# Contents

## Definition

A suspended category is an additive category $C$ equipped with an additive functor $S:C\to C$ called suspension and a class of $S$-sequences called triangles satisfying axioms below. Here one calls an $S$-sequence a sequence of morphisms of the form

$X\stackrel{f}\to Y\stackrel{g}\to Z\stackrel{h}\to S X,$

and morphisms are ladders of the type

$\array{ X&\stackrel{f}\to &Y&\stackrel{g}\to &Z&\stackrel{h}\to& S X\\ a\downarrow&&b\downarrow&&c\downarrow&&\downarrow S a\\ X'&\stackrel{f'}\to &Y'&\stackrel{g'}\to &Z'&\stackrel{h'}\to& S X',\\ }$

where all the squares commute. Axioms:

(SP0) Each sequence isomorphic to a triangle is a triangle.

(SP1) Each sequence of the form $0\to X\stackrel{id}\to X\to S0$ is a triangle.

(SP2) If $X\stackrel{f}\to Y\stackrel{g}\to Z\stackrel{h}\to S X$ is a triangle, then $Y\stackrel{g}\to Z\stackrel{h}\to S X\stackrel{-S f}\to S Y$ is also a triangle.

(SP3) Every diagram of the form

$\array{ X&\stackrel{f}\to &Y&\stackrel{g}\to &Z&\stackrel{h}\to& S X\\ a\downarrow&&b\downarrow&&&&\downarrow S a\\ X'&\stackrel{f'}\to &Y'&\stackrel{g'}\to &Z'&\stackrel{h'}\to& S X'\\ }$

can be completed to a morphism of $S$-sequences.

(SP4) For any two morphisms $X\stackrel{f}\to Y$ and $Y\stackrel{g}\to Z$ there is a commuting diagram

$\array{ X&\stackrel{f}\to &Y&\stackrel{g}\to &Z'&\to& S X\\ =\downarrow&&f\downarrow&&\downarrow&&=\downarrow \\ X&\to &Z&\to &Y'&\to& S X\\ &&\downarrow&&\downarrow&&\downarrow S f\\ & &X'&\stackrel{id}\to &X'&\stackrel{j}\to& S Y\\ &&j\downarrow&&\downarrow&&\\ &&S Y&\stackrel{S i}\to &S Z'&&\\ }$

where the first two rows and the middle two columns are triangles.

## References

Suspended categories were introduced in

• Bernhard Keller, Dieter Vossieck, Sous les catégories dérivées. [Beneath the derived categories] C. R. Acad. Sci. Paris Sér. I Math. 305 (1987), no. 6, 225–228.