Geodesic
Free systems of algebras and ultraclosed classes
Acta mathematica Universitatis Comenianae, Tome 75 (2006) no. 1 
R. Thron; J. Koppitz. Free systems of algebras and ultraclosed classes. Acta mathematica Universitatis Comenianae, Tome 75 (2006) no. 1. http://geodesic.mathdoc.fr/item/AMUC_2006_75_1_a11/
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     author = {R. Thron and J. Koppitz},
     title = {Free systems of algebras and ultraclosed classes},
     journal = {Acta mathematica Universitatis Comenianae},
     year = {2006},
     volume = {75},
     number = {1},
     url = {http://geodesic.mathdoc.fr/item/AMUC_2006_75_1_a11/}
}
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Voir la notice de l'article provenant de la source Comenius University

There is considered the concept of the so-called free system of algebras for an ultraclosed class of algebras of a fixed arithmetic type. Certain free systems exist for such a class if and only if the class is defined by finite disjunctions of identities where the operational symbols are interpreted as operational variables for fundamental operations of an algebra.