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On (co)products of partial combinatory algebras, with an application to pushouts of realizability toposes

Published online by Cambridge University Press:  13 August 2021

Jetze Zoethout Open the ORCID record for Jetze Zoethout [Opens in a new window]
Jetze Zoethout*
Affiliation:
Mathematical Institute, Utrecht University, Utrecht, Netherlands Email: j.zoethout@uu.nl
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Abstract

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We consider two preorder-enriched categories of ordered partial combinatory algebras: OPCA, where the arrows are functional (i.e., projective) morphisms, and OPCA†, where the arrows are applicative morphisms. We show that OPCA has small products and finite biproducts, and that OPCA† has finite coproducts, all in a suitable 2-categorical sense. On the other hand, OPCA† lacks all nontrivial binary products. We deduce from this that the pushout, over Set, of two nontrivial realizability toposes is never a realizability topos. In contrast, we show that nontrivial subtoposes of realizability toposes are closed under pushouts over Set.

Keywords

Partial combinatory algebrarealizabilitytoposes
Type
Paper
Information
Mathematical Structures in Computer Science , Volume 31 , Issue 2 , February 2021 , pp. 214 - 233
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Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
© The Author(s), 2021. Published by Cambridge University Press

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