Gerbe

Summary

In mathematics, a gerbe (/ɜːrb/; French: [ʒɛʁb]) is a construct in homological algebra and topology. Gerbes were introduced by Jean Giraud (Giraud 1971) following ideas of Alexandre Grothendieck as a tool for non-commutative cohomology in degree 2. They can be seen as an analogue of fibre bundles where the fibre is the classifying stack of a group. Gerbes provide a convenient, if highly abstract, language for dealing with many types of deformation questions especially in modern algebraic geometry. In addition, special cases of gerbes have been used more recently in differential topology and differential geometry to give alternative descriptions to certain cohomology classes and additional structures attached to them.

"Gerbe" is a French (and archaic English) word that literally means wheat sheaf.

Definitions edit

Gerbes on a topological space edit

A gerbe on a topological space  [1]: 318  is a stack   of groupoids over   that is locally non-empty (each point   has an open neighbourhood   over which the section category   of the gerbe is not empty) and transitive (for any two objects   and   of   for any open set  , there is an open covering   of   such that the restrictions of   and   to each   are connected by at least one morphism).

A canonical example is the gerbe   of principal bundles with a fixed structure group  : the section category over an open set   is the category of principal  -bundles on   with isomorphism as morphisms (thus the category is a groupoid). As principal bundles glue together (satisfy the descent condition), these groupoids form a stack. The trivial bundle   shows that the local non-emptiness condition is satisfied, and finally as principal bundles are locally trivial, they become isomorphic when restricted to sufficiently small open sets; thus the transitivity condition is satisfied as well.

Gerbes on a site edit

The most general definition of gerbes are defined over a site. Given a site   a  -gerbe  [2][3]: 129  is a category fibered in groupoids   such that

  1. There exists a refinement[4]   of   such that for every object   the associated fibered category   is non-empty
  2. For every   any two objects in the fibered category   are locally isomorphic

Note that for a site   with a final object  , a category fibered in groupoids   is a  -gerbe admits a local section, meaning satisfies the first axiom, if  .

Motivation for gerbes on a site edit

One of the main motivations for considering gerbes on a site is to consider the following naive question: if the Cech cohomology group   for a suitable covering   of a space   gives the isomorphism classes of principal  -bundles over  , what does the iterated cohomology functor   represent? Meaning, we are gluing together the groups   via some one cocycle. Gerbes are a technical response for this question: they give geometric representations of elements in the higher cohomology group  . It is expected this intuition should hold for higher gerbes.

Cohomological classification edit

One of the main theorems concerning gerbes is their cohomological classification whenever they have automorphism groups given by a fixed sheaf of abelian groups  ,[5][2] called a band. For a gerbe   on a site  , an object  , and an object  , the automorphism group of a gerbe is defined as the automorphism group  . Notice this is well defined whenever the automorphism group is always the same. Given a covering  , there is an associated class

 

representing the isomorphism class of the gerbe   banded by  . For example, in topology, many examples of gerbes can be constructed by considering gerbes banded by the group  . As the classifying space   is the second Eilenberg–Maclane space for the integers, a bundle gerbe banded by   on a topological space   is constructed from a homotopy class of maps in

 ,

which is exactly the third singular homology group  . It has been found[6] that all gerbes representing torsion cohomology classes in   are represented by a bundle of finite dimensional algebras   for a fixed complex vector space  . In addition, the non-torsion classes are represented as infinite-dimensional principal bundles   of the projective group of unitary operators on a fixed infinite dimensional separable Hilbert space  . Note this is well defined because all separable Hilbert spaces are isomorphic to the space of square-summable sequences  . The homotopy-theoretic interpretation of gerbes comes from looking at the homotopy fiber square

 

analogous to how a line bundle comes from the homotopy fiber square

 

where  , giving   as the group of isomorphism classes of line bundles on  .

Examples edit

C*-algebras edit

There are natural examples of Gerbes that arise from studying the algebra of compactly supported complex valued functions on a paracompact space  [7]pg 3. Given a cover   of   there is the Cech groupoid defined as

 

with source and target maps given by the inclusions

 

and the space of composable arrows is just

 

Then a degree 2 cohomology class   is just a map

 

We can then form a non-commutative C*-algebra  , which is associated to the set of compact supported complex valued functions of the space

 

It has a non-commutative product given by

 

where the cohomology class   twists the multiplication of the standard  -algebra product.

Algebraic geometry edit

Let   be a variety over an algebraically closed field  ,   an algebraic group, for example  . Recall that a G-torsor over   is an algebraic space   with an action of   and a map  , such that locally on   (in étale topology or fppf topology)   is a direct product  . A G-gerbe over M may be defined in a similar way. It is an Artin stack   with a map  , such that locally on M (in étale or fppf topology)   is a direct product  .[8] Here   denotes the classifying stack of  , i.e. a quotient   of a point by a trivial  -action. There is no need to impose the compatibility with the group structure in that case since it is covered by the definition of a stack. The underlying topological spaces of   and   are the same, but in   each point is equipped with a stabilizer group isomorphic to  .

From two-term complexes of coherent sheaves edit

Every two-term complex of coherent sheaves

 

on a scheme   has a canonical sheaf of groupoids associated to it, where on an open subset   there is a two-term complex of  -modules

 

giving a groupoid. It has objects given by elements   and a morphism   is given by an element   such that

 

In order for this stack to be a gerbe, the cohomology sheaf   must always have a section. This hypothesis implies the category constructed above always has objects. Note this can be applied to the situation of comodules over Hopf-algebroids to construct algebraic models of gerbes over affine or projective stacks (projectivity if a graded Hopf-algebroid is used). In addition, two-term spectra from the stabilization of the derived category of comodules of Hopf-algebroids   with   flat over   give additional models of gerbes that are non-strict.

Moduli stack of stable bundles on a curve edit

Consider a smooth projective curve   over   of genus  . Let   be the moduli stack of stable vector bundles on   of rank   and degree  . It has a coarse moduli space  , which is a quasiprojective variety. These two moduli problems parametrize the same objects, but the stacky version remembers automorphisms of vector bundles. For any stable vector bundle   the automorphism group   consists only of scalar multiplications, so each point in a moduli stack has a stabilizer isomorphic to  . It turns out that the map   is indeed a  -gerbe in the sense above.[9] It is a trivial gerbe if and only if   and   are coprime.

Root stacks edit

Another class of gerbes can be found using the construction of root stacks. Informally, the  -th root stack of a line bundle   over a scheme is a space representing the  -th root of   and is denoted

 [10]pg 52

The  -th root stack of   has the property

 

as gerbes. It is constructed as the stack

 

sending an  -scheme   to the category whose objects are line bundles of the form

 

and morphisms are commutative diagrams compatible with the isomorphisms  . This gerbe is banded by the algebraic group of roots of unity  , where on a cover   it acts on a point   by cyclically permuting the factors of   in  . Geometrically, these stacks are formed as the fiber product of stacks

 

where the vertical map of   comes from the Kummer sequence

 

This is because   is the moduli space of line bundles, so the line bundle   corresponds to an object of the category   (considered as a point of the moduli space).

Root stacks with sections edit

There is another related construction of root stacks with sections. Given the data above, let   be a section. Then the  -th root stack of the pair   is defined as the lax 2-functor[10][11]

 

sending an  -scheme   to the category whose objects are line bundles of the form

 

and morphisms are given similarly. These stacks can be constructed very explicitly, and are well understood for affine schemes. In fact, these form the affine models for root stacks with sections.[11]: 4  Given an affine scheme  , all line bundles are trivial, hence   and any section   is equivalent to taking an element  . Then, the stack is given by the stack quotient

 [11]: 9 

with

 

If   then this gives an infinitesimal extension of  .

Examples throughout algebraic geometry edit

These and more general kinds of gerbes arise in several contexts as both geometric spaces and as formal bookkeeping tools:

Differential geometry edit

  •   and  -gerbes: Jean-Luc Brylinski's approach

History edit

Gerbes first appeared in the context of algebraic geometry. They were subsequently developed in a more traditional geometric framework by Brylinski (Brylinski 1993). One can think of gerbes as being a natural step in a hierarchy of mathematical objects providing geometric realizations of integral cohomology classes.

A more specialised notion of gerbe was introduced by Murray and called bundle gerbes. Essentially they are a smooth version of abelian gerbes belonging more to the hierarchy starting with principal bundles than sheaves. Bundle gerbes have been used in gauge theory and also string theory. Current work by others is developing a theory of non-abelian bundle gerbes.

See also edit

References edit

  1. ^ Basic bundle theory and K-cohomology invariants. Husemöller, Dale. Berlin: Springer. 2008. ISBN 978-3-540-74956-1. OCLC 233973513.{{cite book}}: CS1 maint: others (link)
  2. ^ a b "Section 8.11 (06NY): Gerbes—The Stacks project". stacks.math.columbia.edu. Retrieved 2020-10-27.
  3. ^ Giraud, J. (Jean) (1971). Cohomologie non abélienne. Berlin: Springer-Verlag. ISBN 3-540-05307-7. OCLC 186709.
  4. ^ "Section 7.8 (00VS): Families of morphisms with fixed target—The Stacks project". stacks.math.columbia.edu. Retrieved 2020-10-27.
  5. ^ "Section 21.11 (0CJZ): Second cohomology and gerbes—The Stacks project". stacks.math.columbia.edu. Retrieved 2020-10-27.
  6. ^ Karoubi, Max (2010-12-12). "Twisted bundles and twisted K-theory". arXiv:1012.2512 [math.KT].
  7. ^ Block, Jonathan; Daenzer, Calder (2009-01-09). "Mukai duality for gerbes with connection". arXiv:0803.1529 [math.QA].
  8. ^ Edidin, Dan; Hassett, Brendan; Kresch, Andrew; Vistoli, Angelo (2001). "Brauer groups and quotient stacks". American Journal of Mathematics. 123 (4): 761–777. arXiv:math/9905049. doi:10.1353/ajm.2001.0024. S2CID 16541492.
  9. ^ Hoffman, Norbert (2010). "Moduli stacks of vector bundles on curves and the King–Schofield rationality proof". Cohomological and Geometric Approaches to Rationality Problems: 133–148. arXiv:math/0511660. doi:10.1007/978-0-8176-4934-0_5. ISBN 978-0-8176-4933-3. S2CID 5467668.
  10. ^ a b Abramovich, Dan; Graber, Tom; Vistoli, Angelo (2008-04-13). "Gromov-Witten theory of Deligne-Mumford stacks". arXiv:math/0603151.
  11. ^ a b c Cadman, Charles (2007). "Using stacks to impose tangency conditions on curves" (PDF). Amer. J. Math. 129 (2): 405–427. arXiv:math/0312349. doi:10.1353/ajm.2007.0007. S2CID 10323243.

External links edit

Introductory articles edit

  • Constructions with Bundle Gerbes - Stuart Johnson
  • An Introduction to Gerbes on Orbifolds, Ernesto Lupercio, Bernado Uribe.
  • What is a Gerbe?, by Nigel Hitchin in Notices of the AMS
  • Bundle gerbes, Michael Murray.
  • Moerdijk, Ieke. "Introduction to the Language of Stacks and Gerbes". Retrieved 2007-05-20.

Gerbes in topology edit

  • Homotopy theory of presheaves of simplicial groupoids, Zhi-Ming Luo

Twisted K-theory edit

  • Twisted K-theory and K-theory of bundle gerbes
  • Twisted Bundles and Twisted K-Theory - Karoubi

Applications in string theory edit

  • Stable Singularities in String Theory - contains examples of gerbes in appendix using the Brauer group
  • Branes on Group Manifolds, Gluon Condensates, and twisted K-theory
  • Lectures on Special Lagrangian Submanifolds - Very down-to earth introduction with applications to Mirror symmetry
  • The basic gerbe over a compact simple Lie group - Gives techniques for describing groups such as the String group as a gerbe