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Positive algebras with countable congruence lattices. (English. Russian original) Zbl 0787.08002
Algebra Logic 31, No. 1, 12-23 (1992); translation from Algebra Logika 31, No. 1, 21-37 (1992).
A. I. Mal’tsev was one of the first to investigate algorithmic properties of positive algebras with countable congruence lattices; in particular, he proved constructiveness of positive algebras with finite congruence lattices and constructiveness of finitely generated positive algebras with nonzero congruences of only finite index. W. Baur established constructiveness for associative and commutative rings with identities and Noetherian lattices of ideals. In Algebra Logika 30, No. 3, 293-305 (1991; Zbl 0774.03029), the author constructed examples of nonconstructive positive groupoids with nonzero finite index congruences; descriptions of such algebras were given. All the lattices for all the algebras mentioned above are obviously countable.
In this paper we study the most general properties of positive algebras with countable lattices of congruences. For such algebras, in particular we infer an algebraic criterion of effective infinity, prove local finiteness of noneffectively infinite algebras, and construct an example of an algebra that is finitely defined in a finitely based variety and has Noetherian congruence lattice and an undecidable word problem. We also prove that for any natural $$n$$, there exists a positive algebra with exactly $$n$$ nonrecursively enumerable congruences.

##### MSC:
 08A30 Subalgebras, congruence relations 03D45 Theory of numerations, effectively presented structures 03C57 Computable structure theory, computable model theory 08A50 Word problems (aspects of algebraic structures)
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##### References:
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