Planar ternary ring

In mathematics, an algebraic structure formula_1 consisting of a non-empty set formula_2 and a ternary mapping formula_3 may be called a ternary system. A planar ternary ring (PTR) or ternary field is special type of ternary system used by to construct projective planes by means of coordinates. A planar ternary ring is not a ring in the traditional sense, but any field gives a planar ternary ring where the operation formula_4 is defined by formula_5. Thus, we can think of a planar ternary ring as a generalization of a field where the ternary operation takes the place of both addition and multiplication. In effect, in computer architecture, this ternary operation is known, e.g., as the multiply–accumulate operation (MAC).
There is wide variation in the terminology. Planar ternary rings or ternary fields as defined here have been called by other names in the literature, and the term "planar ternary ring" can mean a variant of the system defined here. The term "ternary ring" often means a planar ternary ring, but it can also simply mean a ternary system.
Definition.
A planar ternary ring is a structure formula_1 where formula_2 is a set containing at least two distinct elements, called 0 and 1, and formula_8 is a mapping which satisfies these five axioms:
When formula_2 is finite, the third and fifth axioms are equivalent in the presence of the fourth.
No other pair (0', 1') in formula_21 can be found such that formula_4 still satisfies the first two axioms.
Binary operations.
Addition.
Define formula_23. The structure formula_24 is a loop with identity element 0.
Multiplication.
Define formula_25. The set formula_26 is closed under this multiplication. The structure formula_27 is also a loop, with identity element 1.
Linear PTR.
A planar ternary ring formula_1 is said to be "linear" if formula_29.
For example, the planar ternary ring associated to a quasifield is (by construction) linear.
Connection with projective planes.
Given a planar ternary ring formula_1, one can construct a projective plane with point set "P" and line set "L" as follows: (Note that formula_31 is an extra symbol not in formula_2.)
Let
Then define, formula_35, the incidence relation formula_36 in this way:
Every projective plane can be constructed in this way, starting with an appropriate planar ternary ring. However, two nonisomorphic planar ternary rings can lead to the construction of isomorphic projective planes.
Conversely, given any projective plane π, by choosing four points, labelled "o", "e", "u", and "v", no three of which lie on the same line, coordinates can be introduced in π so that these special points are given the coordinates: "o" = (0,0), "e" = (1,1), "v" = (formula_31) and "u" = (0). The ternary operation is now defined on the coordinate symbols (except formula_31) by "y" = T("x","a","b") if and only if the point ("x","y") lies on the line which joins ("a") with (0,"b"). The axioms defining a projective plane are used to show that this gives a planar ternary ring. 
Linearity of the PTR is equivalent to a geometric condition holding in the associated projective plane.
Related algebraic structures.
PTR's which satisfy additional algebraic conditions are given other names. These names are not uniformly applied in the literature. The following listing of names and properties is taken from .
A linear PTR whose additive loop is associative (and thus a group), is called a cartesian group. In a cartesian group, the mappings
formula_48, and
formula_49
must be permutations whenever formula_50. Since cartesian groups are groups under addition, we revert to using a simple "+" for the additive operation.
A quasifield is a cartesian group satisfying the right distributive law:
formula_51.
Addition in any quasifield is commutative.
A semifield is a quasifield which also satisfies the left distributive law:
formula_52
A planar nearfield is a quasifield whose multiplicative loop is associative (and hence a group). Not all nearfields are planar nearfields.