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Concept
Quasifield
Résumé
In mathematics, a quasifield is an algebraic structure where and are binary operations on , much like a division ring, but with some weaker conditions. All division rings, and thus all fields, are quasifields.
A quasifield is a structure, where and are binary operations on , satisfying these axioms:
is a group
is a loop, where
(left distributivity)
has exactly one solution for ,
Strictly speaking, this is the definition of a left quasifield. A right quasifield is similarly defined, but satisfies right distributivity instead. A quasifield satisfying both distributive laws is called a semifield, in the sense in which the term is used in projective geometry.
Although not assumed, one can prove that the axioms imply that the additive group is abelian. Thus, when referring to an abelian quasifield, one means that is abelian.
The kernel of a quasifield is the set of all elements such that:
Restricting the binary operations and to , one can shown that is a division ring.
One can now make a vector space of over , with the following scalar multiplication :
As a finite division ring is a finite field by Wedderburn's theorem, the order of the kernel of a finite quasifield is a prime power. The vector space construction implies that the order of any finite quasifield must also be a prime power.
All division rings, and thus all fields, are quasifields.
A (right) near-field that is a (right) quasifield is called a "planar near-field".
The smallest quasifields are abelian and unique. They are the finite fields of orders up to and including eight. The smallest quasifields that are not division rings are the four non-abelian quasifields of order nine; they are presented in and .
Projective plane
Given a quasifield , we define a ternary map by
One can then verify that satisfies the axioms of a planar ternary ring. Associated to is its corresponding projective plane. The projective planes constructed this way are characterized as follows;
the details of this relationship are given in .
Source officielle
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