Redirected from "Isbell adjunction".
Contents
Context
Higher algebra
Higher geometry
Duality
higher geometry Isbell duality higher algebra
Contents
Idea
A general abstract adjunction
relates (higher) presheaves with (higher) copresheaves on a given (higher) category : this is called Isbell conjugation or Isbell duality (after John Isbell).
To the extent that this adjunction descends to presheaves that are (higher) sheaves and copresheaves that are (higher) algebras this duality relates higher geometry with higher algebra.
Objects preserved by the monad of this adjunction are called Isbell self-dual.
Under the interpretation of presheaves as generalized spaces and copresheaves as generalized quantities modeled on (Lawvere 86, see at space and quantity), Isbell duality is the archetype of the duality between geometry and algebra that permeates mathematics (such as Gelfand duality, Stone duality, or the embedding of smooth manifolds into formal duals of R-algebras).
Definition
Let be a good enriching category (a cosmos, i.e. a complete and cocomplete closed symmetric monoidal category).
Let be a small -enriched category.
Write and for the enriched functor categories.
Proposition
There is a -adjunction
where
and
The proof is mostly a tautology after the notation is unwound. The mechanism of the proof may still be of interest and be relevant for generalizations and for less tautological variations of the setup. We therefore spell out several proofs.
Proof A
Use the end-expression for the hom-objects of the enriched functor categories to compute
The following proof does not use ends and needs instead slightly more preparation, but has then the advantage that its structure goes through also in great generality in higher category theory.
Proof B
Notice that
Lemma 1:
because we have a natural isomorphism
by the Yoneda lemma.
From this we get
Lemma 2:
by the sequence of natural isomorphisms
where the first is Lemma 1 and the second the Yoneda lemma.
Since (by what is sometimes called the co-Yoneda lemma) every object may be written as a colimit
over representables we have
In terms of the same diagram of representables it then follows that
Lemma 3:
because using the above colimit representation and the Yoneda lemma we have natural isomorphisms
Using all this we can finally obtain the adjunction in question by the following sequence of natural isomorphisms
The pattern of this proof has the advantage that it goes through in great generality also on higher category theory without reference to a higher notion of enriched category theory.
Example
In the simplest case, namely for an ordinary category , the adjunction between presheaves and copresheaves arises as follows.
The category of presheaves is the free cocompletion of . This means that any functor
to a cocomplete category extends along the Yoneda embedding to a cocontinuous functor
in a manner unique up to natural isomorphism.
Dually, the category of copresheaves is the free completion of . This means that any functor
to a complete category extends along the co-Yoneda embedding to a continuous functor
in a manner unique up to natural isomorphism.
We can apply these ideas to get the functors involved in Isbell duality. The presheaf category has all limits, so we can extend the Yoneda embedding to a continuous functor
from copresheaves to presheaves. Dually, the copresheaf category has all colimits, so we can extend the co-Yoneda embedding to a cocontinuous functor
from presheaves to copresheaves.
Isbell duality says that these are adjoint functors: is right adjoint to .
Properties
Relation to Yoneda embedding
is the left Kan extension of the Yoneda embedding along the contravariant Yoneda embedding, while is the left Kan extension of the contravariant Yoneda embedding along the Yoneda embedding.
The codensity monad of the Yoneda embedding is isomorphic to the monad induced by the Isbell adjunction, (Kock 66, Theorem 4.1 and Di Liberti 19, Theorem 2.7).
Respect for limits
Choose any class of limits in and write for the full subcategory consisting of those functors preserving these limits.
Proposition
The -adjunction does descend to this inclusion, in that we have an adjunction
Proof
Because the hom-functors preserves all limits:
Isbell self-dual objects
Definition
An object or is Isbell-self-dual if
-
is an isomorphism in ;
-
is an isomorphism in , respectively.
Proof
By Proof B , lemma 1 we have a natural isomorphisms in
Therefore we have also the natural isomorphism
where the second step is the Yoneda lemma. Similarly the other way round.
Isbell envelope
See Isbell envelope.
Reflexive completion
See reflexive completion.
Examples and similar dualities
Isbell duality is a template for many other space/algebra-dualities in mathematics.
Function -Algebras on presheaves
Let be any cartesian closed category.
Let be the syntactic category of a -enriched Lawvere theory, that is a -category with finite products such that all objects are generated under products from a single object .
Then write for category of product-preserving functors: the category of -algebras. This comes with the canonical forgetful functor
Write
for the Yoneda embedding.
Definition
Call
the -line object.
Observation
For all we have
In particular
Proof
We have isomorphisms natural in
by the above adjunction and then by the Yoneda lemma.
All this generalizes to the following case:
instead of setting let more generally
be a small full subcategory of -algebras, containing all the free -algebras.
Then the original construction of no longer makes sense, but that in terms of the line object still does
Proposition
Set
and
Then we still have an adjunction
Proof
The first step that is not a definition is the Yoneda lemma. The step after that is the symmetric-closed-monoidal structure of .
Function -algebras on derived -stacks
The structure of our Proof B above goes through in higher category theory.
Formulated in terms of derived stacks over the (∞,1)-category of dg-algebras, this is essentially the argument appearing on page 23 of (Ben-ZviNadler).
Function -algebras on -stacks
for the moment see at function algebras on ∞-stacks.
Function 2-algebras on algebraic stacks
see Tannaka duality for geometric stacks
Gelfand duality
Gelfand duality is the equivalence of categories between (nonunital) commutative C*-algebras and (locally) compact topological spaces. See there for more details.
Serre-Swan theorem
The Serre-Swan theorem says that suitable modules over an commutative C*-algebra are equivalently modules of sections of vector bundles over the Gelfand-dual topological space.
duality between algebra and geometry
geometry | category | dual category | algebra |
---|
topology | | | commutative algebra |
topology | | | comm. C-star-algebra |
noncomm. topology | | | general C-star-algebra |
algebraic geometry | | | commutative ring |
noncomm. algebraic geometry | | | fin. gen. associative algebra |
differential geometry | | | commutative algebra |
supergeometry | | | supercommutative superalgebra |
formal higher supergeometry (super Lie theory) | | | differential graded-commutative superalgebra (“FDAs”) |
in physics:
References
The original articles on Isbell duality and the Isbell envelope are
-
John Isbell, Structure of categories, Bulletin of the American Mathematical Society 72 (1966), 619-655. [euclid:1183528163, ams:1966-72-04/S0002-9904-1966-11541-0]
-
John Isbell, Normal completions of categories, Reports of the Midwest Category Seminar, 47, Springer (1967) 110-155 [doi:10.1007/BFb0074302]
-
Anders Kock. Continuous Yoneda representation of a small category. University of Aarhus, Denmark, 1966. (pdf)
More recent discussion:
-
William Lawvere, p. 17 of Taking categories seriously, Revista Colombiana de Matematicas, XX (1986) 147-178, reprinted as: Reprints in Theory and Applications of Categories, No. 8 (2005) pp. 1-24 (web)
-
Michael Barr, John Kennison, Robert Raphael, Isbell Duality Theory and Applications of Categories 20 15 (2008) 504-542 [tac:20-15]
-
Michael Barr, John Kennison, Robert Raphael, Isbell duality for for modules, Theory and Applications of Categories 22 17 (2009) 401-419 [tac:22-17, pdf]
-
Richard Garner, The Isbell monad, Advances in Mathematics 274 (2015) pp.516-537. (draft)
-
Vaughan Pratt, Communes via Yoneda, from an elementary perspective, Fundamenta Informaticae 103 (2010), 203–218.
-
Ivan Di Liberti, Fosco Loregian, On the Unicity of Formal Category Theories, arXiv:1901.01594 (2019). (abstract)
-
Ivan Di Liberti, Codensity: Isbell duality, pro-objects, compactness and accessibility, (arXiv:1910.01014)
-
Tom Avery, Tom Leinster. Isbell conjugacy and the reflexive completion. Theory and Applications of Categories, 36 12 (2021) 306-347 [tac:36-12, pdf]
(relation to reflexive completion)
-
John Baez, Isbell duality, Notices Amer. Math. Soc. 70 (2022) 140-141 [doi:10.1090/noti2602, pdf]
Isbell conjugacy for (∞,1)-presheaves over the (∞,1)-category of duals of dg-algebras is discussed around page 32 of
in
Isbell self-dual ∞-stacks over duals of commutative associative algebras are called affine stacks. They are characterized as those objects that are small in a sense and local with respect to the cohomology with coefficients in the canonical line object.
A generalization of this latter to -stacks over duals of algebras over arbitrary abelian Lawvere theories is the content of
- Herman Stel, -Stacks and their function algebras – with applications to -Lie theory, master thesis (2010) (web)
See also