String diagram

In category theory, string diagrams are a way of representing morphisms in monoidal categories, or more generally 2-cells in 2-categories. In category theory, string diagrams are a way of representing morphisms in monoidal categories, or more generally 2-cells in 2-categories. The idea is to represent structures of dimension d by structures of dimension 2-d, using the Poincaré duality. Thus,