Computing Isotropy in Grothendieck...

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Motivation and Overview Introducing Higher-Order Isotropy (Sequential) Colimits and Isotropy Profunctors and Isotropy Computing Isotropy in Grothendieck Toposes Sakif Khan University of Ottawa [email protected] August 12, 2016 Sakif Khan CT2016

Transcript of Computing Isotropy in Grothendieck...

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Computing Isotropy in Grothendieck Toposes

Sakif Khan

University of Ottawa

[email protected]

August 12, 2016

Sakif Khan CT2016

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Structure of Talk

1 Motivation and OverviewBrief Review of IsotropyFurther Motivation

2 Introducing Higher-Order Isotropy

3 (Sequential) Colimits and Isotropy

4 Profunctors and Isotropy

Sakif Khan CT2016

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Brief Review of IsotropyFurther Motivation

Two Facets of Grothendieck Toposes

A (Grothendieck) topos E possesses simultaneously two kinds ofproperties: spatial and algebraic. We can deduce a few analogiesbetween these two aspects. Suppose X is an object in E .

Spatial Algebraic

Can take the class of subobjects of X Can take the slice E/XCollection of subobjects forms a poset Collection of automorphisms of E/X → E forms a groupGet a presheaf SubE(−) : Eop → Pos Get a presheaf Z(−) : Eop → Grp

SubE(−) represented by the subobject classifier Ω Z(−) represented by the isotropy group ZΩ is a locale internal to E Z is a group internal to E

Succinctly, the isotropy group of E encodes algebraic informationin much the same way that Ω encodes spatial information.

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Brief Review of IsotropyFurther Motivation

Isotropy Group for Presheaf Toposes

Every Grothendieck topos has an isotropy group associated toit.

In particular, SetCop

contains such a group (for a smallcategory C) Z . In fact Z : Cop → Grp is the functor

Z (C ) = automorphisms of C/C → C

(see [FHS12]).

Explicitly, an element of Z (C ) is a family of automorphismsα : A→ AA∈C coherently lifting down to each other

A A

B B

α

α|f

f f

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Brief Review of IsotropyFurther Motivation

Isotropy Group for Presheaf Toposes II

An easy example of this last point is given by a groupoid: Supposewe take objects A and B in a small groupoid G , an automorphismα : A→ A and a morphism f : B → A. Then we can always lift αalong f to an automorphism of B as in the diagram

A A

B B

α

f

f −1αf

f

by simple conjugation. For conjugation, it is clear that(α|f )|g = α|fg .

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Brief Review of IsotropyFurther Motivation

Isotropy Group for Presheaf Toposes III

The maps in the image of Z (C )→ HomC(C ,C ) are calledisotropy maps on C.

We can take the collection of all isotropy maps I in C.

Induces an obvious quotient C C/I. This is the isotropyquotient of C, whose job is to trivialise the maps in I.

But can C/I itself have non-trivial isotropy maps? Yes!

How about iterated quotients C/In? Also yes. Leads to thenotion of higher-order isotropy (for small categories).

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Brief Review of IsotropyFurther Motivation

Further Motivation

How can we actually tell when a (presheaf topos on a)category possesses higher-order isotropy?

Turning this question around, how might we build a categorywith a desired isotropy rank of some (ordinal) order?

Questions of isotropy rank are also questions about isotropygroups of categories.

Answering the first question answers: how do we computeisotropy groups of categories?

Answering the second question answers: how do we buildcategories with desired isotropy groups? – much the same wayEilenberg-MacLane spaces or Moore spaces are constructed tohave certain homotopy/homology groups.

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Higher-Order Isotropy

Let us make precise the notion of higher-order isotropy.

Definition

Note that for a small category C, we can keep taking isotropyquotients to get a sequence

C C/I C/I2 · · ·

which eventually stabilizes (for simple cardinality reasons) andwhere, for a limit ordinal µ, C/Iµ is the colimit lim←−

α<µ

C/Iα. We say

that C has λth-order isotropy if the chain stabilizes at stage λ.

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

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Isotropy Rank I

How does the isotropy of the category relate to automorphisms init?

Definition

Let C be a small category and ϕ : C → C an automorphism in C.The isotropy rank of ϕ, denoted ||ϕ||C, is defined by

||ϕ||C =

0 if ϕ = 1C∧λ | πλI(ϕ) = 1C if ∃λ > 0 such that πλI(ϕ) = 1C

−∞ otherwise.

Isotropy rank just says at which point in the isotropy chain ϕ getstrivialised.

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Isotropy Rank II

Lemma

The isotropy rank of the category C is the supremum

||C||I :=∨||ϕ||C | ϕ is an automorphism in C ≥ 0.

We also obtain a corresponding notion of preservation.

Definition

A functor F : C→ D preserves isotropy up to rank λ incase ||F (ϕ)||D = ||ϕ||C for all automorphisms ϕ ∈ Mor(C)with ||ϕ||C ≤ λ.

If we also have that ||C||I ≤ ||D||I and F preserves isotropyup to rank ||C||I , then F is said to simply preserve isotropyranks

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Profunctors and Isotropy

Isotropy Rank III

Put differently, a functor which preserves isotropy up to rank λ isone which can be lifted along isotropy quotient maps as indicatedin the diagram

C D

C/I D/I

......

C/Iλ D/Iλ

F

π1I π1

IF/I2

π2I π2

I

F/Iλ

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Building Models of Higher-Order Isotropy

Question

For a given ordinal λ, how can we build a category which hasλth-order isotropy?

Rough Answer

For finite-order isotropy, repeatedly take the collage of certainsimple profunctors. These fit together into a “nice” sequentialdiagram, the colimit of which gives ωth-order isotropy. Repeat forhigher successor and limit ordinals.

So, we need to develop some technology for manipulating/building(isotropy) automorphisms in small categories.

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

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Isotropy-preserving Functors

We will first look at how isotropy interacts with colimits. Butbefore that, a

Definition

A functor C→ D is isotropy-preserving if there is an inducedfunctor on isotropy quotients

C D

C/I D/I

where the vertical arrows are the canonical isotropy quotientfunctors. Moreover, it can be proven that there is at most onehorizontal filler making the square commute.

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Isotropy-preserving Functors II

The notion of isotropy-preservation is much coarser than thatof preserving isotropy up to specified isotropy rank.

Indeed, isotropy-preservation is just preserving isotropy up torank 1.

Let λ be the chain category of ordinals less than λ. Given asequential diagram F : λ→ Cat of categories and ordinalsα ≤ β < λ, we denote by F βα : F (α)→ F (β) the imageF (α ≤ β).

We say that the functor F βα : F (α)→ F (β) is a transition

map if F βα is full and injective on morphisms.

In particular, inclusions of categories are transition maps.

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Sequential colimits I

We are finally ready to state the

Theorem

Sequential colimits under diagrams with isotropy-preservingtransition maps commute with isotropy quotients.

and we obtain a

Corollary

Sequential colimits under diagrams with isotropy-preservinginclusions commute with isotropy quotients.

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Sequential colimits II

So, our corollary says that, under appropriate conditions, we have apicture

C0 C1 C2 · · · C

C0/I C1/I C2/I · · · C/I

where

all vertical arrows are isotropy quotient functors;

the inclusions in the bottom row are induced by theisotropy-preserving property of inclusions in the top row.

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Question

Can we propagate this diagram vertically so that higher quotientsof C are also computed as colimits of higher quotients?

Answer

In general, no. But recall that if isotropy ranks are preserved by afunctor, then we obtain a vertical tower of lifts for that functor.

If our inclusions have this additional property, then indeed,

C0 C1 C2 · · · C

C0/I C1/I C2/I · · · C/I

......

......

...

C0/Iλ C1/Iλ C2/Iλ · · · C/IλSakif Khan CT2016

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Towards Profunctors

Our task of building models of higher-order isotropy has thus beenreduced to constructing a sequential diagram of categories withcertain nice properties – namely, each arrow in the sequence is aninclusion functor which preserves isotropy ranks. One way to buildsuch a sequence is to repeatedly take the collage of certain simplecategories. Let us recall a few basic facts.

Definition

A profunctor H : C 9 D is just a functor Dop × C→ Set. Itcorresponds to a two-sided codiscrete cofibration called the collage

C D

K

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Facts about Profunctors

A profunctor H is corepresentable ifH(d , c) = HomD(d ,F (c)) (and correspondingly on arrows) forsome functor F : C→ D.

The data of a codiscrete cofibration is precisely the data of afunctor K → ∆[1] such that the fibre over 0 is D and over 1 isC. Here, ∆[1] = 0→ 1.

We haveProf Cat/∆[1]

Profcorep Fib/∆[1]

'

'

where Fib indicates fibrations in the ordinary sense.

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Isotropy in Collages

We think of C as being “stacked on top” of D via the (corep)profunctor and the (hetero)morphisms from D to C in K as“connecting” objects in D to those in C.

Morphisms in C and D have isotropy ranks independently ofthe profunctor.

Forcing these morphisms to interact via a profunctor gives acategory – the collage – where isotropy ranks are dependenton both isotropy ranks in C and D.

How do we compute isotropy ranks in K given that we knowisotropy ranks in C and D? – Bit like computing fundamentalgroup of a space obtained by joining together two spaceswhose fundamental groups we already know.

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

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Isotropy in Collages II

The following result makes this intuition precise.

Theorem

Given an object c ∈ C, we have a pullback square

ZK (c) ZD(Fc)

ZC(c) AutC(c)

Ψc

Φc φc

ψc

in Grp.

The two projection maps are somewhat technical to define and weskip it for brevity.

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Lastly, we can relate isotropy quotients of C and D to that of K .

Proposition

The following are equivalent.

Each Φc in the pullback square

ZK (c) ZD(Fc)

ZC(c) X (c)

Ψc

Φc φc

ψc

is surjective.

The isotropy quotient C/I coincides with C/ ∼.

The isotropy quotient K/I is the collage of the functorF/I : C/I → D/I.

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

Profunctors and Isotropy

Thank You

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Motivation and OverviewIntroducing Higher-Order Isotropy(Sequential) Colimits and Isotropy

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References

Jean Benabou.

Distributors at work, June 2000.

Available athttp://www.mathematik.tu-darmstadt.de/ streicher/FIBR/DiWo.pdf.

Jonathan Funk, Pieter Hofstra, and Benjamin Steinberg.

Isotropy and crossed toposes.

Theory and Applications of Categories, 26(24):660–709, November 2012.

Available athttp://www.emis.de/journals/TAC/volumes/26/24/26-24abs.html.

Saunders MacLane and Ieke Moerdijk.

Sheaves in Geometry and Logic: A First Introduction to Topos Theory.

Universitext. Springer, October 1994.

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