aspects of duality in 2-categories · aspects of duality in 2-categories bruce bartlett phd...
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![Page 1: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/1.jpg)
Aspects of duality in 2-categories
Bruce Bartlett
PhD student, supervisor Simon WillertonSheffield University
Categories, Logic and Foundations of PhysicsImperial College14th May 2008
![Page 2: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/2.jpg)
Introduction
Basic Principle (Baez, Dolan, Coecke, Abramsky, ...)
Spacetime and quantum theory make more sense when expressed in theirnatural language — not the language of sets and functions, but thelanguage of (higher) categories with duals.
Category theory allows you to work on structures without the need first topulverize them into set theoretic dust. To give an example from the fieldof architecture, when studying the Notre Dame cathedral in Paris, you tryto understand how the building relates to other cathedrals of the day, andthen to earlier and later cathedrals, and other kinds of ecclesiasticalbuilding. What you don’t do is begin by imagining it reduced to a pile ofmineral fragments. — David Corfield
When in Rome, do as the Romans do. — St Augustine
![Page 3: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/3.jpg)
Introduction
Basic Principle (Baez, Dolan, Coecke, Abramsky, ...)
Spacetime and quantum theory make more sense when expressed in theirnatural language — not the language of sets and functions, but thelanguage of (higher) categories with duals.
Category theory allows you to work on structures without the need first topulverize them into set theoretic dust. To give an example from the fieldof architecture, when studying the Notre Dame cathedral in Paris, you tryto understand how the building relates to other cathedrals of the day, andthen to earlier and later cathedrals, and other kinds of ecclesiasticalbuilding. What you don’t do is begin by imagining it reduced to a pile ofmineral fragments. — David Corfield
When in Rome, do as the Romans do. — St Augustine
![Page 4: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/4.jpg)
Introduction
Basic Principle (Baez, Dolan, Coecke, Abramsky, ...)
Spacetime and quantum theory make more sense when expressed in theirnatural language — not the language of sets and functions, but thelanguage of (higher) categories with duals.
Category theory allows you to work on structures without the need first topulverize them into set theoretic dust. To give an example from the fieldof architecture, when studying the Notre Dame cathedral in Paris, you tryto understand how the building relates to other cathedrals of the day, andthen to earlier and later cathedrals, and other kinds of ecclesiasticalbuilding. What you don’t do is begin by imagining it reduced to a pile ofmineral fragments. — David Corfield
When in Rome, do as the Romans do. — St Augustine
![Page 5: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/5.jpg)
Introduction
Category theory allows you to work on structures without the need first topulverize them into set theoretic dust. To give an example from the fieldof architecture, when studying the Notre Dame cathedral in Paris, you tryto understand how the building relates to other cathedrals of the day, andthen to earlier and later cathedrals, and other kinds of ecclesiasticalbuilding. What you don’t do is begin by imagining it reduced to a pile ofmineral fragments. — David Corfield
When in Rome, do as the Romans do. — St Augustine
Umntu ngumntu ngabantu. — Nguni proverb, Southern Africa.Um︸︷︷︸
A
ntu︸︷︷︸person
ng︸︷︷︸is
um︸︷︷︸a
ntu︸︷︷︸person
ng︸︷︷︸through
aba︸︷︷︸plural
ntu︸︷︷︸person
(Related: Ubuntu — lit. ‘Person-ness’; basic human decency.)
!ke e: /karra //ke — Khoisan proverb, ‘Diverse people unite’?
Stupid is as stupid does. — Forrest Gump’s mum.
![Page 6: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/6.jpg)
Introduction
Category theory allows you to work on structures without the need first topulverize them into set theoretic dust. To give an example from the fieldof architecture, when studying the Notre Dame cathedral in Paris, you tryto understand how the building relates to other cathedrals of the day, andthen to earlier and later cathedrals, and other kinds of ecclesiasticalbuilding. What you don’t do is begin by imagining it reduced to a pile ofmineral fragments. — David Corfield
When in Rome, do as the Romans do. — St Augustine
Umntu ngumntu ngabantu. — Nguni proverb, Southern Africa.
Um︸︷︷︸A
ntu︸︷︷︸person
ng︸︷︷︸is
um︸︷︷︸a
ntu︸︷︷︸person
ng︸︷︷︸through
aba︸︷︷︸plural
ntu︸︷︷︸person
(Related: Ubuntu — lit. ‘Person-ness’; basic human decency.)
!ke e: /karra //ke — Khoisan proverb, ‘Diverse people unite’?
Stupid is as stupid does. — Forrest Gump’s mum.
![Page 7: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/7.jpg)
Introduction
Category theory allows you to work on structures without the need first topulverize them into set theoretic dust. To give an example from the fieldof architecture, when studying the Notre Dame cathedral in Paris, you tryto understand how the building relates to other cathedrals of the day, andthen to earlier and later cathedrals, and other kinds of ecclesiasticalbuilding. What you don’t do is begin by imagining it reduced to a pile ofmineral fragments. — David Corfield
When in Rome, do as the Romans do. — St Augustine
Umntu ngumntu ngabantu. — Nguni proverb, Southern Africa.Um︸︷︷︸
A
ntu︸︷︷︸person
ng︸︷︷︸is
um︸︷︷︸a
ntu︸︷︷︸person
ng︸︷︷︸through
aba︸︷︷︸plural
ntu︸︷︷︸person
(Related: Ubuntu — lit. ‘Person-ness’; basic human decency.)
!ke e: /karra //ke — Khoisan proverb, ‘Diverse people unite’?
Stupid is as stupid does. — Forrest Gump’s mum.
![Page 8: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/8.jpg)
Introduction
Category theory allows you to work on structures without the need first topulverize them into set theoretic dust. To give an example from the fieldof architecture, when studying the Notre Dame cathedral in Paris, you tryto understand how the building relates to other cathedrals of the day, andthen to earlier and later cathedrals, and other kinds of ecclesiasticalbuilding. What you don’t do is begin by imagining it reduced to a pile ofmineral fragments. — David Corfield
When in Rome, do as the Romans do. — St Augustine
Umntu ngumntu ngabantu. — Nguni proverb, Southern Africa.Um︸︷︷︸
A
ntu︸︷︷︸person
ng︸︷︷︸is
um︸︷︷︸a
ntu︸︷︷︸person
ng︸︷︷︸through
aba︸︷︷︸plural
ntu︸︷︷︸person
(Related: Ubuntu — lit. ‘Person-ness’; basic human decency.)
!ke e: /karra //ke — Khoisan proverb, ‘Diverse people unite’?
Stupid is as stupid does. — Forrest Gump’s mum.
![Page 9: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/9.jpg)
Introduction
Category theory allows you to work on structures without the need first topulverize them into set theoretic dust. To give an example from the fieldof architecture, when studying the Notre Dame cathedral in Paris, you tryto understand how the building relates to other cathedrals of the day, andthen to earlier and later cathedrals, and other kinds of ecclesiasticalbuilding. What you don’t do is begin by imagining it reduced to a pile ofmineral fragments. — David Corfield
When in Rome, do as the Romans do. — St Augustine
Umntu ngumntu ngabantu. — Nguni proverb, Southern Africa.Um︸︷︷︸
A
ntu︸︷︷︸person
ng︸︷︷︸is
um︸︷︷︸a
ntu︸︷︷︸person
ng︸︷︷︸through
aba︸︷︷︸plural
ntu︸︷︷︸person
(Related: Ubuntu — lit. ‘Person-ness’; basic human decency.)
!ke e: /karra //ke — Khoisan proverb, ‘Diverse people unite’?
Stupid is as stupid does. — Forrest Gump’s mum.
![Page 10: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/10.jpg)
Introduction
Category theory allows you to work on structures without the need first topulverize them into set theoretic dust. To give an example from the fieldof architecture, when studying the Notre Dame cathedral in Paris, you tryto understand how the building relates to other cathedrals of the day, andthen to earlier and later cathedrals, and other kinds of ecclesiasticalbuilding. What you don’t do is begin by imagining it reduced to a pile ofmineral fragments. — David Corfield
When in Rome, do as the Romans do. — St Augustine
Umntu ngumntu ngabantu. — Nguni proverb, Southern Africa.Um︸︷︷︸
A
ntu︸︷︷︸person
ng︸︷︷︸is
um︸︷︷︸a
ntu︸︷︷︸person
ng︸︷︷︸through
aba︸︷︷︸plural
ntu︸︷︷︸person
(Related: Ubuntu — lit. ‘Person-ness’; basic human decency.)
!ke e: /karra //ke — Khoisan proverb, ‘Diverse people unite’?
Stupid is as stupid does. — Forrest Gump’s mum.
![Page 11: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/11.jpg)
Introduction
nCob Hilb
objects (n − 1)-dim space H fin dim Hilbert space
morphisms n-dim spacetime
H1
A��
H2
linear map
monoidal H ⊗ H
duals forobjects
H
duals formorphisms
H2
A∗
��H1
adjoint
For instance, quantum entanglement:
6= +Cψ��
H ⊗ H
6=Cψ1��
H
+Cψ2��
H
![Page 12: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/12.jpg)
Introduction
nCob Hilb
objects (n − 1)-dim space H fin dim Hilbert space
morphisms n-dim spacetime
H1
A��
H2
linear map
monoidal H ⊗ H
duals forobjects
H
duals formorphisms
H2
A∗
��H1
adjoint
For instance, quantum entanglement:
6= +Cψ��
H ⊗ H
6=Cψ1��
H
+Cψ2��
H
![Page 13: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/13.jpg)
Introduction: Reminder on Cobordism Hypothesis
Baez and Dolan proposed that a unitary extended n-dimensional TQFT isa unitary representation of the cobordism n-category on the n-category ofn-Hilbert spaces:
Z : nCob→ nHilb
Cobordism Hyopthesis (Baez,Dolan)
nCob is the free weak n-category with duals on one object.
So understanding duality in higher categories will aid our understanding ofspacetime and quantum theory!
Skip extra stuff
![Page 14: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/14.jpg)
Introduction: Reminder on Cobordism Hypothesis
Baez and Dolan proposed that a unitary extended n-dimensional TQFT isa unitary representation of the cobordism n-category on the n-category ofn-Hilbert spaces:
Z : nCob→ nHilb
Cobordism Hyopthesis (Baez,Dolan)
nCob is the free weak n-category with duals on one object.
So understanding duality in higher categories will aid our understanding ofspacetime and quantum theory!
Skip extra stuff
![Page 15: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/15.jpg)
Introduction: Reminder on Cobordism Hypothesis
Baez and Dolan proposed that a unitary extended n-dimensional TQFT isa unitary representation of the cobordism n-category on the n-category ofn-Hilbert spaces:
Z : nCob→ nHilb
Cobordism Hyopthesis (Baez,Dolan)
nCob is the free weak n-category with duals on one object.
So understanding duality in higher categories will aid our understanding ofspacetime and quantum theory!
Skip extra stuff
![Page 16: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/16.jpg)
Introduction: Reminder on Cobordism Hypothesis
Baez and Dolan proposed that a unitary extended n-dimensional TQFT isa unitary representation of the cobordism n-category on the n-category ofn-Hilbert spaces:
Z : nCob→ nHilb
Cobordism Hyopthesis (Baez,Dolan)
nCob is the free weak n-category with duals on one object.
So understanding duality in higher categories will aid our understanding ofspacetime and quantum theory!
Skip extra stuff
![Page 17: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/17.jpg)
Cobordism Hypothesis: Extra
Cobordism Hyopthesis (Baez,Dolan)
nCob is the free weak n-category with duals on one object.
This means that an n-dimensional TQFT should be determined by Z (pt),the n-Hilbert space assigned to a point. For example in the untwisted 3dChern-Simons theory associated to a Lie group G , it seems that
Z (pt) = 2Rep(G ).
Indeed, at least when G is finite:
Theorem (BB, SW).
BunG (G )︸ ︷︷ ︸Z(S1)
braided mon' Dim 2Rep(G )︸ ︷︷ ︸category of weak transformations
and modifications of identity 2-functor
.
![Page 18: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/18.jpg)
Cobordism Hypothesis: Extra
Cobordism Hyopthesis (Baez,Dolan)
nCob is the free weak n-category with duals on one object.
This means that an n-dimensional TQFT should be determined by Z (pt),the n-Hilbert space assigned to a point.
For example in the untwisted 3dChern-Simons theory associated to a Lie group G , it seems that
Z (pt) = 2Rep(G ).
Indeed, at least when G is finite:
Theorem (BB, SW).
BunG (G )︸ ︷︷ ︸Z(S1)
braided mon' Dim 2Rep(G )︸ ︷︷ ︸category of weak transformations
and modifications of identity 2-functor
.
![Page 19: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/19.jpg)
Cobordism Hypothesis: Extra
Cobordism Hyopthesis (Baez,Dolan)
nCob is the free weak n-category with duals on one object.
This means that an n-dimensional TQFT should be determined by Z (pt),the n-Hilbert space assigned to a point. For example in the untwisted 3dChern-Simons theory associated to a Lie group G , it seems that
Z (pt) = 2Rep(G ).
Indeed, at least when G is finite:
Theorem (BB, SW).
BunG (G )︸ ︷︷ ︸Z(S1)
braided mon' Dim 2Rep(G )︸ ︷︷ ︸category of weak transformations
and modifications of identity 2-functor
.
![Page 20: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/20.jpg)
Cobordism Hypothesis: Extra
Cobordism Hyopthesis (Baez,Dolan)
nCob is the free weak n-category with duals on one object.
This means that an n-dimensional TQFT should be determined by Z (pt),the n-Hilbert space assigned to a point. For example in the untwisted 3dChern-Simons theory associated to a Lie group G , it seems that
Z (pt) = 2Rep(G ).
Indeed, at least when G is finite:
Theorem (BB, SW).
BunG (G )︸ ︷︷ ︸Z(S1)
braided mon' Dim 2Rep(G )︸ ︷︷ ︸category of weak transformations
and modifications of identity 2-functor
.
![Page 21: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/21.jpg)
2. String diagram notation for 2-categories
Note — string diagrams work perfectly well for weak 2-categories, as longas the the parenthesis scheme of the input and output 1-morphisms areunderstood in each context, eg.:
unique interpretation7→by coherence
[Fn ◦ (Fn−1 ◦ Fn−2)
]◦ · · · ◦ [F2 ◦ F1]
��Gm ◦ · · · ◦
[G3 ◦ (G2 ◦ G1)
]
![Page 22: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/22.jpg)
2. String diagram notation for 2-categories
Note — string diagrams work perfectly well for weak 2-categories, as longas the the parenthesis scheme of the input and output 1-morphisms areunderstood in each context, eg.:
unique interpretation7→by coherence
[Fn ◦ (Fn−1 ◦ Fn−2)
]◦ · · · ◦ [F2 ◦ F1]
��Gm ◦ · · · ◦
[G3 ◦ (G2 ◦ G1)
]
![Page 23: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/23.jpg)
Ambiijunction groupoid I
An ambidextrous adjoint of a morphism F : A→ B in a 2-category is amorphism F ∗ : B → A equipped with unit and counit 2-morphismsexpressing F ∗ as a right adjoint of F , and unit and counit 2-morphismsexpressing F ∗ as a left adjoint of F :
〈F ∗〉 ≡
(, , , ,
)Taken together these form the ambijunction groupoid Amb(F ) of F , wherea morphism γ : 〈F ∗〉 → 〈(F ∗)′〉 is defined to be an invertible 2-morphism
such that(, , , ,
)=
(, , , ,
).
![Page 24: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/24.jpg)
Ambiijunction groupoid I
An ambidextrous adjoint of a morphism F : A→ B in a 2-category is amorphism F ∗ : B → A equipped with unit and counit 2-morphismsexpressing F ∗ as a right adjoint of F , and unit and counit 2-morphismsexpressing F ∗ as a left adjoint of F :
〈F ∗〉 ≡
(, , , ,
)
Taken together these form the ambijunction groupoid Amb(F ) of F , wherea morphism γ : 〈F ∗〉 → 〈(F ∗)′〉 is defined to be an invertible 2-morphism
such that(, , , ,
)=
(, , , ,
).
![Page 25: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/25.jpg)
Ambiijunction groupoid I
An ambidextrous adjoint of a morphism F : A→ B in a 2-category is amorphism F ∗ : B → A equipped with unit and counit 2-morphismsexpressing F ∗ as a right adjoint of F , and unit and counit 2-morphismsexpressing F ∗ as a left adjoint of F :
〈F ∗〉 ≡
(, , , ,
)Taken together these form the ambijunction groupoid Amb(F ) of F , wherea morphism γ : 〈F ∗〉 → 〈(F ∗)′〉 is defined to be an invertible 2-morphism
such that(, , , ,
)=
(, , , ,
).
![Page 26: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/26.jpg)
Ambijunction groupoid II
We write [Amb(F )] for the isomorphism classes in Amb(F ).
Properties of the ambijunction groupoid
If Amb(F ) is not empty, then
There is at most one arrow between any two ambidextrousadjunctions in Amb(F ).
The group Aut(F ) of automorphisms of F acts freely and transitivelyon [Amb(F )].
An automorphism α : F → F acts on an ambidextrous adjoint by twistingthe right unit and counit maps,
α · [F ∗] =
[, , , ,
].
![Page 27: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/27.jpg)
Ambijunction groupoid II
We write [Amb(F )] for the isomorphism classes in Amb(F ).
Properties of the ambijunction groupoid
If Amb(F ) is not empty, then
There is at most one arrow between any two ambidextrousadjunctions in Amb(F ).
The group Aut(F ) of automorphisms of F acts freely and transitivelyon [Amb(F )].
An automorphism α : F → F acts on an ambidextrous adjoint by twistingthe right unit and counit maps,
α · [F ∗] =
[, , , ,
].
![Page 28: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/28.jpg)
Even-handed structures
Suppose θ : F ⇒ G is a 2-morphism, and that choices of ambidextrousadjoints 〈F ∗〉 and 〈G ∗〉 have been made. The right and left daggers of θare:
θ† = †θ =
Whether θ† =† θ only depends on [F ∗] and [G ∗].We say that a 2-category C has ambidextrous adjoints if every morphismhas an (unspecified) ambidextrous adjoint.
Definition
An even-handed structure on a 2-category with ambidextrous adjoints is achoice F [∗] ∈ [Amb(F )] for every morphism F , such that:
1 id[∗] = class of trivial ambidextrous adjunction for all identity 1-cells,
2 (G ◦ F )[∗] = F [∗] ◦ G [∗] for all composable pairs of morphisms, and
3 θ† =† θ for every 2-morphism θ : F ⇒ G , provided they are computedusing ambidextrous adjoints from the classes F [∗] and G [∗].
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Even-handed structures
Suppose θ : F ⇒ G is a 2-morphism, and that choices of ambidextrousadjoints 〈F ∗〉 and 〈G ∗〉 have been made. The right and left daggers of θare:
θ† = †θ =
Whether θ† =† θ only depends on [F ∗] and [G ∗].
We say that a 2-category C has ambidextrous adjoints if every morphismhas an (unspecified) ambidextrous adjoint.
Definition
An even-handed structure on a 2-category with ambidextrous adjoints is achoice F [∗] ∈ [Amb(F )] for every morphism F , such that:
1 id[∗] = class of trivial ambidextrous adjunction for all identity 1-cells,
2 (G ◦ F )[∗] = F [∗] ◦ G [∗] for all composable pairs of morphisms, and
3 θ† =† θ for every 2-morphism θ : F ⇒ G , provided they are computedusing ambidextrous adjoints from the classes F [∗] and G [∗].
![Page 30: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/30.jpg)
Even-handed structures
Suppose θ : F ⇒ G is a 2-morphism, and that choices of ambidextrousadjoints 〈F ∗〉 and 〈G ∗〉 have been made. The right and left daggers of θare:
θ† = †θ =
Whether θ† =† θ only depends on [F ∗] and [G ∗].We say that a 2-category C has ambidextrous adjoints if every morphismhas an (unspecified) ambidextrous adjoint.
Definition
An even-handed structure on a 2-category with ambidextrous adjoints is achoice F [∗] ∈ [Amb(F )] for every morphism F , such that:
1 id[∗] = class of trivial ambidextrous adjunction for all identity 1-cells,
2 (G ◦ F )[∗] = F [∗] ◦ G [∗] for all composable pairs of morphisms, and
3 θ† =† θ for every 2-morphism θ : F ⇒ G , provided they are computedusing ambidextrous adjoints from the classes F [∗] and G [∗].
![Page 31: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/31.jpg)
Even-handed structures
Suppose θ : F ⇒ G is a 2-morphism, and that choices of ambidextrousadjoints 〈F ∗〉 and 〈G ∗〉 have been made. The right and left daggers of θare:
θ† = †θ =
Whether θ† =† θ only depends on [F ∗] and [G ∗].We say that a 2-category C has ambidextrous adjoints if every morphismhas an (unspecified) ambidextrous adjoint.
Definition
An even-handed structure on a 2-category with ambidextrous adjoints is achoice F [∗] ∈ [Amb(F )] for every morphism F , such that:
1 id[∗] = class of trivial ambidextrous adjunction for all identity 1-cells,
2 (G ◦ F )[∗] = F [∗] ◦ G [∗] for all composable pairs of morphisms, and
3 θ† =† θ for every 2-morphism θ : F ⇒ G , provided they are computedusing ambidextrous adjoints from the classes F [∗] and G [∗].
![Page 32: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/32.jpg)
Even-handed structures
Suppose θ : F ⇒ G is a 2-morphism, and that choices of ambidextrousadjoints 〈F ∗〉 and 〈G ∗〉 have been made. The right and left daggers of θare:
θ† = †θ =
Whether θ† =† θ only depends on [F ∗] and [G ∗].We say that a 2-category C has ambidextrous adjoints if every morphismhas an (unspecified) ambidextrous adjoint.
Definition
An even-handed structure on a 2-category with ambidextrous adjoints is achoice F [∗] ∈ [Amb(F )] for every morphism F , such that:
1 id[∗] = class of trivial ambidextrous adjunction for all identity 1-cells,
2 (G ◦ F )[∗] = F [∗] ◦ G [∗] for all composable pairs of morphisms, and
3 θ† =† θ for every 2-morphism θ : F ⇒ G , provided they are computedusing ambidextrous adjoints from the classes F [∗] and G [∗].
So... an even-handed structure on a 2-category is an ‘even-handedtrivialization of the ambijunction gerbe’:
Analagous to Murray and Singer’s refomulation of a spin structure on amanifold as a trivialization of the spin gerbe.
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Examples: Monoidal categories, remarks
Consider a monoidal category C (a one-object 2-category) with ambidex-trous duals, eg. Rep(G ). Some preliminary remarks:
The set of even-handed structures on C is a torsor for the groupAut⊗(idC ), which acts by twisting the right unit and counit maps asbefore.
The conventional paradigm for duals in monoidal categories is tochoose a specific right dual V ? for each object, and define a pivotalstructure on C as a monoidal natural isomorphism γ : id⇒ ??.
In good cases, an even-handed structure [∗] gives rise to a ringhomomorphism
dim[∗] : K (C )→ End(1)
[V ] 7→ .
If C is a semisimple linear category, [∗] is characterized by dim[∗].
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Examples: Monoidal categories, remarks
Consider a monoidal category C (a one-object 2-category) with ambidex-trous duals, eg. Rep(G ). Some preliminary remarks:
The set of even-handed structures on C is a torsor for the groupAut⊗(idC ), which acts by twisting the right unit and counit maps asbefore.
The conventional paradigm for duals in monoidal categories is tochoose a specific right dual V ? for each object, and define a pivotalstructure on C as a monoidal natural isomorphism γ : id⇒ ??.
In good cases, an even-handed structure [∗] gives rise to a ringhomomorphism
dim[∗] : K (C )→ End(1)
[V ] 7→ .
If C is a semisimple linear category, [∗] is characterized by dim[∗].
![Page 35: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/35.jpg)
Examples: Monoidal categories, remarks
Consider a monoidal category C (a one-object 2-category) with ambidex-trous duals, eg. Rep(G ). Some preliminary remarks:
The set of even-handed structures on C is a torsor for the groupAut⊗(idC ), which acts by twisting the right unit and counit maps asbefore.
The conventional paradigm for duals in monoidal categories is tochoose a specific right dual V ? for each object, and define a pivotalstructure on C as a monoidal natural isomorphism γ : id⇒ ??.
In good cases, an even-handed structure [∗] gives rise to a ringhomomorphism
dim[∗] : K (C )→ End(1)
[V ] 7→ .
If C is a semisimple linear category, [∗] is characterized by dim[∗].
![Page 36: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/36.jpg)
Examples: Monoidal categories, remarks
Consider a monoidal category C (a one-object 2-category) with ambidex-trous duals, eg. Rep(G ). Some preliminary remarks:
The set of even-handed structures on C is a torsor for the groupAut⊗(idC ), which acts by twisting the right unit and counit maps asbefore.
The conventional paradigm for duals in monoidal categories is tochoose a specific right dual V ? for each object, and define a pivotalstructure on C as a monoidal natural isomorphism γ : id⇒ ??.
In good cases, an even-handed structure [∗] gives rise to a ringhomomorphism
dim[∗] : K (C )→ End(1)
[V ] 7→ .
If C is a semisimple linear category, [∗] is characterized by dim[∗].
![Page 37: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/37.jpg)
Examples: Monoidal categories, remarks
Consider a monoidal category C (a one-object 2-category) with ambidex-trous duals, eg. Rep(G ). Some preliminary remarks:
The set of even-handed structures on C is a torsor for the groupAut⊗(idC ), which acts by twisting the right unit and counit maps asbefore.
The conventional paradigm for duals in monoidal categories is tochoose a specific right dual V ? for each object, and define a pivotalstructure on C as a monoidal natural isomorphism γ : id⇒ ??.
Lemma
For each choice of system of right duals ? on C , there is a canonicalbijection{
Even-handed structures on C ,considered as a one object 2-category
}∼={
Pivotal structures onC with respect to ?
}.
Moreover, this bijection is natural with respect to changing the system ofright duals ?.
In good cases, an even-handed structure [∗] gives rise to a ringhomomorphism
dim[∗] : K (C )→ End(1)
[V ] 7→ .
If C is a semisimple linear category, [∗] is characterized by dim[∗].
![Page 38: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/38.jpg)
Examples: Monoidal categories, remarks
Consider a monoidal category C (a one-object 2-category) with ambidex-trous duals, eg. Rep(G ). Some preliminary remarks:
The set of even-handed structures on C is a torsor for the groupAut⊗(idC ), which acts by twisting the right unit and counit maps asbefore.
The conventional paradigm for duals in monoidal categories is tochoose a specific right dual V ? for each object, and define a pivotalstructure on C as a monoidal natural isomorphism γ : id⇒ ??.
In good cases, an even-handed structure [∗] gives rise to a ringhomomorphism
dim[∗] : K (C )→ End(1)
[V ] 7→ .
If C is a semisimple linear category, [∗] is characterized by dim[∗].
![Page 39: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/39.jpg)
Examples: Monoidal categories, remarks
Consider a monoidal category C (a one-object 2-category) with ambidex-trous duals, eg. Rep(G ). Some preliminary remarks:
The set of even-handed structures on C is a torsor for the groupAut⊗(idC ), which acts by twisting the right unit and counit maps asbefore.
The conventional paradigm for duals in monoidal categories is tochoose a specific right dual V ? for each object, and define a pivotalstructure on C as a monoidal natural isomorphism γ : id⇒ ??.
In good cases, an even-handed structure [∗] gives rise to a ringhomomorphism
dim[∗] : K (C )→ End(1)
[V ] 7→ .
If C is a semisimple linear category, [∗] is characterized by dim[∗].
![Page 40: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/40.jpg)
Examples: Monoidal categories (one-object 2-categories)
1 For the monoidal category (G , ω) coming from a 3-cocycleω ∈ Z 3(G ,U(1)),{
Even-handed structureson (G , ω)
}∼={
Group homomorphismsf : G → U(1)
}.
2 Suppose C is a monoidal category where every object has a rightdual. If C can be equipped with a braiding σ, then{
Even-handed structureson C
}∼={
Pretwists on Cwith respect to σ
}.
3 For fusion categories (semisimple rigid monoidal k-linear categories),some mysteries remain:
What are the equations which have the symmetry group
Aut⊗(id) = {θi ∈ k× : θi = θjθk whenever Xi appears in Xj ⊗ Xk}?
Will they ensure that an even-handed structure always exists?
![Page 41: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/41.jpg)
Examples: Monoidal categories (one-object 2-categories)
1 For the monoidal category (G , ω) coming from a 3-cocycleω ∈ Z 3(G ,U(1)),{
Even-handed structureson (G , ω)
}∼={
Group homomorphismsf : G → U(1)
}.
2 Suppose C is a monoidal category where every object has a rightdual. If C can be equipped with a braiding σ, then{
Even-handed structureson C
}∼={
Pretwists on Cwith respect to σ
}.
3 For fusion categories (semisimple rigid monoidal k-linear categories),some mysteries remain:
What are the equations which have the symmetry group
Aut⊗(id) = {θi ∈ k× : θi = θjθk whenever Xi appears in Xj ⊗ Xk}?
Will they ensure that an even-handed structure always exists?
![Page 42: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/42.jpg)
Examples: Monoidal categories (one-object 2-categories)
1 For the monoidal category (G , ω) coming from a 3-cocycleω ∈ Z 3(G ,U(1)),{
Even-handed structureson (G , ω)
}∼={
Group homomorphismsf : G → U(1)
}.
2 Suppose C is a monoidal category where every object has a rightdual. If C can be equipped with a braiding σ, then{
Even-handed structureson C
}∼={
Pretwists on Cwith respect to σ
}.
θ ∈ Aut(id) is a pretwist with respect to σ if
= .
Gives even-handed structure
V [∗] =
, , , ,
.
3 For fusion categories (semisimple rigid monoidal k-linear categories),some mysteries remain:
What are the equations which have the symmetry group
Aut⊗(id) = {θi ∈ k× : θi = θjθk whenever Xi appears in Xj ⊗ Xk}?
Will they ensure that an even-handed structure always exists?
![Page 43: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/43.jpg)
Examples: Monoidal categories (one-object 2-categories)
1 For the monoidal category (G , ω) coming from a 3-cocycleω ∈ Z 3(G ,U(1)),{
Even-handed structureson (G , ω)
}∼={
Group homomorphismsf : G → U(1)
}.
2 Suppose C is a monoidal category where every object has a rightdual. If C can be equipped with a braiding σ, then{
Even-handed structureson C
}∼={
Pretwists on Cwith respect to σ
}.
3 For fusion categories (semisimple rigid monoidal k-linear categories),some mysteries remain:
What are the equations which have the symmetry group
Aut⊗(id) = {θi ∈ k× : θi = θjθk whenever Xi appears in Xj ⊗ Xk}?
Will they ensure that an even-handed structure always exists?
![Page 44: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/44.jpg)
Examples: Monoidal categories (one-object 2-categories)
1 For the monoidal category (G , ω) coming from a 3-cocycleω ∈ Z 3(G ,U(1)),{
Even-handed structureson (G , ω)
}∼={
Group homomorphismsf : G → U(1)
}.
2 Suppose C is a monoidal category where every object has a rightdual. If C can be equipped with a braiding σ, then{
Even-handed structureson C
}∼={
Pretwists on Cwith respect to σ
}.
3 For fusion categories (semisimple rigid monoidal k-linear categories),some mysteries remain:
What are the equations which have the symmetry group
Aut⊗(id) = {θi ∈ k× : θi = θjθk whenever Xi appears in Xj ⊗ Xk}?
Will they ensure that an even-handed structure always exists?
![Page 45: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/45.jpg)
Examples: Monoidal categories (one-object 2-categories)
1 For the monoidal category (G , ω) coming from a 3-cocycleω ∈ Z 3(G ,U(1)),{
Even-handed structureson (G , ω)
}∼={
Group homomorphismsf : G → U(1)
}.
2 Suppose C is a monoidal category where every object has a rightdual. If C can be equipped with a braiding σ, then{
Even-handed structureson C
}∼={
Pretwists on Cwith respect to σ
}.
3 For fusion categories (semisimple rigid monoidal k-linear categories),some mysteries remain:
What are the equations which have the symmetry group
Aut⊗(id) = {θi ∈ k× : θi = θjθk whenever Xi appears in Xj ⊗ Xk}?
Will they ensure that an even-handed structure always exists?
The manifest invariants of a fusion category are the fusion ring K (C )and Muger’s squared norms di of the simple objects:
di = ...this is independent of choice of 〈X ∗i 〉.
If an even-handed structure exists, then dim[∗] : K (C )→ k satisfies:
It is a ring homomorphism taking nonzero values on simple objects,
dim[Xi ] dim[X ∗i ] = di for all simple objects.
Do these equations have Aut⊗(id) as their symmetry group? Tomake an even-handed structure, crucially need:
= δpq
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Even-handed structures on sub-2-categories of Cat
If F : A→ B is a functor between categories, express right and leftadjunctions via
φ : Hom(Fx , y)∼=→ Hom(x ,F ∗y)
ψ : Hom(F ∗y , x)∼=→ Hom(y ,Fx)
For θ : F ⇒ G , can express θ†,†θ : G ∗ ⇒ F ∗ as follows:
post(θ†y ) = φF ◦ pre(θx) ◦ φ-1G
pre(†θy ) = ψ-1G ◦ post(θx) ◦ ψF
In this way, an even-handed structure on a sub-2-category C ⊆ Cat withambidextrous adjoints translates into a system of bijective maps
ΨF ,F∗ : Adj(F a F ∗)→ Adj(F ∗ a F )
which transform correctly under isomorphism 2-cells, respect composition,and satisfy the even-handed equation:
post−1(φF ◦ pre(θ) ◦ φ−1
G
)= pre−1
(Ψ(φG )−1 ◦ post(θ) ◦Ψ(φF )
)
![Page 47: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/47.jpg)
Even-handed structures on sub-2-categories of Cat
If F : A→ B is a functor between categories, express right and leftadjunctions via
φ : Hom(Fx , y)∼=→ Hom(x ,F ∗y)
ψ : Hom(F ∗y , x)∼=→ Hom(y ,Fx)
For θ : F ⇒ G , can express θ†,†θ : G ∗ ⇒ F ∗ as follows:
post(θ†y ) = φF ◦ pre(θx) ◦ φ-1G
pre(†θy ) = ψ-1G ◦ post(θx) ◦ ψF
In this way, an even-handed structure on a sub-2-category C ⊆ Cat withambidextrous adjoints translates into a system of bijective maps
ΨF ,F∗ : Adj(F a F ∗)→ Adj(F ∗ a F )
which transform correctly under isomorphism 2-cells, respect composition,and satisfy the even-handed equation:
post−1(φF ◦ pre(θ) ◦ φ−1
G
)= pre−1
(Ψ(φG )−1 ◦ post(θ) ◦Ψ(φF )
)
![Page 48: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/48.jpg)
Even-handed structures on sub-2-categories of Cat
If F : A→ B is a functor between categories, express right and leftadjunctions via
φ : Hom(Fx , y)∼=→ Hom(x ,F ∗y)
ψ : Hom(F ∗y , x)∼=→ Hom(y ,Fx)
For θ : F ⇒ G , can express θ†,†θ : G ∗ ⇒ F ∗ as follows:
post(θ†y ) = φF ◦ pre(θx) ◦ φ-1G
pre(†θy ) = ψ-1G ◦ post(θx) ◦ ψF
In this way, an even-handed structure on a sub-2-category C ⊆ Cat withambidextrous adjoints translates into a system of bijective maps
ΨF ,F∗ : Adj(F a F ∗)→ Adj(F ∗ a F )
which transform correctly under isomorphism 2-cells, respect composition,and satisfy the even-handed equation:
post−1(φF ◦ pre(θ) ◦ φ−1
G
)= pre−1
(Ψ(φG )−1 ◦ post(θ) ◦Ψ(φF )
)
![Page 49: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/49.jpg)
Even-handed structures on sub-2-categories of Cat
If F : A→ B is a functor between categories, express right and leftadjunctions via
φ : Hom(Fx , y)∼=→ Hom(x ,F ∗y)
ψ : Hom(F ∗y , x)∼=→ Hom(y ,Fx)
For θ : F ⇒ G , can express θ†,†θ : G ∗ ⇒ F ∗ as follows:
post(θ†y ) = φF ◦ pre(θx) ◦ φ-1G
pre(†θy ) = ψ-1G ◦ post(θx) ◦ ψF
In this way, an even-handed structure on a sub-2-category C ⊆ Cat withambidextrous adjoints translates into a system of bijective maps
ΨF ,F∗ : Adj(F a F ∗)→ Adj(F ∗ a F )
which transform correctly under isomorphism 2-cells, respect composition,and satisfy the even-handed equation:
post−1(φF ◦ pre(θ) ◦ φ−1
G
)= pre−1
(Ψ(φG )−1 ◦ post(θ) ◦Ψ(φF )
)
![Page 50: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/50.jpg)
Even-handed structures from traces on linear categories
Definition
A trace on a k-linear category is a linear map Trx : End(x)→ k for eachobject satisfying:
Trx(gf ) = Try (fg).
Nondegeneracy: s : Hom(x , y)∼=→ Hom(y , x)∨.
Turn right adjoints into left adjoints:
Hom(F ∗y , x)
sA��
φT// Hom(y ,Fx)
sB��
Hom(x ,F ∗y)∨φ∨
// Hom(Fx , y)∨
φT := s -1B ◦ φ∨ ◦ sA
Theorem
If each category in C ⊆ LinearCatk comes equipped with a trace, then
sending φΨ7→ φT gives an even handed structure on C.
![Page 51: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/51.jpg)
Even-handed structures from traces on linear categories
Definition
A trace on a k-linear category is a linear map Trx : End(x)→ k for eachobject satisfying:
Trx(gf ) = Try (fg).
Nondegeneracy: s : Hom(x , y)∼=→ Hom(y , x)∨.
Turn right adjoints into left adjoints:
Hom(F ∗y , x)
sA��
φT// Hom(y ,Fx)
sB��
Hom(x ,F ∗y)∨φ∨
// Hom(Fx , y)∨
φT := s -1B ◦ φ∨ ◦ sA
Theorem
If each category in C ⊆ LinearCatk comes equipped with a trace, then
sending φΨ7→ φT gives an even handed structure on C.
![Page 52: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/52.jpg)
Even-handed structures from traces on linear categories
Definition
A trace on a k-linear category is a linear map Trx : End(x)→ k for eachobject satisfying:
Trx(gf ) = Try (fg).
Nondegeneracy: s : Hom(x , y)∼=→ Hom(y , x)∨.
Remark
A trace on C is the same thing as a symmetric monoidal functor
PlanarCobOb(C) → Vectk
7→ Hom(x , y)
7→Hom(x , y)⊗ Hom(y , z)
◦��Hom(x , z)
7→Hom(x , x)
Trx��k
7→1_��∑
i fi⊗f ietc.
Turn right adjoints into left adjoints:
Hom(F ∗y , x)
sA��
φT// Hom(y ,Fx)
sB��
Hom(x ,F ∗y)∨φ∨
// Hom(Fx , y)∨
φT := s -1B ◦ φ∨ ◦ sA
Theorem
If each category in C ⊆ LinearCatk comes equipped with a trace, then
sending φΨ7→ φT gives an even handed structure on C.
![Page 53: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/53.jpg)
Even-handed structures from traces on linear categories
Definition
A trace on a k-linear category is a linear map Trx : End(x)→ k for eachobject satisfying:
Trx(gf ) = Try (fg).
Nondegeneracy: s : Hom(x , y)∼=→ Hom(y , x)∨.
Turn right adjoints into left adjoints:
Hom(F ∗y , x)
sA��
φT// Hom(y ,Fx)
sB��
Hom(x ,F ∗y)∨φ∨
// Hom(Fx , y)∨
φT := s -1B ◦ φ∨ ◦ sA
Theorem
If each category in C ⊆ LinearCatk comes equipped with a trace, then
sending φΨ7→ φT gives an even handed structure on C.
![Page 54: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/54.jpg)
Even-handed structures from traces on linear categories
Definition
A trace on a k-linear category is a linear map Trx : End(x)→ k for eachobject satisfying:
Trx(gf ) = Try (fg).
Nondegeneracy: s : Hom(x , y)∼=→ Hom(y , x)∨.
Turn right adjoints into left adjoints:
Hom(F ∗y , x)
sA��
φT// Hom(y ,Fx)
sB��
Hom(x ,F ∗y)∨φ∨
// Hom(Fx , y)∨
φT := s -1B ◦ φ∨ ◦ sA
Theorem
If each category in C ⊆ LinearCatk comes equipped with a trace, then
sending φΨ7→ φT gives an even handed structure on C.
![Page 55: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/55.jpg)
Even-handed structures from traces on linear categories
Definition
A trace on a k-linear category is a linear map Trx : End(x)→ k for eachobject satisfying:
Trx(gf ) = Try (fg).
Nondegeneracy: s : Hom(x , y)∼=→ Hom(y , x)∨.
Turn right adjoints into left adjoints:
Hom(F ∗y , x)
sA��
φT// Hom(y ,Fx)
sB��
Hom(x ,F ∗y)∨φ∨
// Hom(Fx , y)∨
φT := s -1B ◦ φ∨ ◦ sA
Theorem
If each category in C ⊆ LinearCatk comes equipped with a trace, then
sending φΨ7→ φT gives an even handed structure on C.
![Page 56: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/56.jpg)
Example: The 2-category of 2-Hilbert spaces
Definition (Baez)
A 2-Hilbert space is an abelian Hilb-category H equipped with antilinearmaps ∗ : Hom(x , y)→ Hom(y , x) compatible with composition and innerproducts. They form a 2-category 2Hilb.
A good example is the category Rep(G ) of unitary representations ofa compact group.
Every 2-Hilbert space comes equipped with a canonical trace,f 7→ (idx , f ).
Lemma
The resulting even-handed structure on 2Hilb can be expressed as
φT = ∗ φ∗ ∗ .
where φ∗ is the adjoint linear map of φ.
![Page 57: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/57.jpg)
Example: The 2-category of 2-Hilbert spaces
Definition (Baez)
A 2-Hilbert space is an abelian Hilb-category H equipped with antilinearmaps ∗ : Hom(x , y)→ Hom(y , x) compatible with composition and innerproducts. They form a 2-category 2Hilb.
A good example is the category Rep(G ) of unitary representations ofa compact group.
Every 2-Hilbert space comes equipped with a canonical trace,f 7→ (idx , f ).
Lemma
The resulting even-handed structure on 2Hilb can be expressed as
φT = ∗ φ∗ ∗ .
where φ∗ is the adjoint linear map of φ.
![Page 58: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/58.jpg)
Example: The 2-category of 2-Hilbert spaces
Definition (Baez)
A 2-Hilbert space is an abelian Hilb-category H equipped with antilinearmaps ∗ : Hom(x , y)→ Hom(y , x) compatible with composition and innerproducts. They form a 2-category 2Hilb.
A good example is the category Rep(G ) of unitary representations ofa compact group.
Every 2-Hilbert space comes equipped with a canonical trace,f 7→ (idx , f ).
Lemma
The resulting even-handed structure on 2Hilb can be expressed as
φT = ∗ φ∗ ∗ .
where φ∗ is the adjoint linear map of φ.
![Page 59: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/59.jpg)
Example: The 2-category of 2-Hilbert spaces
Definition (Baez)
A 2-Hilbert space is an abelian Hilb-category H equipped with antilinearmaps ∗ : Hom(x , y)→ Hom(y , x) compatible with composition and innerproducts. They form a 2-category 2Hilb.
A good example is the category Rep(G ) of unitary representations ofa compact group.
Every 2-Hilbert space comes equipped with a canonical trace,f 7→ (idx , f ).
Lemma
The resulting even-handed structure on 2Hilb can be expressed as
φT = ∗ φ∗ ∗ .
where φ∗ is the adjoint linear map of φ.
![Page 60: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/60.jpg)
Example: The 2-category of 2-Hilbert spaces
Definition (Baez)
A 2-Hilbert space is an abelian Hilb-category H equipped with antilinearmaps ∗ : Hom(x , y)→ Hom(y , x) compatible with composition and innerproducts. They form a 2-category 2Hilb.
A good example is the category Rep(G ) of unitary representations ofa compact group.
Every 2-Hilbert space comes equipped with a canonical trace,f 7→ (idx , f ).
Lemma
The resulting even-handed structure on 2Hilb can be expressed as
φT = ∗ φ∗ ∗ .
where φ∗ is the adjoint linear map of φ.
![Page 61: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/61.jpg)
Example: The 2-category of 2-Hilbert spaces
Definition (Baez)
A 2-Hilbert space is an abelian Hilb-category H equipped with antilinearmaps ∗ : Hom(x , y)→ Hom(y , x) compatible with composition and innerproducts. They form a 2-category 2Hilb.
A good example is the category Rep(G ) of unitary representations ofa compact group.
Every 2-Hilbert space comes equipped with a canonical trace,f 7→ (idx , f ).
Lemma
The resulting even-handed structure on 2Hilb can be expressed as
φT = ∗ φ∗ ∗ .
where φ∗ is the adjoint linear map of φ.
Similar to the formula for d∗ on acompact Riemannian manifold,
d∗ = ∗ d ∗
![Page 62: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/62.jpg)
Example: The 2-category of 2-Hilbert spaces
Definition (Baez)
A 2-Hilbert space is an abelian Hilb-category H equipped with antilinearmaps ∗ : Hom(x , y)→ Hom(y , x) compatible with composition and innerproducts. They form a 2-category 2Hilb.
A good example is the category Rep(G ) of unitary representations ofa compact group.
Every 2-Hilbert space comes equipped with a canonical trace,f 7→ (idx , f ).
Lemma
The resulting even-handed structure on 2Hilb can be expressed as
φT = ∗ φ∗ ∗ .
where φ∗ is the adjoint linear map of φ.
Similar to the formula for d∗ on acompact Riemannian manifold,
d∗ = ∗ d ∗
Indeed, can prove that in the semisimple context,Even-handed structures on
a full sub-2-categoryS ⊂ SLCatk
∼=
Weightings on thesimple objects in S
up to a global scale factor
.
This is similar to
dim{harmonic spinors on a spin manifold} is a conformal invariant.
![Page 63: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/63.jpg)
Example: The 2-category CYau
X an n-dimensional Calabi-Yau manifold. Have the graded derivedcategory D(X ):
An object is a bounded complex E of coherent sheaves on X .
HomD(X )(E ,F ) :=⊕
i
HomD(X )(E ,F [i ]).
Serre duality gives D(X ) a supertrace,
Tr : HomnD(X )(E ,E )→ C.
Definition
The 2-category CYau has graded derived categories D(X ) of Calabi-Yaumanifolds as objects. A morphism is a functor F : D(X )→ D(Y ) naturallyisomorphic to an integral kernel. A 2-morphism is a natural transformation.
Corollary
The 2-category CYau comes equipped with a canonical even-handedstructure arising from Serre duality on each D(X ).
![Page 64: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/64.jpg)
Example: The 2-category CYau
X an n-dimensional Calabi-Yau manifold. Have the graded derivedcategory D(X ):
An object is a bounded complex E of coherent sheaves on X .
HomD(X )(E ,F ) :=⊕
i
HomD(X )(E ,F [i ]).
Serre duality gives D(X ) a supertrace,
Tr : HomnD(X )(E ,E )→ C.
Definition
The 2-category CYau has graded derived categories D(X ) of Calabi-Yaumanifolds as objects. A morphism is a functor F : D(X )→ D(Y ) naturallyisomorphic to an integral kernel. A 2-morphism is a natural transformation.
Corollary
The 2-category CYau comes equipped with a canonical even-handedstructure arising from Serre duality on each D(X ).
![Page 65: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/65.jpg)
Example: The 2-category CYau
X an n-dimensional Calabi-Yau manifold. Have the graded derivedcategory D(X ):
An object is a bounded complex E of coherent sheaves on X .
HomD(X )(E ,F ) :=⊕
i
HomD(X )(E ,F [i ]).
Serre duality gives D(X ) a supertrace,
Tr : HomnD(X )(E ,E )→ C.
Definition
The 2-category CYau has graded derived categories D(X ) of Calabi-Yaumanifolds as objects. A morphism is a functor F : D(X )→ D(Y ) naturallyisomorphic to an integral kernel. A 2-morphism is a natural transformation.
Corollary
The 2-category CYau comes equipped with a canonical even-handedstructure arising from Serre duality on each D(X ).
![Page 66: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/66.jpg)
Example: The 2-category CYau
X an n-dimensional Calabi-Yau manifold. Have the graded derivedcategory D(X ):
An object is a bounded complex E of coherent sheaves on X .
HomD(X )(E ,F ) :=⊕
i
HomD(X )(E ,F [i ]).
Serre duality gives D(X ) a supertrace,
Tr : HomnD(X )(E ,E )→ C.
Definition
The 2-category CYau has graded derived categories D(X ) of Calabi-Yaumanifolds as objects. A morphism is a functor F : D(X )→ D(Y ) naturallyisomorphic to an integral kernel. A 2-morphism is a natural transformation.
Corollary
The 2-category CYau comes equipped with a canonical even-handedstructure arising from Serre duality on each D(X ).
![Page 67: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/67.jpg)
Example: The 2-category CYau
X an n-dimensional Calabi-Yau manifold. Have the graded derivedcategory D(X ):
An object is a bounded complex E of coherent sheaves on X .
HomD(X )(E ,F ) :=⊕
i
HomD(X )(E ,F [i ]).
Serre duality gives D(X ) a supertrace,
Tr : HomnD(X )(E ,E )→ C.
Definition
The 2-category CYau has graded derived categories D(X ) of Calabi-Yaumanifolds as objects. A morphism is a functor F : D(X )→ D(Y ) naturallyisomorphic to an integral kernel. A 2-morphism is a natural transformation.
Corollary
The 2-category CYau comes equipped with a canonical even-handedstructure arising from Serre duality on each D(X ).
![Page 68: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/68.jpg)
Summary
Investigating duality in 2-categories from the paradigm of TQFT.
The concept of an even-handed structure
is economicalis flexiblerequires no awkward fixed choices of adjoints, so fits examples like2Hilb and CYau.goes hand in hand with a powerful string-diagram calculus
The concept of an even-handed structure, and the ‘moduli space’ ofeven-handed structures on a given 2-category, have geometricovertones.
Personal motivation: needed an even-handed structure on 2Hilb tomake the 2-character into a functor
χ : 2Rep(G )︸ ︷︷ ︸homotopy category
→ BunG (G )
![Page 69: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/69.jpg)
Summary
Investigating duality in 2-categories from the paradigm of TQFT.
The concept of an even-handed structure
is economicalis flexiblerequires no awkward fixed choices of adjoints, so fits examples like2Hilb and CYau.goes hand in hand with a powerful string-diagram calculus
The concept of an even-handed structure, and the ‘moduli space’ ofeven-handed structures on a given 2-category, have geometricovertones.
Personal motivation: needed an even-handed structure on 2Hilb tomake the 2-character into a functor
χ : 2Rep(G )︸ ︷︷ ︸homotopy category
→ BunG (G )
![Page 70: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/70.jpg)
Summary
Investigating duality in 2-categories from the paradigm of TQFT.
The concept of an even-handed structure
is economicalis flexiblerequires no awkward fixed choices of adjoints, so fits examples like2Hilb and CYau.goes hand in hand with a powerful string-diagram calculus
The concept of an even-handed structure, and the ‘moduli space’ ofeven-handed structures on a given 2-category, have geometricovertones.
Personal motivation: needed an even-handed structure on 2Hilb tomake the 2-character into a functor
χ : 2Rep(G )︸ ︷︷ ︸homotopy category
→ BunG (G )
![Page 71: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/71.jpg)
Summary
Investigating duality in 2-categories from the paradigm of TQFT.
The concept of an even-handed structure
is economical
is flexiblerequires no awkward fixed choices of adjoints, so fits examples like2Hilb and CYau.goes hand in hand with a powerful string-diagram calculus
The concept of an even-handed structure, and the ‘moduli space’ ofeven-handed structures on a given 2-category, have geometricovertones.
Personal motivation: needed an even-handed structure on 2Hilb tomake the 2-character into a functor
χ : 2Rep(G )︸ ︷︷ ︸homotopy category
→ BunG (G )
![Page 72: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/72.jpg)
Summary
Investigating duality in 2-categories from the paradigm of TQFT.
The concept of an even-handed structure
is economicalis flexible
requires no awkward fixed choices of adjoints, so fits examples like2Hilb and CYau.goes hand in hand with a powerful string-diagram calculus
The concept of an even-handed structure, and the ‘moduli space’ ofeven-handed structures on a given 2-category, have geometricovertones.
Personal motivation: needed an even-handed structure on 2Hilb tomake the 2-character into a functor
χ : 2Rep(G )︸ ︷︷ ︸homotopy category
→ BunG (G )
![Page 73: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/73.jpg)
Summary
Investigating duality in 2-categories from the paradigm of TQFT.
The concept of an even-handed structure
is economicalis flexiblerequires no awkward fixed choices of adjoints, so fits examples like2Hilb and CYau.
goes hand in hand with a powerful string-diagram calculus
The concept of an even-handed structure, and the ‘moduli space’ ofeven-handed structures on a given 2-category, have geometricovertones.
Personal motivation: needed an even-handed structure on 2Hilb tomake the 2-character into a functor
χ : 2Rep(G )︸ ︷︷ ︸homotopy category
→ BunG (G )
![Page 74: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/74.jpg)
Summary
Investigating duality in 2-categories from the paradigm of TQFT.
The concept of an even-handed structure
is economicalis flexiblerequires no awkward fixed choices of adjoints, so fits examples like2Hilb and CYau.goes hand in hand with a powerful string-diagram calculus
The concept of an even-handed structure, and the ‘moduli space’ ofeven-handed structures on a given 2-category, have geometricovertones.
Personal motivation: needed an even-handed structure on 2Hilb tomake the 2-character into a functor
χ : 2Rep(G )︸ ︷︷ ︸homotopy category
→ BunG (G )
![Page 75: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/75.jpg)
Summary
Investigating duality in 2-categories from the paradigm of TQFT.
The concept of an even-handed structure
is economicalis flexiblerequires no awkward fixed choices of adjoints, so fits examples like2Hilb and CYau.goes hand in hand with a powerful string-diagram calculus
The concept of an even-handed structure, and the ‘moduli space’ ofeven-handed structures on a given 2-category, have geometricovertones.
Personal motivation: needed an even-handed structure on 2Hilb tomake the 2-character into a functor
χ : 2Rep(G )︸ ︷︷ ︸homotopy category
→ BunG (G )
![Page 76: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/76.jpg)
Summary
Investigating duality in 2-categories from the paradigm of TQFT.
The concept of an even-handed structure
is economicalis flexiblerequires no awkward fixed choices of adjoints, so fits examples like2Hilb and CYau.goes hand in hand with a powerful string-diagram calculus
The concept of an even-handed structure, and the ‘moduli space’ ofeven-handed structures on a given 2-category, have geometricovertones.
Personal motivation: needed an even-handed structure on 2Hilb tomake the 2-character into a functor
χ : 2Rep(G )︸ ︷︷ ︸homotopy category
→ BunG (G )
![Page 77: Aspects of duality in 2-categories · Aspects of duality in 2-categories Bruce Bartlett PhD student, supervisor Simon Willerton She eld University Categories, Logic and Foundations](https://reader033.vdocuments.us/reader033/viewer/2022042018/5e7641036897115b2814012e/html5/thumbnails/77.jpg)
Reference
J. Baez and J. Dolan, Higher dimensional algebra and topologicalquantum field theory, arXiv:q-alg/9503002.J. Baez, Higher dimensional algebra II: 2-Hilbert spaces,arXiv:q-alg/9609018.M. Muger, From Subfactors to Categories and Topology I. Frobeniusalgebras in and Morita equivalence of tensor categories,arXiv:math/0111204.P. Etingof, D. Nikshych and V. Ostrik, On fusion categories,arXiv:math/0203060v9.A. Caldararu and S. Willerton, The Mukai Pairing, I : A categoricalapproach, arXiv/0707.2052.M. Boyarchenko, notes for the series of lectures ‘Introduction tomodular categories’,www.math.uchicago.edu/~mitya/langlands.html.N. Ganter, M. Kapranov, Representation and character theory in2-categories, arXiv/math.KT/0602510.J. Roberts and S. Willerton, On the Rozansky-Witten weight systems,arXiv.org/math/0602653.