Medial magma

In abstract algebra, a medial magma or medial groupoid is a magma or groupoid (that is, a set with a binary operation) which satisfies the identity

, or more simply
For the triple product, see Median algebra.

for all x, y, u and v, using the convention that juxtaposition denotes the same operation but has higher precedence. This identity has been variously called medial, abelian, alternation, transposition, interchange, bi-commutative, bisymmetric, surcommutative, entropic etc.[1]

Any commutative semigroup is a medial magma, and a medial magma has an identity element if and only if it is a commutative monoid. Another class of semigroups forming medial magmas are normal bands.[2] Medial magmas need not be associative: for any nontrivial abelian group with operation + and integers m ≠ n, the new binary operation defined by yields a medial magma which in general is neither associative nor commutative.

Using the categorical definition of product, for a magma M, one may define the Cartesian square magma M × M with the operation

(x, y) ∙ (u, v) = (x ∙ u, y ∙ v) .

The binary operation ∙ of M, considered as a mapping from M × M to M, maps (x, y) to x ∙ y, (u, v) to u ∙ v, and (x ∙ u, y ∙ v)  to (x ∙ u) ∙ (y ∙ v) . Hence, a magma M is medial if and only if its binary operation is a magma homomorphism from M × M to M. This can easily be expressed in terms of a commutative diagram, and thus leads to the notion of a medial magma object in a category with a Cartesian product. (See the discussion in auto magma object.)

If f and g are endomorphisms of a medial magma, then the mapping  f∙g defined by pointwise multiplication

is itself an endomorphism. It follows that the set End(M) of all endomorphisms of a medial magma M is itself a medial magma.

Bruck–Murdoch–Toyoda theorem

The Bruck–Murdoch-Toyoda theorem provides the following characterization of medial quasigroups. Given an abelian group A and two commuting automorphisms φ and ψ of A, define an operation ∗ on A by

x ∗ y = φ(x) + ψ(y) + c,

where c some fixed element of A. It is not hard to prove that A forms a medial quasigroup under this operation. The Bruck–Toyoda theorem states that every medial quasigroup is of this form, i.e. is isomorphic to a quasigroup defined from an abelian group in this way.[3] In particular, every medial quasigroup is isotopic to an abelian group.

The result was obtained independently in 1941 by D.C. Murdoch and K. Toyoda. It was then rediscovered by Bruck in 1944.

Generalizations

The term medial or (more commonly) entropic is also used for a generalization to multiple operations. An algebraic structure is an entropic algebra[4] if every two operations satisfy a generalization of the medial identity. Let f and g be operations of arity m and n, respectively. Then f and g are required to satisfy

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See also

References
  1. Historical comments Archived 2011-07-18 at the Wayback Machine J.Jezek and T.Kepka: Medial groupoids Rozpravy CSAV, Rada mat. a prir. ved 93/2 (1983), 93 pp
  2. Yamada, Miyuki (1971), "Note on exclusive semigroups", Semigroup Forum, 3 (1): 160–167, doi:10.1007/BF02572956.
  3. Kuzʹmin, E. N. & Shestakov, I. P. (1995). "Non-associative structures". Algebra VI. Encyclopaedia of Mathematical Sciences. 6. Berlin, New York: Springer-Verlag. pp. 197–280. ISBN 978-3-540-54699-3.
  4. Davey, B. A.; Davis, G. (1985). "Tensor products and entropic varieties". Algebra Universalis. 21: 68–88. doi:10.1007/BF01187558.
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