Planar ternary ring
In mathematics, an algebraic structure consisting of a non-empty set and a ternary mapping may be called a ternary system. A planar ternary ring (PTR) or ternary field is special type of ternary system used by Hall (1943) to construct projective planes by means of coordinates. A planar ternary ring is not a ring in the traditional sense.
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.
A planar ternary ring is a structure where is a set containing at least two distinct elements, called 0 and 1, and is a mapping which satisfies these five axioms:
- , there is a unique such that : ;
- , there is a unique , such that ; and
- , the equations have a unique solution .
When is finite, the third and fifth axioms are equivalent in the presence of the fourth.
No other pair (0', 1') in can be found such that still satisfies the first two axioms.
Define . The set is closed under this multiplication. The structure is also a loop, with identity element 1.
Connection with projective planes
- , and
Then define, , the incidence relation 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 = () and u = (0). The ternary operation is now defined on the coordinate symbols (except ) 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 Dembowski (1968, p. 129).
must be permutations whenever . Since cartesian groups are groups under addition, we revert to using a simple "+" for the additive operation.
A semifield is a quasifield which also satisfies the left distributive law:
A planar nearfield is a quasifield whose multiplicative loop is associative (and hence a group). Not all nearfields are planar nearfields.
- Hughes & Piper 1973, p. 118, Theorem 5.4
- In the literature there are two versions of this definition. This is the form used by Hall (1959, p. 355), Albert & Sandler (1968, p. 50), and Dembowski (1968, p. 128), while is used by Hughes & Piper (1973, p. 117), Pickert (1975, p. 38), and Stevenson (1972, p. 274). The difference comes from the alternative ways these authors coordinatize the plane.
- R. H. Bruck, Recent Advances in the Foundations of Euclidean Plane Geometry, (1955) Appendix I.
- Hall 1943, p.247 Theorem 5.4
- This can be done in several ways. A short description of the method used by Hall (1943) can be found in Dembowski (1968, p. 127).
- Dembowski 1968, p. 129
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- Hughes, Daniel R.; Piper, Fred C. (1973), Projective Planes, Graduate Texts in Mathematics (6), New York: Springer-Verlag, ISBN 0387900446, MR 0333959
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- Stevenson, Frederick (1972), Projective Planes, San Francisco: W.H. Freeman and Company, ISBN 071670443-9