Congruence-permutable algebra

In universal algebra, a congruence-permutable algebra is an algebra whose congruences commute under composition. This symmetry has several equivalent characterizations, which lend to the analysis of such algebras. Many familiar varieties of algebras, such as the variety of groups, consist of congruence-permutable algebras, but some, like the variety of lattices, have members that are not congruence-permutable.

Definition

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Given an algebra  , a pair of congruences   are said to permute when  .[1]: 121  An algebra   is called congruence-permutable when each pair of congruences of   permute.[1]: 122  A variety of algebras   is referred to as congruence-permutable when every algebra in   is congruence-permutable.[1]: 122 

Properties

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In 1954 Maltsev gave two other conditions that are equivalent to the one given above defining a congruence-permutable variety of algebras. This initiated the study of congruence-permutable varieties.[1]: 122 

Theorem (Maltsev, 1954)

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Suppose that   is a variety of algebras. The following are equivalent:

  1. The variety   is congruence-permutable.
  2. The free algebra on   generators in   is congruence-permutable.
  3. There is a ternary term   such that
     .

Such a term is called a Maltsev term and congruence-permutable varieties are also known as Maltsev varieties in his honor.[1]: 122 

Examples

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Most classical varieties in abstract algebra, such as groups[1]: 123 , rings[1]: 123 , and Lie algebras[citation needed] are congruence-permutable. Any variety that contains a group operation is congruence-permutable, and the Maltsev term is  .[citation needed]

Nonexamples

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Viewed as a lattice the chain with three elements is not congruence-permutable and hence neither is the variety of lattices.[1]: 123 

References

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  1. ^ a b c d e f g h Bergman, Clifford (2011). Universal Algebra: Fundamentals and Selected Topics. Chapman and Hall/CRC. ISBN 978-1-4398-5129-6.