## Abstract

Generalizing results of Jónsson and Tarski, Maddux introduced the notion of a *pair-dense* relation algebra and proved that every pair-dense relation algebra is representable. The notion of a pair below the identity element is readily definable within the equational framework of relation algebras. The notion of a triple, a quadruple, or more generally, an element of size (or measure) \(n>2\) is not definable within this framework, and therefore it seems at first glance that Maddux’s theorem cannot be generalized. It turns out, however, that a very far-reaching generalization of Maddux’s result is possible if one is willing to go outside of the equational framework of relation algebras, and work instead within the framework of the first-order theory. Moreover, this generalization sheds a great deal of light not only on Maddux’s theorem, but on the earlier results of Jónsson and Tarski. In the present paper, we define the notion of an atom below the identity element in a relation algebra having measure *n* for an arbitrary cardinal number \(n>0\), and we define a relation algebra to be *measurable* if it’s identity element is the sum of atoms each of which has some (finite or infinite) measure. The main purpose of the present paper is to construct a large class of new examples of *group relation algebras* using systems of groups and corresponding systems of quotient isomorphisms (instead of the classic example of using a single group and forming its complex algebra), and to prove that each of these algebras is an example of a measurable set relation algebra. In a subsequent paper, the class of examples will be greatly expanded by adding a third ingredient to the mix, namely systems of “shifting” cosets. The expanded class of examples—called *coset relation algebras*—will be large enough to prove a representation theorem saying that every atomic, measurable relation algebra is essentially isomorphic to a coset relation algebra.

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## Acknowledgements

The author is very much indebted to Dr. Hajnal Andréka, of the Alfréd Rényi Mathematical Institute in Budapest, for carefully reading a draft of this paper and making many extremely helpful suggestions.

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*This article is dedicated to Bjarni Jónsson.*

This article is part of the topical collection “In memory of Bjarni Jónsson” edited by J.B. Nation.

This research was partially supported by Mills College.

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Givant, S. Relation algebras and groups.
*Algebra Univers.* **79**, 16 (2018). https://doi.org/10.1007/s00012-018-0505-0

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DOI: https://doi.org/10.1007/s00012-018-0505-0