# Tannakian formalism

__: Monoidal category__

**Short description**In mathematics, a **Tannakian category** is a particular kind of monoidal category *C*, equipped with some extra structure relative to a given field *K*. The role of such categories *C* is to generalise the category of linear representations of an algebraic group *G* defined over *K*. A number of major applications of the theory have been made, or might be made in pursuit of some of the central conjectures of contemporary algebraic geometry and number theory.

The name is taken from Tadao Tannaka and Tannaka–Krein duality, a theory about compact groups *G* and their representation theory. The theory was developed first in the school of Alexander Grothendieck. It was later reconsidered by Pierre Deligne, and some simplifications made. The pattern of the theory is that of Grothendieck's Galois theory, which is a theory about finite permutation representations of groups *G* which are profinite groups.

The gist of the theory is that the fiber functor Φ of the Galois theory is replaced by an exact and faithful **tensor functor** *F* from *C* to the category of finite-dimensional vector spaces over *K*. The group of natural transformations of Φ to itself, which turns out to be a profinite group in the Galois theory, is replaced by the group *G* of natural transformations of *F* into itself, that respect the tensor structure. This is in general not an algebraic group but a more general group scheme that is an inverse limit of algebraic groups (pro-algebraic group), and *C* is then found to be equivalent to the category of finite-dimensional linear representations of *G*.

More generally, it may be that fiber functors *F* as above only exists to categories of finite dimensional vector spaces over non-trivial extension fields *L/K*.
In such cases the group scheme *G* is replaced by a gerbe [math]\displaystyle{ \mathcal G }[/math] on the fpqc site of Spec(*K*), and *C* is then equivalent to the category of (finite-dimensional) representations of [math]\displaystyle{ \mathcal G }[/math].

## Formal definition of Tannakian categories

Let *K* be a field and *C* a *K*-linear abelian rigid tensor (i.e., a symmetric monoidal) category such that [math]\displaystyle{ \mathrm{End}(\mathbf{1})\cong K }[/math]. Then *C* is a **Tannakian category** (over *K*) if there is an extension field *L* of *K* such that there exists a *K*-linear exact and faithful tensor functor (i.e., a strong monoidal functor) *F* from *C* to the category of finite dimensional *L*-vector spaces.
A Tannakian category over *K* is **neutral** if such exact faithful tensor functor *F* exists with *L=K*.^{[1]}

## Applications

The construction is used in cases where a Hodge structure or l-adic representation is to be considered in the light of group representation theory. For example, the Mumford–Tate group and motivic Galois group are potentially to be recovered from one cohomology group or Galois module, by means of a mediating Tannakian category it generates.

Those areas of application are closely connected to the theory of motives. Another place in which Tannakian categories have been used is in connection with the Grothendieck–Katz p-curvature conjecture; in other words, in bounding monodromy groups.

The Geometric Satake equivalence establishes an equivalence between representations of the Langlands dual group [math]\displaystyle{ {}^L G }[/math] of a reductive group *G* and certain equivariant perverse sheaves on the affine Grassmannian associated to *G*. This equivalence provides a non-combinatorial construction of the Langlands dual group. It is proved by showing that the mentioned category of perverse sheaves is a Tannakian category and identifying its Tannaka dual group with [math]\displaystyle{ {}^L G }[/math].

## Extensions

(Wedhorn 2004) has established partial Tannaka duality results in the situation where the category is *R*-linear, where *R* is no longer a field (as in classical Tannakian duality), but certain valuation rings.

## References

- ↑ (Saavedra Rivano 1972)

- Deligne, Pierre (2007), "Catégories tannakiennes",
*The Grothendieck Festschrift*,**II**, Birkhauser, pp. 111–195, ISBN 9780817645755, https://books.google.com/books?id=GCK3eIC97y4C&pg=PA111 - Deligne, Pierre; Milne, James (1982), "Tannakian categories", in Deligne, Pierre; Milne, James; Ogus, Arthur et al.,
*Hodge Cycles, Motives, and Shimura Varieties*, Lecture Notes in Mathematics,**900**, Springer, pp. 101–228, ISBN 978-3-540-38955-2, http://www.jmilne.org/math/xnotes/tc.html - Saavedra Rivano, Neantro (1972),
*Catégories Tannakiennes*, Lecture Notes in Mathematics,**265**, Springer, ISBN 978-3-540-37477-0, https://www.springer.com/gp/book/9783540058441 - Wedhorn, Torsten (2004), "On Tannakian duality over valuation rings",
*Journal of Algebra***282**(2): 575-609, doi:10.1016/j.jalgebra.2004.07.024

## Further reading

- M. Larsen and R. Pink. Determining representations from invariant dimensions. Invent. math., 102:377–389, 1990.

Original source: https://en.wikipedia.org/wiki/Tannakian formalism.
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