Congruence n-permutable

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An algebra is congruence n-permutable if for all congruence relations θ,φ of the algebra θoφoθoφo... = φoθoφoθo..., where n congruences appear on each side of the equation.

A class of algebras is congruence n-permutable if each of its members is congruence n-permutable.

The term congruence permutable is short for congruence 2-permutable, i.e. θoφ = φoθ.

Congruence permutability holds for many 'classical' varieties such as groups, rings and vector spaces.

Congruence n-permutability is characterized by a Mal'cev condition...

For n = 2, a variety is congruence permutable iff there exists a term p(x,y,z) such that the identities p(x,z,z) = x = p(z,z,x) hold in the variety.