Abstract
The class \(\mathsf{TPA}\) of t rue p airing a lgebras is defined to be the class of relation algebras expanded with concrete set theoretical projection functions. The main results of the present paper is that neither the equational theory of \(\mathsf{TPA}\) nor the first order theory of \(\mathsf{TPA}\) are decidable. Moreover, we show that the set of all equations valid in \(\mathsf{TPA}\) is exactly on the \(\Pi ^1_1\) level. We consider the class \(\mathsf{TPA}^-\) of the relation algebra reducts of \(\mathsf{TPA}\)’s, as well. We prove that the equational theory of \(\mathsf{TPA}^-\) is much simpler, namely, it is recursively enumerable. We also give motivation for our results and some connections to related work.
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Research supported by Hungarian National Foundation for Scientific Research Grant Numbers T030314, T034861, and T035192.
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Mikulás, S., Sain, I. & Simon, A. Complexity of equational theory of relational algebras with standard projection elements. Synthese 192, 2159–2182 (2015). https://doi.org/10.1007/s11229-015-0689-1
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DOI: https://doi.org/10.1007/s11229-015-0689-1