This document provides an overview of representation theory of monoidal categories, also known as cell theory for monoidal categories. It begins with an introduction motivating the generalization of representation theory from groups to monoids and monoidal categories. It then discusses examples of finitary and fiat monoidal categories, including representation categories of finite groups and diagram categories. The document concludes with an introduction to cells in monoidal categories, which are equivalence classes that measure information loss.
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Representation theory of monoidal categories
1. Representation theory of monoidal categories
Or: Cell theory for monoidal categories
Daniel Tubbenhauer
Part 1: Reps of monoids; Part 2: Reps of algebras
August 2022
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 1 / 7
2. Where do we want to go?
Groups Monoids
Fusion mon-
oidal cats
Fiat mon-
oidal cats
Group reps Monoid reps
Fusion reps Fiat reps
generalize
“categorify”
rep theory
I Green, Clifford, Munn, Ponizovskiı̆ ∼1940+
+ + many others
Representation theory of (finite) monoids
I Goal Find some categorical analog
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 2 / 7
3. Where do we want to go?
Groups Monoids
Fusion mon-
oidal cats
Fiat mon-
oidal cats
Group reps Monoid reps
Fusion reps Fiat reps
generalize
“categorify”
rep theory
I Green, Clifford, Munn, Ponizovskiı̆ ∼1940+
+ + many others
Representation theory of (finite) monoids
I Goal Find some categorical analog
“Categorify”
!
is motivated by
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 2 / 7
4. Where do we want to go?
Stasheff polytopes ⇒ I can ignore associators
I Today Cell theory for monoidal categories
I Instead of Rep(G, K) we study Rep(Rep(G, K))
I Examples we discuss Rep(G, K) and S (V ⊗d
|d ∈ N) (“diagram cats”)
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 2 / 7
5. Where do we want to go?
Stasheff polytopes ⇒ I can ignore associators
I Today Cell theory for monoidal categories
I Instead of Rep(G, K) we study Rep(Rep(G, K))
I Examples we discuss Rep(G, K) and S (V ⊗d
|d ∈ N) (“diagram cats”)
The categories in this talk
Categories are monoidal (strict or nonstrict, I won’t be very careful)
Categories are K-linear over some field K
Categories are additive ⊕
Categories are idempotent complete ⊂
⊕
Hom spaces are finite dimensional dimK < ∞
Categories have finitely many indecomposable objects (up to iso)
Not always, but sometimes categories have dualities ?
(rigid, pivotal etc.)
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 2 / 7
6. Where do we want to go?
Stasheff polytopes ⇒ I can ignore associators
I Today Cell theory for monoidal categories
I Instead of Rep(G, K) we study Rep(Rep(G, K))
I Examples we discuss Rep(G, K) and S (V ⊗d
|d ∈ N) (“diagram cats”)
The categories in this talk
Categories are monoidal (strict or nonstrict, I won’t be very careful)
Categories are K-linear over some field K
Categories are additive ⊕
Categories are idempotent complete ⊂
⊕
Hom spaces are finite dimensional dimK < ∞
Categories have finitely many indecomposable objects (up to iso)
Not always, but sometimes categories have dualities ?
(rigid, pivotal etc.)
Everything has a bicategory version
but I completely ignore that!
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 2 / 7
7. Where do we want to go?
Stasheff polytopes ⇒ I can ignore associators
I Today Cell theory for monoidal categories
I Instead of Rep(G, K) we study Rep(Rep(G, K))
I Examples we discuss Rep(G, K) and S (V ⊗d
|d ∈ N) (“diagram cats”)
The categories in this talk
Categories are monoidal (strict or nonstrict, I won’t be very careful)
Categories are K-linear over some field K
Categories are additive ⊕
Categories are idempotent complete ⊂
⊕
Hom spaces are finite dimensional dimK < ∞
Categories have finitely many indecomposable objects (up to iso)
Not always, but sometimes categories have dualities ?
(rigid, pivotal etc.)
Everything has a bicategory version
but I completely ignore that!
Examples
V ec
V ecG /V ecS for a finite group G/monoid S
Rep(G, C), P roj(G, K) or I nj(G, K) for a finite group G
Rep(G, K) for a finite group G sometimes works (details in a sec)
Rep(S, K) for a finite monoid S sometimes (but rarely) works
Categories S (V ⊗d
|d ∈ N) with ⊗-generator V sometimes work (details later)
Quotients of tilting module categories
Projective functor categories CA
Soergel bimodules S bim for finite Coxeter types
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 2 / 7
8. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = Rep(G, K)
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces
I S often has infinitely many indecomposable objects
I S has dualities
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
9. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = Rep(G, K)
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces
I S often has infinitely many indecomposable objects
I S has dualities
finitary
fiat
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
10. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = Rep(S, K)
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces
I S often has infinitely many indecomposable objects (even for K = C)
I S has no dualities in general
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
11. Finitary/fiat monoidal cats
Z1 ! Z2 ! Z3 !
Z4 ! Z5 !
I Take G = Z/5Z and K = F5, then K[G] ∼
= K[X]/(X5
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has five indecomposable objects ⇒ fiat
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
12. Finitary/fiat monoidal cats
Z2l :
Z2l+1 :
I Take G = Z/2Z×Z/2Z and K = F2, then K[G] ∼
= K[X, Y ]/(X2
, Y 2
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has infinitely many indecomposable objects ⇒ not fiat
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
13. Finitary/fiat monoidal cats
Z2l :
Z2l+1 :
I Take G = Z/2Z×Z/2Z and K = F2, then K[G] ∼
= K[X, Y ]/(X2
, Y 2
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has infinitely many indecomposable objects ⇒ not fiat
Theorem (Higman ∼1954)
Rep(G, K) is fiat if and only if either
(a) char(K) does not divide |G|
or
(b) char(K) = p divides |G| and the p-Sylow subgroups of G are cyclic
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
14. Finitary/fiat monoidal cats
Z2l :
Z2l+1 :
I Take G = Z/2Z×Z/2Z and K = F2, then K[G] ∼
= K[X, Y ]/(X2
, Y 2
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has infinitely many indecomposable objects ⇒ not fiat
Theorem (Higman ∼1954)
Rep(G, K) is fiat if and only if either
(a) char(K) does not divide |G|
or
(b) char(K) = p divides |G| and the p-Sylow subgroups of G are cyclic
Examples and nonexamples
Rep(S3, F2), Rep(Dodd, F2) are fiat
Rep(S4, F2), Rep(Deven, F2) are not fiat
Blue circle = cyclic subgroups, green = 2-Sylows
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
15. Finitary/fiat monoidal cats
Z2l :
Z2l+1 :
I Take G = Z/2Z×Z/2Z and K = F2, then K[G] ∼
= K[X, Y ]/(X2
, Y 2
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has infinitely many indecomposable objects ⇒ not fiat
Theorem (Higman ∼1954)
Rep(G, K) is fiat if and only if either
(a) char(K) does not divide |G|
or
(b) char(K) = p divides |G| and the p-Sylow subgroups of G are cyclic
Together with P roj(G, K) and I nj(G, K) (these are always fiat )
Higman’s theorem provides many examples of fiat categories
A Higman theorem for monoids is widely open
but one shouldn’t expect it too be very nice, e.g.
Tn has finite representation type over C ⇔ n ≤ 4
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
16. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = S (V ⊗d
|d ∈ N) (+ K-linear + ⊕ + ⊂
⊕ ) for some nice V
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces (
I S often has infinitely many indecomposable objects
I S has dualities ( ) depends but is easy to check
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
17. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = S (V ⊗d
|d ∈ N) (+ K-linear + ⊕ + ⊂
⊕ ) for some nice V
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces (
I S often has infinitely many indecomposable objects
I S has dualities ( ) depends but is easy to check
Almost examples
Temperley–Lieb (TL), Brauer or Deligne categories
and other diagram categories in the same spirit
Catch These usually have infinitely many indecomposable objects
⇒ truncate these appropriately
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
18. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = S (V ⊗d
|d ∈ N) (+ K-linear + ⊕ + ⊂
⊕ ) for some nice V
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces (
I S often has infinitely many indecomposable objects
I S has dualities ( ) depends but is easy to check
Example/Theorem (Alperin, Kovács ∼1979)
“Finite TL”, i.e. V any simple of G = SL2(Fpk ) over characteristic p
S (V ⊗d
|d ∈ N) is fiat , e.g. p = 5, K = F5, k = 2, V = (F25)2
:
simples in Rep(G, K):
indecomposables in Rep(G, K):
indecomposables in S (V ⊗d
|d ∈ N):
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
19. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = S (V ⊗d
|d ∈ N) (+ K-linear + ⊕ + ⊂
⊕ ) for some nice V
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces (
I S often has infinitely many indecomposable objects
I S has dualities ( ) depends but is easy to check
Example/Theorem (folklore)
V any 2d simple of a finite group G
S (V ⊗d
|d ∈ N) is finitary ,
e.g. K = F2, V the two dim simple of G = D6:
simples in Rep(G, K):
indecomposables in Rep(G, K):
indecomposables in S (V ⊗d
|d ∈ N):
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
20. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = S (V ⊗d
|d ∈ N) (+ K-linear + ⊕ + ⊂
⊕ ) for some nice V
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces (
I S often has infinitely many indecomposable objects
I S has dualities ( ) depends but is easy to check
Algebraic modules à la Alperin
provide many examples of finitary/fiat “diagram lookalike cats”
The state of the arts for algebraic modules is roughly the same as for algebraic numbers:
there are some results, but not so many
In the monoid case next to nothing is known
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
21. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = S (V ⊗d
|d ∈ N) (+ K-linear + ⊕ + ⊂
⊕ ) for some nice V
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces (
I S often has infinitely many indecomposable objects
I S has dualities ( ) depends but is easy to check
Example/Theorem (Craven ∼2013)
V any simple of M11 in characteristic 2
S (V ⊗d
|d ∈ N) is finitary ,
e.g. V the 10 dim simple of G = M11:
simples in Rep(G, K):
indecomposables in Rep(G, K):
indecomposables in S (V ⊗d
|d ∈ N):
There are many similar results known, but they all look a bit random, e.g.
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
22. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I H-cells = intersections of left and right cells
I Slogan Cells measure information loss
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
23. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I H-cells = intersections of left and right cells
I Slogan Cells measure information loss
Green cells in categories
B = {X, Y, Z, ...} set of indecomposables of a finitary monoidal category S
⊂
⊕ = is direct summand of
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
24. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
25. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example (Rep(S3, C))
Indecomposable objects Z1
∼
= 1 ! , Z2 ! , Z3 !
1 ⊂
⊕ ⊗ ⇒ is in the lowest cell
1 ⊂
⊕ ⊗ ⇒ is in the lowest cell
Only one cell
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
26. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example (Rep(S3, C))
Indecomposable objects Z1
∼
= 1 ! , Z2 ! , Z3 !
1 ⊂
⊕ ⊗ ⇒ is in the lowest cell
1 ⊂
⊕ ⊗ ⇒ is in the lowest cell
Only one cell
Example (Rep(G, C))
1 ⊂
⊕ Z ⊗ Z∗
⇒ Z is in the lowest cell
Only one cell
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
27. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example (Rep(S3, C))
Indecomposable objects Z1
∼
= 1 ! , Z2 ! , Z3 !
1 ⊂
⊕ ⊗ ⇒ is in the lowest cell
1 ⊂
⊕ ⊗ ⇒ is in the lowest cell
Only one cell
Example (Rep(G, C))
1 ⊂
⊕ Z ⊗ Z∗
⇒ Z is in the lowest cell
Only one cell
Example (semisimple + dual (replaces ( )−1
))
1 ⊂
⊕ Z ⊗ Z∗
⇒ Z is in the lowest cell
Only one cell
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
28. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example (S (V ⊗d
|d ∈ N) for the 2d simple S3 rep over F2)
Indecomposable objects Z1
∼
= 1 ! , Z2 ! , Z3 = P(1)
⊗ ∼
= Z3 ⊕ Z3
⊗ Z3
∼
= ⊕ Z3
Z3 ⊗ Z3
∼
= ⊕
Two cells
Z2, Z3
1
Jt
Jb
SH
∼
= ??
SH
∼
= V ec
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
29. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example (S (V ⊗d
|d ∈ N) for the 2d simple S3 rep over F2)
Indecomposable objects Z1
∼
= 1 ! , Z2 ! , Z3 = P(1)
⊗ ∼
= Z3 ⊕ Z3
⊗ Z3
∼
= ⊕ Z3
Z3 ⊗ Z3
∼
= ⊕
Two cells
Z2, Z3
1
Jt
Jb
SH
∼
= ??
SH
∼
= V ec
In general, for S ⊂ Rep(G, K)
the top J cell is the cell of projectives
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
30. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example (S (V ⊗d
|d ∈ N) for the 2d simple S3 rep over F2)
Indecomposable objects Z1
∼
= 1 ! , Z2 ! , Z3 = P(1)
⊗ ∼
= Z3 ⊕ Z3
⊗ Z3
∼
= ⊕ Z3
Z3 ⊗ Z3
∼
= ⊕
Two cells
Z2, Z3
1
Jt
Jb
SH
∼
= ??
SH
∼
= V ec
In general, for S ⊂ Rep(G, K)
the top J cell is the cell of projectives
Warning
For S ⊂ Rep(S, K)
the top J cell is usually not the cell of projectives
Dualities are helpful
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
31. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example/theorem (folklore)
S (V ⊗d
|d ∈ N) for “finite TL” over Fpk
There are (k + 1) cells
Zpk −1, ..., Z2pk −2
.
.
.
Zp3−1, ..., Zp4−2
Zp2−1, ..., Zp3−2
Zp−1, ..., Zp2−2
Z0 = 1, ..., Zp−2
Jt
J3
J2
J1
Jb
SH
∼
= V erpk
SH
∼
= V erp3
SH
∼
= V erp2
SH
∼
= V erp
SH
∼
= V er
where V er is the semisimplification of SL2(Fp) tilting modules
and the other SH are “higher” Verlinde cats
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
32. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example (projective functors)
A some reasonable algebra, 1 = e1 + e2 primitive orthogonal idempotents
CA finitary monoidal category of projective functors + id functor
There are 2 cells
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
33. Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example (Soergel bimodules)
S bim is fiat monoidal category for finite Coxeter type
Cells = p cells
For type B2 (dihedral group D4) one has e.g.
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
34. Reps of monoidal cats
Frobenius: act on linear spaces
Schur: act on projective spaces
Varying the source/target gives slightly different theories
I Start with examples In a sec
I Choose the type of categories you want to represent Finitary/fiat monoidal
I Choose the type of categories you want as a target Finitary
I Build a theory Depends crucially on the setting
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
35. Reps of monoidal cats
Frobenius: act on linear spaces
Schur: act on projective spaces
Varying the source/target gives slightly different theories
I Start with examples In a sec
I Choose the type of categories you want to represent Finitary/fiat monoidal
I Choose the type of categories you want as a target Finitary
I Build a theory Depends crucially on the setting
Some flavors, varying source/target
Categorical reps of groups (subfactors, fusion cats, etc.)
à la Jones, Ocneanu, Popa, others ∼1990
Categorical reps of Lie groups/Lie algebras
à la Chuang–Rouquier, Khovanov–Lauda, others ∼2000
Categorical reps of algebras ( abelian , tensor cats, etc.)
à la Etingof, Nikshych, Ostrik, others ∼2000
Categorical reps of monoids/algebras ( additive , finitary/fiat monoidal cats, etc.)
à la Mazorchuk, Miemietz, others ∼2010
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
36. Reps of monoidal cats
I Let S = Rep(G, K)
I The regular cat module M: S → End(S ):
M //
f
M ⊗
f ⊗
N // N ⊗
I The decategorification is an N -module
Example (G = S3, K = C)
Z1
∼
= 1 ! , Z2 ! , Z3 !
[M(Z1)] !
1 0 0
0 1 0
0 0 1
, [M(Z2)] !
0 1 0
1 1 1
0 1 0
, [M(Z3)] !
0 0 1
0 1 0
1 0 0
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
37. Reps of monoidal cats
I Let K ⊂ G be a subgroup
I Rep(K, K) is a cat module of Rep(G, K) via
M(K, 1) = ResG
K ⊗ : Rep(G, K) → End Rep(K, K)
,
M //
f
ResG
K (M) ⊗
ResG
K (f )⊗
N // ResG
K (N) ⊗
I The decategorifications are N -modules
Example (G = S3, K = S2, K = C, M = M(K, 1))
→ , → ⊕ , →
[M(Z1)] !
1 0
0 1
, [M(Z2)] !
1 1
1 1
, [M(Z3)] !
0 1
1 0
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
38. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
39. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
M(K, ϕ) are solutions to equations on the Grothendieck level
and
the categorical level
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
40. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
M(K, ϕ) are solutions to equations on the Grothendieck level
and
the categorical level
Goal
Find some setting where M(K, ϕ) naturally fit into
(I really like them!)
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
41. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Source/target
I want finitary/fiat categories to act
My target categories are finitary
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
42. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Source/target
I want finitary/fiat categories to act
My target categories are finitary
Decat
M is called transitive if it is nonzero and is generated by any nonzero X
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
43. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Source/target
I want finitary/fiat categories to act
My target categories are finitary
Decat
M is called transitive if it is nonzero and is generated by any nonzero X
Cat
M is called simple (transitive) if there are no nontrivial S -stable ideals
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
44. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Source/target
I want finitary/fiat categories to act
My target categories are finitary
Decat
M is called transitive if it is nonzero and is generated by any nonzero X
Cat
M is called simple (transitive) if there are no nontrivial S -stable ideals
Example (Rep(S3, C) and M = M(S3, φ))
M is transitive because T = Z1 ⊕ Z2 ⊕ Z3 has a connected action matrix
T !
1 1 1
1 2 1
1 1 1
!
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
45. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Source/target
I want finitary/fiat categories to act
My target categories are finitary
Decat
M is called transitive if it is nonzero and is generated by any nonzero X
Cat
M is called simple (transitive) if there are no nontrivial S -stable ideals
Example (Rep(S3, C) and M = M(S3, φ))
M is transitive because T = Z1 ⊕ Z2 ⊕ Z3 has a connected action matrix
T !
1 1 1
1 2 1
1 1 1
!
Example (Rep(S3, C) and M = M(S3, φ))
M is simple because its transitive and hom spaces are boring
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
46. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Theorem (Mazorchuk–Miemietz ∼2014)
In the correct framework
cat reps satisfy a (weak) Jordan–Hölder theorem wrt simple cat reps
(weak = get transitive subquotients and kill ideals)
Goal
For fixed S , find the periodic table of simple cat reps
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
47. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Theorem (Ocneanu ∼1990, folklore)
Completeness
All simples of Rep(G, C) are of the form M(K, ϕ).
Non-redundancy
We have M(K, ϕ) ∼
= M(K0
, ϕ0
) ⇔ the subgroups and cocycles are conjugate
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
48. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Theorem (Ocneanu ∼1990, folklore)
Completeness
All simples of Rep(G, C) are of the form M(K, ϕ).
Non-redundancy
We have M(K, ϕ) ∼
= M(K0
, ϕ0
) ⇔ the subgroups and cocycles are conjugate
Example (G = S3 at the top, G = S4 at the bottom)
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
49. Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Example/theorem (Etingof, Ostrik ∼2003)
The Hopf algebra T = hg, z|gn
= 1, zn
= 0, gz = ζzgi
for a primitive complex nth root of unity ζ ∈ C
T is the Taft algebra (a well known but nasty example in Hopf algebras)
Rep(T, C) is fiat monoidal with two cells
Rep(T, C) has infinitely many simple reps
but only finitely many Grothendieck classes of simple reps
There are infinity many twists of the actions
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
50. Cells and reps of monoidal cats
Clifford, Munn, Ponizovskiı̆ ∼1940+
+ H-reduction
There is a one-to-one correspondence
simples with
apex J (e)
one-to-one
←
−
−
−
−
→
simples of (any)
H(e) ⊂ J (e)
Reps of monoids are controlled by H(e) cells
I We already have cell theory in monoidal cats
I Goal Find an H-reduction in the monoidal setup
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 6 / 7
51. Cells and reps of monoidal cats
Clifford, Munn, Ponizovskiı̆ ∼1940+
+ H-reduction
There is a one-to-one correspondence
simples with
apex J (e)
one-to-one
←
−
−
−
−
→
simples of (any)
H(e) ⊂ J (e)
Reps of monoids are controlled by H(e) cells
I We already have cell theory in monoidal cats
I Goal Find an H-reduction in the monoidal setup
Duflo involution
D = D(L) is Duflo if it satisfies the universal property:
∃ γ : D → 1 such that
Fγ : FD → F right splits (Fγ ◦ s = idF ) for all F ∈ L
“Duflo involution = nonnegative pseudo idempotent”
Having a Duflo involution implies that L has a
nonnegative pseudo idempotent
= coefficients from N wrt the basis of classes of indecomposables
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 6 / 7
52. Cells and reps of monoidal cats
Clifford, Munn, Ponizovskiı̆ ∼1940+
+ H-reduction
There is a one-to-one correspondence
simples with
apex J (e)
one-to-one
←
−
−
−
−
→
simples of (any)
H(e) ⊂ J (e)
Reps of monoids are controlled by H(e) cells
I We already have cell theory in monoidal cats
I Goal Find an H-reduction in the monoidal setup
Duflo involution
D = D(L) is Duflo if it satisfies the universal property:
∃ γ : D → 1 such that
Fγ : FD → F right splits (Fγ ◦ s = idF ) for all F ∈ L
“Duflo involution = nonnegative pseudo idempotent”
Having a Duflo involution implies that L has a
nonnegative pseudo idempotent
= coefficients from N wrt the basis of classes of indecomposables
Example (Rep(G, C))
The unique Duflo involution is 1
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 6 / 7
53. Cells and reps of monoidal cats
Clifford, Munn, Ponizovskiı̆ ∼1940+
+ H-reduction
There is a one-to-one correspondence
simples with
apex J (e)
one-to-one
←
−
−
−
−
→
simples of (any)
H(e) ⊂ J (e)
Reps of monoids are controlled by H(e) cells
I We already have cell theory in monoidal cats
I Goal Find an H-reduction in the monoidal setup
Duflo involution
D = D(L) is Duflo if it satisfies the universal property:
∃ γ : D → 1 such that
Fγ : FD → F right splits (Fγ ◦ s = idF ) for all F ∈ L
“Duflo involution = nonnegative pseudo idempotent”
Having a Duflo involution implies that L has a
nonnegative pseudo idempotent
= coefficients from N wrt the basis of classes of indecomposables
Example (Rep(G, C))
The unique Duflo involution is 1
Example (S bim of dihedral type, n odd)
pseudo idempotents (left) and nonnegative pseudo idempotent (right):
(Recall from the exercises that b12 − b1212± was a pseudo idempotent)
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 6 / 7
54. Cells and reps of monoidal cats
Clifford, Munn, Ponizovskiı̆ ∼1940+
+ H-reduction
There is a one-to-one correspondence
simples with
apex J (e)
one-to-one
←
−
−
−
−
→
simples of (any)
H(e) ⊂ J (e)
Reps of monoids are controlled by H(e) cells
I We already have cell theory in monoidal cats
I Goal Find an H-reduction in the monoidal setup
Example/theorem (folklore)
S (V ⊗d
|d ∈ N) for “finite TL” over Fpk
There are (k + 1) cells
Zpk −1, ..., Z2pk −2
.
.
.
Zp3−1, ..., Zp4−2
Zp2−1, ..., Zp3−2
Zp−1, ..., Zp2−2
Z0 = 1, ..., Zp−2
Jt
J3
J2
J1
Jb
SH
∼
= V erpk
SH
∼
= V erp3
SH
∼
= V erp2
SH
∼
= V erp
SH
∼
= V er
The Steinberg modules Zpj −1 are the Duflo involutions
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 6 / 7
55. Cells and reps of monoidal cats
In spirit of Clifford, Munn, Ponizovskiı̆ ∼1940+
+ H-reduction
There is a one-to-one correspondence (currently only proven in the fiat case)
simples with
apex J
one-to-one
←
−
−
−
−
→
simples of
SH
Reps are controlled by the SH categories
I Each simple has a unique maximal J where having a pseudo idempotent is
replaced by Duflo involutions Apex
I This implies (smod means the category of simples):
S -smodJ ' SH-smod
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 6 / 7
56. Cells and reps of monoidal cats
In spirit of Clifford, Munn, Ponizovskiı̆ ∼1940+
+ H-reduction
There is a one-to-one correspondence (currently only proven in the fiat case)
simples with
apex J
one-to-one
←
−
−
−
−
→
simples of
SH
Reps are controlled by the SH categories
I Each simple has a unique maximal J where having a pseudo idempotent is
replaced by Duflo involutions Apex
I This implies (smod means the category of simples):
S -smodJ ' SH-smod
Example (Rep(G, C))
H-reduction is not really a reduction and we need Ocneanu’s classification
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 6 / 7
57. Cells and reps of monoidal cats
In spirit of Clifford, Munn, Ponizovskiı̆ ∼1940+
+ H-reduction
There is a one-to-one correspondence (currently only proven in the fiat case)
simples with
apex J
one-to-one
←
−
−
−
−
→
simples of
SH
Reps are controlled by the SH categories
I Each simple has a unique maximal J where having a pseudo idempotent is
replaced by Duflo involutions Apex
I This implies (smod means the category of simples):
S -smodJ ' SH-smod
Example (Rep(G, C))
H-reduction is not really a reduction and we need Ocneanu’s classification
Example (S bim)
H-reduction reduces the classification problem a lot
but one needs extra work to complete it (the SH are complicated)
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 6 / 7
58. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = Rep(G, K)
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces
I S often has infinitely many indecomposable objects
I S has dualities
finitary
fiat
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
Finitary/fiat monoidal cats
Z1 ! Z2 ! Z3 !
Z4 ! Z5 !
I Take G = Z/5Z and K = F5, then K[G] ∼
= K[X]/(X5
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has five indecomposable objects ⇒ fiat
finitary
fiat
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
Finitary/fiat monoidal cats
Z2l :
Z2l+1 :
I Take G = Z/2Z×Z/2Z and K = F2, then K[G] ∼
= K[X, Y ]/(X2
, Y 2
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has infinitely many indecomposable objects ⇒ not fiat
finitary
fiat
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
Finitary/fiat monoidal cats
Z2l :
Z2l+1 :
I Take G = Z/2Z×Z/2Z and K = F2, then K[G] ∼
= K[X, Y ]/(X2
, Y 2
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has infinitely many indecomposable objects ⇒ not fiat
finitary
fiat
Theorem (Higman ∼1954)
Rep(G, K) is fiat if and only if either
(a) char(K) does not divide |G|
or
(b) char(K) = p divides |G| and the p-Sylow subgroups of G are cyclic
Examples and nonexamples
Rep(S3, F2), Rep(Dodd, F2) are fiat
Rep(S4, F2), Rep(Deven, F2) are not fiat
Blue circle = cyclic subgroups, green = 2-Sylows
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = S (V ⊗d
|d ∈ N) (+ K-linear + ⊕ + ⊂
⊕ ) for some nice V
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces (
I S often has infinitely many indecomposable objects
I S has dualities ( ) depends but is easy to check
finitary
fiat
Almost examples
Temperley–Lieb (TL), Brauer or Deligne categories
and other diagram categories in the same spirit
Catch These usually have infinitely many indecomposable objects
⇒ truncate these appropriately
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example/theorem (folklore)
S (V ⊗d
|d ∈ N) for “finite TL” over Fpk
There are (k + 1) cells
Zpk −1, ..., Z2pk −2
.
.
.
Zp3−1, ..., Zp4−2
Zp2−1, ..., Zp3−2
Zp−1, ..., Zp2−2
Z0 = 1, ..., Zp−2
Jt
J3
J2
J1
Jb
SH
∼
= V erpk
SH
∼
= V erp3
SH
∼
= V erp2
SH
∼
= V erp
SH
∼
= V er
where V er is the semisimplification of SL2(Fp) tilting modules
and the other SH are “higher” Verlinde cats
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Theorem (Ocneanu ∼1990, folklore)
Completeness
All simples of Rep(G, C) are of the form M(K, ϕ).
Non-redundancy
We have M(K, ϕ) ∼
= M(K0
, ϕ0
) ⇔ the subgroups and cocycles are conjugate
Example (G = S3 at the top, G = S4 at the bottom)
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Example/theorem (Etingof, Ostrik ∼2003)
The Hopf algebra T = hg, z|gn
= 1, zn
= 0, gz = ζzgi
for a primitive complex nth root of unity ζ ∈ C
T is the Taft algebra (a well known but nasty example in Hopf algebras)
Rep(T, C) is fiat monoidal with two cells
Rep(T, C) has infinitely many simple reps
but only finitely many Grothendieck classes of simple reps
There are infinity many twists of the actions
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
Cells and reps of monoidal cats
In spirit of Clifford, Munn, Ponizovskiı̆ ∼1940+
+ H-reduction
There is a one-to-one correspondence (currently only proven in the fiat case)
simples with
apex J
one-to-one
←
−
−
−
−
→
simples of
SH
Reps are controlled by the SH categories
I Each simple has a unique maximal J where having a pseudo idempotent is
replaced by Duflo involutions Apex
I This implies (smod means the category of simples):
S -smodJ ' SH-smod
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 6 / 7
There is still much to do...
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 7 / 7
59. Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = Rep(G, K)
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces
I S often has infinitely many indecomposable objects
I S has dualities
finitary
fiat
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
Finitary/fiat monoidal cats
Z1 ! Z2 ! Z3 !
Z4 ! Z5 !
I Take G = Z/5Z and K = F5, then K[G] ∼
= K[X]/(X5
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has five indecomposable objects ⇒ fiat
finitary
fiat
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
Finitary/fiat monoidal cats
Z2l :
Z2l+1 :
I Take G = Z/2Z×Z/2Z and K = F2, then K[G] ∼
= K[X, Y ]/(X2
, Y 2
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has infinitely many indecomposable objects ⇒ not fiat
finitary
fiat
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
Finitary/fiat monoidal cats
Z2l :
Z2l+1 :
I Take G = Z/2Z×Z/2Z and K = F2, then K[G] ∼
= K[X, Y ]/(X2
, Y 2
)
I Rep(G, K) has one simple object Z1 = 1
I Rep(G, K) has infinitely many indecomposable objects ⇒ not fiat
finitary
fiat
Theorem (Higman ∼1954)
Rep(G, K) is fiat if and only if either
(a) char(K) does not divide |G|
or
(b) char(K) = p divides |G| and the p-Sylow subgroups of G are cyclic
Examples and nonexamples
Rep(S3, F2), Rep(Dodd, F2) are fiat
Rep(S4, F2), Rep(Deven, F2) are not fiat
Blue circle = cyclic subgroups, green = 2-Sylows
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
Finitary/fiat monoidal cats
Reps
!
matter
simple ! elements
indecomposable ! compounds
I Let S = S (V ⊗d
|d ∈ N) (+ K-linear + ⊕ + ⊂
⊕ ) for some nice V
I S is monoidal
I S is K-linear
I S is additive
I S is idempotent complete
I S has fin dim hom spaces (
I S often has infinitely many indecomposable objects
I S has dualities ( ) depends but is easy to check
finitary
fiat
Almost examples
Temperley–Lieb (TL), Brauer or Deligne categories
and other diagram categories in the same spirit
Catch These usually have infinitely many indecomposable objects
⇒ truncate these appropriately
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 3 / 7
Cells in monoidal cats
The categorical cell orders and equivalences for the set of indecomposables B:
Left, right and two-sided cells (a.k.a. L, R and J-cells): equivalence classes
I Get monoidal semicategories SJ , SH by killing higher order terms
I I tell you later which ones are “idempotent”
Example/theorem (folklore)
S (V ⊗d
|d ∈ N) for “finite TL” over Fpk
There are (k + 1) cells
Zpk −1, ..., Z2pk −2
.
.
.
Zp3−1, ..., Zp4−2
Zp2−1, ..., Zp3−2
Zp−1, ..., Zp2−2
Z0 = 1, ..., Zp−2
Jt
J3
J2
J1
Jb
SH
∼
= V erpk
SH
∼
= V erp3
SH
∼
= V erp2
SH
∼
= V erp
SH
∼
= V er
where V er is the semisimplification of SL2(Fp) tilting modules
and the other SH are “higher” Verlinde cats
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 4 / 7
Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Theorem (Ocneanu ∼1990, folklore)
Completeness
All simples of Rep(G, C) are of the form M(K, ϕ).
Non-redundancy
We have M(K, ϕ) ∼
= M(K0
, ϕ0
) ⇔ the subgroups and cocycles are conjugate
Example (G = S3 at the top, G = S4 at the bottom)
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
Reps of monoidal cats
I Let ϕ ∈ H2
(K, C∗
), and M(K, ϕ) be the category of projective K-modules
with Schur multiplier ϕ, i.e. a vector spaces V with ρ: K → End(V ) such that
ρ(g)ρ(h) = ϕ(g, h)ρ(gh), for all g, h ∈ K
I Note that M(K, 1) = Rep(K) and
⊗: M(K, ϕ) M(K, ψ) → M(K, ϕψ)
I M(K, ϕ) is also a cat module of S :
Rep(G, C) M(K, ϕ)
ResG
K Id
−
−
−
−
−
−
→ Rep(K) M(K, ϕ)
⊗
−
→ M(K, ϕ)
I The decategorifications are N -modules – the same ones from before!
Example/theorem (Etingof, Ostrik ∼2003)
The Hopf algebra T = hg, z|gn
= 1, zn
= 0, gz = ζzgi
for a primitive complex nth root of unity ζ ∈ C
T is the Taft algebra (a well known but nasty example in Hopf algebras)
Rep(T, C) is fiat monoidal with two cells
Rep(T, C) has infinitely many simple reps
but only finitely many Grothendieck classes of simple reps
There are infinity many twists of the actions
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 5 / 7
Cells and reps of monoidal cats
In spirit of Clifford, Munn, Ponizovskiı̆ ∼1940+
+ H-reduction
There is a one-to-one correspondence (currently only proven in the fiat case)
simples with
apex J
one-to-one
←
−
−
−
−
→
simples of
SH
Reps are controlled by the SH categories
I Each simple has a unique maximal J where having a pseudo idempotent is
replaced by Duflo involutions Apex
I This implies (smod means the category of simples):
S -smodJ ' SH-smod
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 6 / 7
Thanks for your attention!
Cell theory for monoidal categories Representation theory of monoidal categories August 2022 7 / 7