Mathematical Logic Quarterly 55 (2):138-153 (2009)

Abstract
Built on the foundations laid by Peirce, Schröder, and others in the 19th century, the modern development of relation algebras started with the work of Tarski and his colleagues [21, 22]. They showed that relation algebras can capture strong first-order theories like ZFC, and so their equational theory is undecidable. The less expressive class WA of weakly associative relation algebras was introduced by Maddux [7]. Németi [16] showed that WA's have a decidable universal theory. There has been extensive research on increasing the expressive power of WA by adding new operations [1, 4, 11, 13, 20]. Extensions of this kind usually also have decidable universal theories. Here we give an example – extending WA's with set-theoretic projection elements – where this is not the case. These “logical” connectives are set-theoretic counterparts of the axiomatic quasi-projections that have been investigated in the representation theory of relation algebras [22, 6, 19]. We prove that the quasi-equational theory of the extended class PWA is not recursively enumerable. By adding the difference operator D one can turn WA and PWA to discriminator classes where each universal formula is equivalent to some equation. Hence our result implies that the projections turn the decidable equational theory of “WA + D ” to non-recursively enumerable
Keywords decision problems  quasi‐projections  Weakly associative relation algebras
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DOI 10.1002/malq.200710074
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References found in this work BETA

On the Calculus of Relations.Alfred Tarski - 1941 - Journal of Symbolic Logic 6 (3):73-89.
Finitary Algebraic Logic.Roger D. Maddux - 1989 - Zeitschrift fur mathematische Logik und Grundlagen der Mathematik 35 (4):321-332.
Finitary Algebraic Logic.Roger D. Maddux - 1989 - Mathematical Logic Quarterly 35 (4):321-332.

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