# Tilting theory

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It turns out that there are applications of our functors which make use of the analogous transformations which we like to think of as a change of basis for a fixed root- system - a tilting of the axes relative to the roots which results in a different subset of roots lying in the positive cone. ... For this reason, and because the word 'tilt' inflects easily, we call our functors tilting functors or simply tilts.

Brenner & Butler (1980, p.103)

In mathematics, specifically representation theory, tilting theory describes a way to relate the module categories of two algebras using so-called tilting modules and associated tilting functors. Here, the second algebra is the endomorphism algebra of a tilting module over the first algebra.

Tilting theory was motivated by the introduction of reflection functors by Bernšteĭn, Gelfand & Ponomarev (1973); these functors were used to relate representations of two quivers. These functors were reformulated by Auslander, Platzeck & Reiten (1979), and generalized by Brenner & Butler (1980) who introduced tilting functors. Happel & Ringel (1982) defined tilted algebras and tilting modules as further generalizations of this.

## Definitions

Suppose that A is a finite-dimensional unital associative algebra over some field. A finitely-generated right A-module T is called a tilting module if it has the following three properties:

Given such a tilting module, we define the endomorphism algebra B = EndA(T). This is another finite-dimensional algebra, and T is a finitely-generated left B-module. The tilting functors HomA(T,−), Ext1
A
(T,−), −⊗BT and TorB
1
(−,T) relate the category mod-A of finitely-generated right A-modules to the category mod-B of finitely-generated right B-modules.

In practice one often considers hereditary finite dimensional algebras A because the module categories over such algebras are fairly well understood. The endomorphism algebra of a tilting module over a hereditary finite dimensional algebra is called a tilted algebra.

## Facts

Suppose A is a finite-dimensional algebra, T is a tilting module over A, and B = EndA(T). Write F=HomA(T,−), F′=Ext1
A
(T,−), G=−⊗BT, and G′=TorB
1
(−,T). F is right adjoint to G and F′ is right adjoint to G′.

Brenner & Butler (1980) showed that tilting functors give equivalences between certain subcategories of mod-A and mod-B. Specifically, if we define the two subcategories $\mathcal{F}=\ker(F)$ and $\mathcal{T}=\ker(F')$ of A-mod, and the two subcategories $\mathcal{X}=\ker(G)$ and $\mathcal{Y}=\ker(G')$ of B-mod, then $(\mathcal{T},\mathcal{F})$ is a torsion pair in A-mod (i.e. $\mathcal{T}$ and $\mathcal{F}$ are maximal subcategories with the property $\operatorname{Hom}(\mathcal{T},\mathcal{F})=0$; this implies that every M in A-mod admits a natural short exact sequence $0\to U\to M\to V\to 0$ with U in $\mathcal{T}$ and V in $\mathcal{F}$) and $(\mathcal{X},\mathcal{Y})$ is a torsion pair in B-mod. Further, the restrictions of the functors F and G yield inverse equivalences between $\mathcal{T}$ and $\mathcal{Y}$, while the restrictions of F′ and G′ yield inverse equivalences between $\mathcal{F}$ and $\mathcal{X}$. (Note that these equivalences switch the order of the torsion pairs $(\mathcal{T},\mathcal{F})$ and $(\mathcal{X},\mathcal{Y})$.)

Tilting theory may be seen as a generalization of Morita equivalence which is recovered if T is a projective generator; in that case $\mathcal{T}=\operatorname{mod}-A$ and $\mathcal{Y}=\operatorname{mod}-B$.

If A has finite global dimension, then B also has finite global dimension, and the difference of F and F' induces an isometry between the Grothendieck groups K0(A) and K0(B).

In case A is hereditary (i.e. B is a tilted algebra), the global dimension of B is at most 2, and the torsion pair $(\mathcal{X},\mathcal{Y})$ splits, i.e. every indecomposable object of B-mod is either in $\mathcal{X}$ or in $\mathcal{Y}$.

Happel (1988) and Cline, Parshall, Scott (1986) showed that in general A and B are derived equivalent (i.e. the derived categories Db(A-mod) and Db(B-mod) are equivalent as triangulated categories).

## Generalizations and extensions

A generalized tilting module over the finite-dimensional algebra A is a right A-module T with the following three properties:

• T has finite projective dimension.
• Exti
A
(T,T) = 0 for all i>0.
• There is an exact sequence $0\to A\to T_1\to \dots\to T_n\to 0$ where the Ti are finite direct sums of direct summands of T.

These generalized tilting modules also yield derived equivalences between A and B, where B=EndA(T).

Rickard (1989) extended the results on derived equivalence by proving that two finite-dimensional algebras R and S are derived equivalent if and only if S is the endomorphism algebra of a "tilting complex" over R. Tilting complexes are generalizations of generalized tilting modules. A version of this theorem is valid for arbitrary rings R and S.

Happel, Reiten, Smalø (1996) defined tilting objects in hereditary abelian categories in which all Hom- and Ext-spaces are finite-dimensional over some algebraically closed field k. The endomorphism algebras of these tilting objects are the quasi-tilted algebras, a generalization of tilted algebras. The quasi-tilted algebras over k are precisely the finite-dimensional algebras over k of global dimension ≤ 2 such that every indecomposable module either has projective dimension ≤ 1 or injective dimension ≤ 1. Happel (2001) classified the hereditary abelian categories that can appear in the above construction.

Colpi & Fuller (2007) defined tilting objects T in an arbitrary abelian category C; their definition requires that C contain the direct sums of arbitrary (possibly infinite) numbers of copies of T, so this is not a direct generalization of the finite-dimensional situation considered above. Given such a tilting object with endomorphism ring R, they establish tilting functors that provide equivalences between a torsion pair in C and a torsion pair in R-Mod, the category of all R-modules.

From the theory of cluster algebras came the definition of cluster category and cluster tilted algebra associated to a hereditary algebra A. A cluster tilted algebra arises from a tilted algebra as a certain semidirect product, and the cluster category of A summarizes all the module categories of cluster tilted algebras arising from A.