mathlib documentation


Triangles #

This file contains the definition of triangles in an additive category with an additive shift. It also defines morphisms between these triangles.

TODO: generalise this to n-angles in n-angulated categories as in

A triangle in C is a sextuple (X,Y,Z,f,g,h) where X,Y,Z are objects of C, and f : X ⟶ Y, g : Y ⟶ Z, h : Z ⟶ X⟦1⟧ are morphisms in C. See

A triangle (X,Y,Z,f,g,h) in C is defined by the morphisms f : X ⟶ Y, g : Y ⟶ Z and h : Z ⟶ X⟦1⟧.


A morphism of triangles (X,Y,Z,f,g,h) ⟶ (X',Y',Z',f',g',h') in C is a triple of morphisms a : X ⟶ X', b : Y ⟶ Y', c : Z ⟶ Z' such that a ≫ f' = f ≫ b, b ≫ g' = g ≫ c, and a⟦1⟧' ≫ h = h' ≫ c. In other words, we have a commutative diagram:

     f      g      h
  X  ───> Y  ───> Z  ───> X1
  a      b      c       a1⟧'
  V       V       V        V
  X' ───> Y' ───> Z' ───> X'1
     f'     g'     h'