Copyright © 2006 Elsevier B.V. All rights reserved.
Available online 20 July 2006.
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Abstract
Presheaf categories are well-known to be varieties of algebras and covarieties of coalgebras. We prove the converse: if a category is a variety as well as a covariety, then it is a presheaf category. Our main result is that all coalgebras on a set functor H form a presheaf category iff H is a reduction of a polynomial functor.
Keywords: Variety; Covariety; Presheaf category
Mathematical subject codes: 18C05; 18B20; 08B99; 18C20






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