In mathematics, especially category theory, a discrete category is a category whose only morphisms are the identity morphisms. It is the simplest kind of category. Specifically a category C is discrete if
- homC(X, X) = {idX} for all objects X
- homC(X, Y) = ∅ for all objects X ≠ Y
Since by axioms, there is always the identity morphism between the same object, the above is equivalent to saying
- |homC(X, Y)| is 1 when X = Y and 0 when X is not equal to Y.
Clearly, any class of objects defines a discrete category when augmented with identity maps.
Any subcategory of a discrete category is discrete. Also, a category is discrete if and only if all of its subcategories are full.
The limit of any functor from a discrete category into another category is called a product, while the colimit is called a coproduct.
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