Triangulated category

In mathematics, a triangulated category is a category together with additional structure, a "translation functor" and a class of "distinguished triangles". Prominent examples are the derived category of an abelian category (more generally, the homotopy category of a stable ∞-category) and the stable homotopy category of spectra, both of which carry the structure of a triangulated category in a natural fashion. The distinguished triangles are reminiscent of the long exact sequences of homology; they play a role akin to that of short exact sequences in abelian categories.

A t-category is a triangulated category with a t-structure.

History

The notion of a derived category was introduced by Jean-Louis Verdier (1963) in his Ph.D. thesis, based on the ideas of Grothendieck. He also defined the notion of a triangulated category, based upon the observation that a derived category had some special "triangles", by writing down axioms for the basic properties of these triangles. A very similar set of axioms was written down at about the same time by Dold and Puppe (1961).

Definition

A translation functor on a category D is an automorphism (or for some authors, an auto-equivalence) T from D to D. One usually uses the notation $X[n] = T^n X$ and likewise for morphisms from X to Y.

A triangle (X, Y, Z, u, v, w) consists of 3 objects X, Y, and Z, together with morphisms u : XY, v : YZ and w : ZX[1]. Triangles are generally written in the unravelled form:

$X \xrightarrow{u} Y \xrightarrow{v} Z \xrightarrow {w} X[1]$

or

$X \xrightarrow{u} Y \xrightarrow{v} Z \xrightarrow {w}$

for short.

A triangulated category is an additive category D with a translation functor and a class of triangles, called distinguished triangles, satisfying the following properties (TR 1), (TR 2), (TR 3) and (TR 4). (These axioms are not entirely independent, since (TR 3) can be derived from the others.[1])

TR 1

• For any object X, the following triangle is distinguished:
$X \overset{\text{id}}{\to} X \to 0 \to \cdot$
• For any morphism u : XY, there is an object Z (called a mapping cone of the morphism u) fitting into a distinguished triangle
$X \xrightarrow{u} Y \to Z \to \cdot$
• Any triangle isomorphic to a distinguished triangle is distinguished. This means that if
$X \xrightarrow{u} Y \xrightarrow{v} Z \xrightarrow{w} X[1]$
is a distinguished triangle, and f : XX′, g : YY′, and h : ZZ′ are isomorphisms, then
$X' \xrightarrow{guf^{-1}} Y'\xrightarrow{hvg^{-1}} Z' \xrightarrow {f[1]wh^{-1}} X'[1]$
is also a distinguished triangle.

TR 2

If

$X \xrightarrow{u} Y \xrightarrow{v} Z \xrightarrow {w} X[1]$

is a distinguished triangle, then so are the two rotated triangles

$Y \xrightarrow{v} Z \xrightarrow{w} X[1] \xrightarrow{-u[1]} Y[1]$

and

$Z[-1] \xrightarrow{-w[-1]} X \xrightarrow{u} Y \xrightarrow{v} Z.\$

TR 3

Given two distinguished triangles and a map between the first morphisms in each triangle, there exists a morphism between the third objects in each of the two triangles that makes everything commute. This means that in the following diagram (where the two rows are distinguished triangles and f and g form the map of morphisms such that gu = u′f) there exists some map h (not necessarily unique) making all the squares commute:

TR 4: The octahedral axiom

Suppose we have morphisms u : XY and v : YZ, so that we also have a composed morphism vu : XZ. Form distinguished triangles for each of these three morphisms according to TR 1. The octahedral axiom states (roughly) that the three mapping cones can be made into the vertices of a distinguished triangle so that "everything commutes".

More formally, given distinguished triangles

$X \xrightarrow{u\,} Y \xrightarrow{j} Z' \xrightarrow {k}$
$Y \xrightarrow{v\,} Z \xrightarrow{l} X' \xrightarrow {i}$
$X \xrightarrow{vu} Z \xrightarrow{m} Y' \xrightarrow {n}$

there exists a distinguished triangle

$Z' \xrightarrow{f} Y' \xrightarrow{g} X' \xrightarrow {h}$

such that

$l=gm,\quad k=nf,\quad h=j[1]i,\quad ig=u[1]n,\quad fj=mv.$

This axiom is called the "octahedral axiom" because drawing all the objects and morphisms gives the skeleton of an octahedron, four of whose faces are distinguished triangles. The presentation here is Verdier's own, and appears, complete with octahedral diagram, in (Hartshorne 1966). In the following diagram, u and v are the given morphisms, and the primed letters are the cones of various maps (chosen so that every distinguished triangle has an X, a Y, and a Z letter). Various arrows have been marked with [1] to indicate that they are of "degree 1"; e.g. the map from Z′ to X is in fact from Z′ to T(X). The octahedral axiom then asserts the existence of maps f and g forming a distinguished triangle, and so that f and g form commutative triangles in the other faces that contain them:

Two different pictures appear in (Beilinson, Bernstein & Deligne 1982) (Gelfand and Manin (2006) also present the first one). The first presents the upper and lower pyramids of the above octahedron and asserts that given a lower pyramid, we can fill in an upper pyramid so that the two paths from Y to Y′, and from Y′ to Y, are equal (this condition is omitted, perhaps erroneously, from Hartshorne's presentation). The triangles marked + are commutative and those marked "d" are distinguished:

The second diagram is a more innovative presentation. Distinguished triangles are presented linearly, and the diagram emphasizes the fact that the four triangles in the "octahedron" are connected by a series of maps of triangles, where three triangles (namely, those completing the morphisms from X to Y, from Y to Z, and from X to Z) are given and the existence of the fourth is claimed. We pass between the first two by "pivoting" about X, to the third by pivoting about Z, and to the fourth by pivoting about X′. All enclosures in this diagram are commutative (both trigons and the square) but the other commutative square, expressing the equality of the two paths from Y′ to Y, is not evident. All the arrows pointing "off the edge" are degree 1:

This last diagram also illustrates a useful intuitive interpretation of the octahedral axiom. Since in triangulated categories, triangles play the role of exact sequences, we can pretend that $Z' = Y/X, Y' = Z/X\$ in which case the existence of the last triangle expresses on the one hand

$X' = Z/Y\$ (looking at the triangle $Y \to Z \to X' \to$ ), and
$X' = Y'/Z'\$ (looking at the triangle $Z' \to Y' \to X' \to$ ).

Putting these together, the octahedral axiom asserts the "third isomorphism theorem":

$(Z/X)/(Y/X) = Z/Y.$

When the triangulated category is K(A) for some abelian category A, and when X, Y, Z are objects of A placed in degree 0 in their eponymous complexes, and when the maps XY, YZ are injections in A, then the cones are literally the above quotients, and the pretense becomes truth.

Finally, Neeman (2001) gives a way of expressing the octahedral axiom using a two dimensional commutative diagram with 4 rows and 4 columns.Beilinson, Bernstein, and Deligne (1982) also give generalizations of the octahedral axiom.

Are there better axioms?

Some experts suspect (see, for example, (Gelfand & Manin 2006, Introduction, Chapter IV)) that triangulated categories are not really the "correct" concept. The essential reason is that the mapping cone of a morphism is unique only up to a non-unique isomorphism. In particular the mapping cone of a morphism does not in general depend functorially on the morphism (note the non-uniqueness in axiom (TR 3), for example). This non-uniqueness is a potential source of errors, among other things preventing in many cases a triangulated category from being the derived category of its core (with respect to a particular t-structure). The axioms do however seem to work adequately in practice, and there is currently no convincing replacement. A few proposals have been developed, however, such as derivators that Grothendieck described in his long, unfinished and unpublished manuscript from 1991.

On the other hand, the homotopy category of a stable ∞-category is canonically triangulated. Moreover, a stable ∞-category naturally encodes a whole hierarchy of compatibilities for its homotopy category, at the bottom of which sits the octahedral axiom (see Lurie, Higher Algebra, Ch. 1). Thus, it is strictly stronger to give the data of a stable ∞-category than to give the data of a triangulation of its homotopy category; however, in practice nearly all triangulated categories that arise are essentially given by definition as stable ∞-categories.

Examples

1. Vector spaces (over a field) form an elementary triangulated category in which X[1]=X for all X. A distinguished triangle is a sequence $X \to Y \to Z \to X \to Y$ which is exact at X, Y and Z.

2. If A is an abelian category, then the homotopy category K(A) has as objects all complexes of objects of A, and as morphisms the homotopy classes of morphisms of complexes. Then K(A) is a triangulated category; the distinguished triangles consist of triangles isomorphic to a morphism with its mapping cone (in the sense of chain complexes). It is possible to create variations, using complexes that are bounded on the left, or on the right, or on both sides.

3. The derived category of A is also a triangulated category; it is created from K(A) by localizing at the class of quasi-isomorphisms, a process we now describe.

Under some reasonable conditions on the localizing set S, a localization of a triangulated category is also triangulated. In particular, these conditions are:

• S is closed under all translations, and
• For any two triangles $X \to Y \to Z \to X[1],$ $X' \to Y' \to Z' \to X'[1]$ and arrows $X \to X', Y \to Y'$ as in the axioms, if these arrows are both in S then the promised arrow $Z \to Z'$ completing the map of triangles is also in S.

S is then said to be "compatible with the triangulation". It is not hard to see that this is the case when S is the class of quasi-isomorphisms in K(A), so in particular the derived category of A, which is the localization of K(A) with respect to quasi-isomorphisms, is triangulated.

4. The topologist's stable homotopy category is another example of a triangulated category. The objects are spectra, the suspension is the translation functor, and the cofibration sequences are the distinguished triangles.

5. In modular representation theory of a finite group $G$, the stable module category is yet another example. Its objects are the representations of $G$ and the morphisms are the usual ones modulo those that factor via projective (injective) objects. More generally, such construction is possible for any Frobenius algebra.

Properties

Suppose D is a triangulated category.

Given a distinguished triangle

$X \xrightarrow{u} Y \xrightarrow{v} Z \xrightarrow {w} X[1]$

in D, the composition of any two of the involved morphisms is 0, i.e. vu=0, wv=0, u[1]w=0, etc.

Given a morphism u:XY, TR 1 guarantees the existence of a mapping cone Z completing a distinguished triangle. Any two mapping cones of u are isomorphic, however the isomorphism is not unique.

Every monomorphism in D is a section and every epimorphism is a retraction.

Cohomology in triangulated categories

Triangulated categories admit a notion of cohomology and every triangulated category includes a large number of cohomological functors. By definition, a functor F from a triangulated category D into an abelian category A is a cohomological functor if for every distinguished triangle

$X \xrightarrow{u} Y \xrightarrow{v} Z \xrightarrow{w},\$

which can be written as the doubly infinite sequence of morphisms

$\cdots \to X \xrightarrow{u} Y \xrightarrow{v} Z \xrightarrow{w} X[1] \xrightarrow{u[1]} \cdots,\$

the following sequence (obtained by applying F to this one) is a long exact sequence:

$\cdots \to F(X) \to F(Y) \to F(Z) \to F(X[1]) \to \cdots.\$

In a general triangulated category we are guaranteed that the functors $\operatorname{Hom}(A, \text{--}), \operatorname{Hom}(\text{--}, A),$ for any object A, are cohomological, with values in the category of abelian groups (the latter is a contravariant functor, which we view as taking values in the opposite category, also abelian). That is, we have for example an exact sequence (for the above triangle)

$\cdots \to \operatorname{Hom}(A, X[i]) \to \operatorname{Hom}(A, Y[i]) \to \operatorname{Hom}(A, Z[i]) \to \operatorname{Hom}(A, X[i + 1]) \to \cdots.\$

The functors are also written

$\operatorname{Ext}^i(A, X) = \operatorname{Hom}(A, X[i])$

in analogy with the Ext functors in derived categories. Thus we have the familiar sequence

$\cdots \to \operatorname{Ext}^i(A, X) \to \operatorname{Ext}^i(A, Y) \to \operatorname{Ext}^i(A, Z) \to \operatorname{Ext}^{i + 1}(A, X) \to \cdots.\$

Exact functors and equivalences

An exact functor (also called triangulated functor) from a triangulated category D to a triangulated category E is an additive functor F : DE which, loosely speaking, commutes with translation and maps distinguished triangles to distinguished triangles.

Specifically, the exact functor comes with a natural isomorphism η : FT → TF (where the first T denotes the translation functor of D and the second T denotes the translation functor of E), such that whenever

$X \xrightarrow{u} Y \xrightarrow{v} Z \xrightarrow {w} X[1]$

is a distinguished triangle in D,

$F(X) \xrightarrow{F(u)} F(Y) \xrightarrow{F(v)} F(Z) \xrightarrow {\eta_X F(w)} F(X)[1]$

is a distinguished triangle in E.

An exact equivalence is an exact functor F : DE that is also an equivalence of categories; in this case there exists an exact functor G : ED such that FG and GF are naturally isomorphic to the respective identify functors. D and E are called equivalent as triangulated categories; for most practical purposes they are identical.

t-structures

Further information: t-structure

Verdier introduced triangulated categories in order to place derived categories in a category-theoretic context: for every abelian category A there exists a triangulated category D(A), containing A as a full subcategory (the "0-complexes" concentrated in cohomological degree 0), and in which we can construct derived functors. Different abelian categories can give rise to equivalent derived categories, so that it is impossible to reconstruct A from the triangulated category D(A).

A partial solution to this problem, is to impose a t-structure on the triangulated category D. Different t-structures on D will give rise to different abelian categories inside it. This notion was presented in (Beilinson, Bernstein & Deligne 1982).

References

Part of Verdier's 1963 thesis is reprinted in "SGA 4 1/2" :

and the entire thesis was published in Astérisque and is distributed by the American Mathematical Society in North America as

• Verdier, Jean-Louis (1963), "Des Catégories Dérivées des Catégories Abéliennes", Astérisque (Société Mathématique de France, Marseilles, published 1996) 239

The material is also presented in English in

Axioms similar to Verdier's were presented in:

• Dold, A.; Puppe, D. (1961), "Homologie nicht-additiver Funktoren", Annales de l'Institut Fourier (Université de Grenoble) 11: 201–312, ISSN 0373-0956

Some textbooks that discuss triangulated categories are:

The first section of the following paper discusses (but assumes familiarity with) the axioms of a triangulated category and introduces the notion of a t-structure:

Herein is a concise introduction with applications: