Extensive category

In mathematics, an extensive category is a category C with finite coproducts that are disjoint and well-behaved with respect to pullbacks. Equivalently, C is extensive if the coproduct functor from the product of the slice categories C/X × C/Y to the slice category C/(X + Y) is an equivalence of categories for all objects X and Y of C.^[1]

Examples[edit]

The categories Set and Top of sets and topological spaces, respectively, are extensive categories.^[2] More generally, the category of presheaves on any small category is extensive.^[2]

The category CRing^op of affine schemes is extensive.

References[edit]

^ Carboni, Aurelio; Lack, Stephen; Walters, R.F.C. (1993). "Introduction to extensive and distributive categories". Journal of Pure and Applied Algebra. 84 (2): 145–158. doi:10.1016/0022-4049(93)90035-R.
^ Jump up to: ^a ^b Pedicchio, Maria Cristina; Tholen, Walter (2004). Categorical Foundations: Special Topics in Order, Topology, Algebra, and Sheaf Theory. Cambridge University Press. ISBN 978-0-521-83414-8. Retrieved 4 April 2018.

External links[edit]

Extensive category at the nLab

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[1] Carboni, Aurelio; Lack, Stephen; Walters, R.F.C. (1993). "Introduction to extensive and distributive categories". Journal of Pure and Applied Algebra. 84 (2): 145–158. doi:10.1016/0022-4049(93)90035-R.

[cite2-2] Jump up to: ^a ^b Pedicchio, Maria Cristina; Tholen, Walter (2004). Categorical Foundations: Special Topics in Order, Topology, Algebra, and Sheaf Theory. Cambridge University Press. ISBN 978-0-521-83414-8. Retrieved 4 April 2018.

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