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On congruence extension properties for ordered algebras

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Abstract

We introduce four extension properties (CEP, QEP, SCEP and SQEP) for ordered algebras, similar to the congruence extension property (CEP) and the strong congruence extension property of usual (unordered) algebras. All four properties turn out to have a description in terms of commutative squares or pullback diagrams. We then use these categorical descriptions to prove an ordered analogue of the well-known relation TP = AP + CEP, namely that a variety of ordered algebras has the ordered transferability property if and only if it has the ordered amalgamation property and QEP.

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Acknowledgments

We are very grateful to Professor George Janelidze for pointing out the idea behind Theorem 4.2. We also wish to thank the referee for useful suggestions. This research was supported by Estonian Institutional Research Project IUT20-57. The second named author is a recepient of the Estonian Science Foundation’s MOBILITAS research grant MJD 198.

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Correspondence to Valdis Laan.

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Laan, V., Nasir, S. & Tart, L. On congruence extension properties for ordered algebras. Period Math Hung 71, 92–111 (2015). https://doi.org/10.1007/s10998-015-0088-x

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