.