Associator

In abstract algebra, the term associator is used in different ways as a measure of the nonassociativity of an algebraic structure.

Ring theory

For a nonassociative ring or algebra $R$, the associator is the multilinear map $[\cdot,\cdot,\cdot] : R \times R \times R \to R$ given by

$[x,y,z] = (xy)z - x(yz).\,$

Just as the commutator measures the degree of noncommutativity, the associator measures the degree of nonassociativity of $R$. It is identically zero for an associative ring or algebra.

The associator in any ring obeys the identity

$w[x,y,z] + [w,x,y]z = [wx,y,z] - [w,xy,z] + [w,x,yz].\,$

The associator is alternating precisely when $R$ is an alternative ring.

The associator is symmetric in its two rightmost arguments when $R$ is a pre-Lie algebra.

The nucleus is the set of elements that associate with all others: that is, the n in R such that

$[n,R,R] = [R,n,R] = [R,R,n] = \{0\} \ .$

It turns out that any two of $([n,R,R],[R,n,R] , [R,R,n])$ being $\{0\}$ implies that the third is also the zero set.

Quasigroup theory

A quasigroup Q is a set with a binary operation $\cdot : Q\times Q\to Q$ such that for each a,b in Q, the equations $a\cdot x = b$ and $y\cdot a = b$ have unique solutions x,y in Q. In a quasigroup Q, the associator is the map $(\cdot,\cdot,\cdot) : Q\times Q\times Q\to Q$ defined by the equation

$(a\cdot b)\cdot c=(a\cdot (b\cdot c))\cdot (a,b,c)$

for all a,b,c in Q. As with its ring theory analog, the quasigroup associator is a measure of nonassociativity of Q.

Higher-dimensional algebra

In higher-dimensional algebra, where there may be non-identity morphisms between algebraic expressions, an associator is an isomorphism

$a_{x,y,z} : (xy)z \mapsto x(yz).$

Category theory

In category theory, the associator expresses the associative properties of the internal product functor in monoidal categories.

See also

• Commutator
• Non-associative algebra
• Quasi-bialgebra – discusses the Drinfeld associator

References

• Bremner and Hentzel. "Identities for the Associator in Alternative Algebras" (PDF).
• Schafer, Richard D. (1995) [1966]. An Introduction to Nonassociative Algebras. Dover. ISBN 0-486-68813-5.