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Two addenda to the author's ‘Transfinite constructions’

Published online by Cambridge University Press:  17 April 2009

G.M. Kelly
Affiliation:
Department of Pure Mathematics, University of Sydney, Sydney, New South Wales 2006, Australia.
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Abstract

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Since the author's article “A unified treatment of transfinite constructions …”, in Volume 22 (198O) of this Bulletin, had an encyclopaedic goal, he now takes the opportunity to answer two further questions raised since that article was submitted. The lesser of these asks whether the only pointed endofunctors for which every action is an isomorphism are the well-pointed ones, at least when the endofunctor is cocontinuous; a counter-example provides a negative answer. The more important question concerns the reflexion from the comma-category T/A into the category of algebras for the pointed endofunctor T of A, and the algebra-reflexion sequence which converges to this reflexion; and asks for simplified descriptions in the special case where T is cocontinuous. We give closed formulas in this case, both for the reflexion and for the sequence which converges to it. The reader may wonder why we care about the approximating sequence when we have a closed formula for the reflexion; the answer is that, in certain applications, we need to separate the roles of finite colimits and filtered ones.

Type
Research Article
Copyright
Copyright © Australian Mathematical Society 1982

References

[1]Kelly, G.M., “A unified treatment of transfinite constructions for free algebras, free monoids, colimits, associated sheaves, and so on”, Bull. Austral. Math. Soc. 22 (1980), 183.CrossRefGoogle Scholar