# nLab triangulated category

### Context

#### Homological algebra

homological algebra

and

nonabelian homological algebra

diagram chasing

### Theorems

#### Stable homotopy theory

stable homotopy theory

# Contents

## Idea

Triangulated categories were introduced by Jean-Louis Verdier under the supervision of Grothendieck, motivated by the triangulated structure on derived categories.

A triangulated category is a category equipped with a notion of suspension objects/loop space objects for all of its objects such that in terms of these every morphism fits into a sequence that behaves like a homotopy fiber sequence.

More precisely, a triangulated category is a category that behaves like the homotopy category of a stable (∞,1)-category. Indeed, most examples of triangulated categories that arise in practice appear this way, and in fact often from stable model categories. Notice that the definition of stable (∞,1)-category is very simple and much simpler than the definition of triangulated category, def. 1 below.

Therefore, all the structure and properties of a triangulated category is best understood as a 1-categorical shadow (the decategorification) of the corresponding properties of stable (∞,1)-categories.

A central class of examples of triangulated categories are the derived categories $D(\mathcal{A})$ of abelian categories $\mathcal{A}$. These are the homotopy categories of the (∞,1)-categories of chain complexes in $\mathcal{A}$.

Triangulated categories are sufficient for some purposes, and can be easier to work with than the stable (∞,1)-categories that they come from, but – as with every quotient construction – often one needs more information than is present in the triangulated category, especially concerning the computation of homotopy limits and homotopy colimits: the ordinary limits and colimits and other universal constructions in a triangulated category generally have no useful interpretation.

Accordingly, there is a series on notions that refine that of a triangulated category, approximating more and more of the full structure of a stable (∞,1)-category:

## Definition

The traditional definition of triangulated category is the following. But see remark 1 below.

###### Definition

A triangulated category is

TR0: every triangle isomorphic to a distinguished triangle is itself a distinguished triangle;

TR1: the triangle

$X \stackrel{Id_X}{\to} X \to 0 \to T X$

is a distinguished triangle;

TR2: for all $f : X \to Y$, there exists a distinguished triangle

$X \stackrel{f}{\to} Y \to Z \to TX \,;$

TR3: a triangle

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

is a distinguished triangle precisely if

$Y \stackrel{-g}{\to} Z \stackrel{-h}{\to} T X \stackrel{-T(f)}{\to} T Y$

is a distinguished triangle;

TR4: given two distinguished triangles

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

and

$X' \stackrel{f'}{\to} Y' \stackrel{g'}{\to} Z' \stackrel{h'}{\to} T X'$

and given morphisms $\alpha$ and $\beta$ in

$\array{ X &\stackrel{f}{\to}& Y \\ \downarrow^\alpha && \downarrow^\beta \\ X' &\stackrel{f'}{\to}& Y' }$

there exists a morphism $\gamma : Z \to Z'$ extending this to a morphism of distinguished triangles in that the diagram

$\array{ X &\stackrel{f}{\to}& Y &\stackrel{g}{\to}& Z &\stackrel{h}{\to}& T X \\ \downarrow^\alpha && \downarrow^\beta && \downarrow^{\exists \gamma} && \downarrow^{T(\alpha)} \\ X' &\stackrel{f'}{\to}& Y' &\stackrel{g'}{\to}& Z' &\stackrel{h'}{\to}& T X' }$

commutes;

TR5: given three distinguished triangles of the form

\begin{aligned} & X \stackrel{f}{\to} Y \stackrel{h}{\to} Y/X \stackrel{}{\to} T X \\ & Y \stackrel{g}{\to} Z \stackrel{k}{\to} Z/Y \stackrel{}{\to} T Y \\ & X \stackrel{g \circ f}{\to} Z \stackrel{l}{\to} Z/X \stackrel{}{\to} T X \end{aligned}

there exists a distinguished triangle

$Y/X \stackrel{u}{\to} Z/X \stackrel{v}{\to} Z/Y \stackrel{w}{\to} T (Y/X)$

such that the following big diagram commutes

$\array{ X &&\stackrel{g \circ f}{\to}&& Z &&\stackrel{k}{\to}&& Z/Y &&\stackrel{k}{\to}&& T (Y/X) \\ & {}_{f}\searrow && \nearrow_{g} && \searrow^{l} && \nearrow_{v} && \searrow^{} && \nearrow_{T(h)} \\ && Y &&&& Z/X &&&& T Y \\ &&& \searrow^{h} && \nearrow_{u} && \searrow^{} && \nearrow_{T(f)} \\ &&&& Y/X &&\stackrel{}{\to}&& T X }$
###### Remark

This classical definition is actually redundant; TR4 and one direction of TR3 follow from the remaining axioms. See (May).

###### Remark

In the context of triangulated categories the translation functor $T : C \to C$ is often called the suspension functor and denoted $(-)[1] : X \mapsto X[1]$ (in an algebraic context) or $S$ or $\Sigma$ (in a topological context). However unlike “translation functor”, suspension functor is also the term used when the invertibility is not assumed, cf. suspended category.

###### Remark

If $(f,g,h)$ is a distinguished triangle, then $(f,g,-h)$ is not generally distinguished, although it is “exact” (induces long exact sequences in homology and cohomology). However, $(f,-g,-h)$ is always distinguished, since it is isomorphic to $(f,g,h)$:

$\array{ X & \xrightarrow{f} & Y & \xrightarrow{g} & Z & \xrightarrow{h} & T X\\ ^{id}\downarrow && ^{id} \downarrow && ^{-1} \downarrow && \downarrow^{id}\\ X & \xrightarrow{f} & Y & \xrightarrow{-g} & Z & \xrightarrow{-h} & T X}$

## References

The original reference is the thesis of Verdier:

• Verdier, Jean-Louis, Des Catégories Dérivées des Catégories Abéliennes, Astérisque (Paris: Société Mathématique de France) 239. Available in electronic format courtesy of Georges Maltsiniotis.

A comprehensive monograph is

• Amnon Neeman, Triangulated Categories , Princeton University Press (2001)

and a survey is in section 10 of

section 3 of

A survey of formalisms used in stable homotopy theory to present the triangulated homotopy category of a stable (∞,1)-category is in

Discussion of the redundancy in the traditional definition of triangulated category is in

• Peter May, The additivity of traces in triangulated categories, (pdf)

There was also some discussion at the nForum.

Revised on May 27, 2014 02:13:12 by Urs Schreiber (82.136.246.44)