Example 4.2.13. Given a category \mathcal{C} and an object X\in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}) we define the category of objects over X, denoted \mathcal{C}/X as follows. The objects of \mathcal{C}/X are morphisms Y\to X for some Y\in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}). Morphisms between objects Y\to X and Y'\to X are morphisms Y\to Y' in \mathcal{C} that make the obvious diagram commute. Note that there is a functor p_ X : \mathcal{C}/X\to \mathcal{C} which simply forgets the morphism. Moreover given a morphism f : X'\to X in \mathcal{C} there is an induced functor F : \mathcal{C}/X' \to \mathcal{C}/X obtained by composition with f, and p_ X\circ F = p_{X'}.
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