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We illustrate the formula $ (\downarrow p)x = \Gamma_!(x/p) $, which gives the reflection $\downarrow p$ of a category $p : P \to X$ over $X$ in discrete fibrations. One of its proofs is based on a ``complement operator" which takes a discrete fibration $A$ to the functor $\neg A$, right adjoint to $\Gamma_!(A\times-):Cat/X \to Set$ and valued in discrete opfibrations.Some consequences and applications are presented.
@article{TAC_2007_19_a1, author = {Claudio Pisani}, title = {Components, complements and the reflection formula}, journal = {Theory and applications of categories}, pages = {19--40}, publisher = {mathdoc}, volume = {19}, year = {2007}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2007_19_a1/} }
Claudio Pisani. Components, complements and the reflection formula. Theory and applications of categories, CT2006, Tome 19 (2007), pp. 19-40. http://geodesic.mathdoc.fr/item/TAC_2007_19_a1/