nLab
finite product

Contents

Contents

Definition

A finite product is a product (Cartesian product) of a finite number of factors.

Finite products are generated from the empty product (the terminal object) and binary products.

Similarly a finite coproduct is a coproduct of a finite number of summands. This is generated from the empty coproduct (the initial object) and binary coproducts.

Properties

Example

(categories with finite products are cosifted)

Let 𝒞\mathcal{C} be a small category which has finite products. Then 𝒞\mathcal{C} is a cosifted category, equivalently its opposite category 𝒞 op\mathcal{C}^{op} is a sifted category, equivalently colimits over 𝒞 op\mathcal{C}^{op} with values in Set are sifted colimits, equivalently colimits over 𝒞 op\mathcal{C}^{op} with values in Set commute with finite products, as follows:

For X,Y[𝒞 op,Set]\mathbf{X}, \mathbf{Y} \in [\mathcal{C}^{op}, Set] to functors on the opposite category of 𝒞\mathcal{C} (hence two presheaves on 𝒞\mathcal{C}) we have a natural isomorphism

lim𝒞 op(X×Y)(lim𝒞 opX)×(lim𝒞 opY). \underset{\underset{\mathcal{C}^{op}}{\longrightarrow}}{\lim} \left( \mathbf{X} \times \mathbf{Y} \right) \;\simeq\; \left( \underset{\underset{\mathcal{C}^{op}}{\longrightarrow}}{\lim} \mathbf{X} \right) \times \left( \underset{\underset{\mathcal{C}^{op}}{\longrightarrow}}{\lim} \mathbf{Y} \right) \,.

Last revised on March 31, 2019 at 11:27:30. See the history of this page for a list of all contributions to it.