# Medial magma

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For the triple product, see Median algebra.

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

$(x \cdot y) \cdot (u \cdot v) = (x \cdot u) \cdot (y \cdot v)$, or more simply, $xy\cdot uv = xu\cdot yv$

using the convention that juxtaposition denotes the same operation but has higher precedence. A magma or groupoid is an algebraic structure that generalizes a group. 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 the normal bands.[2] Medial magmas need not be associative: for any nontrivial abelian group and integers mn, replacing the group operation $x+y$ with the binary operation $x \cdot y = mx+ny$ yields a medial magma which in general is neither associative nor commutative.

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

(x, y) ∙ (u, v) = (xu, yv) .

The binary operation of M, considered as a function on M × M, maps (x, y) to xy, (u, v) to uv, and (xu, yv)  to (xu) ∙ (yv) . 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 fg defined by pointwise multiplication

$(f\cdot g)(x) = f(x)\cdot g(x)$

is itself an endomorphism.

## 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

$f( g(x_{11}, \ldots, x_{1n}), \ldots, g(x_{m1}, \ldots, x_{mn}) ) = g( f(x_{11}, \ldots, x_{m1}), \ldots, f(x_{1n}, \ldots, x_{mn}) ).$

## References

1. ^ Historical comments 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. and 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. doi:10.1007/BF01187558. edit
• D.C. Murdoch, Structure of abelian quasigroups. Trans. Amer. Math. Soc, 1941, 47, p. 134-138.
• K. Toyoda, On axioms of linear functions. Proc. Imp. Acad. Tokyo, 1941, 17, p. 221-227
• R. H. Bruck, Some results in the theory of quasigroups, Trans. Amer. Math. Soc, 1944, 55.