Twisted Calabi-Yau and Artin-Schelter regular properties ... · Manuel L. Reyes BIRS | 9/15/2016...
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Twisted Calabi-Yau and Artin-Schelter regular propertiesfor locally finite graded algebras
Manuel L. Reyes
BIRS — 9/15/2016
Joint work with Daniel Rogalski
Manny Reyes Twisted CY and AS regular properties September 15, 2016 1 / 32
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1 Noncommutative polynomial algebras: two candidates
2 Basics of twisted Calabi-Yau algebras
3 Locally finite algebras
4 “Generalized AS regular” versus twisted CY algebras
5 Twisted CY algebras in dimensions 1 and 2
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Noncommutative polynomial algebras
What kind of noncommutative graded algebras A deserve to be viewed as“noncommutative polynomials”? (k = an arbitrary field.)
Notes: (1) We allow GKdim(A) =∞.(2) Our graded algebras are all N-graded: A =
⊕∞n=0 An.
Two candidates:
1 Artin-Schelter regular algebras
2 Graded twisted Calabi-Yau algebras
Q: How do these compare?
Same if A is connected: A0 = k .
Today’s talk: What happens when A is not connected?
Manny Reyes Twisted CY and AS regular properties September 15, 2016 3 / 32
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Noncommutative polynomial algebras
What kind of noncommutative graded algebras A deserve to be viewed as“noncommutative polynomials”? (k = an arbitrary field.)
Notes: (1) We allow GKdim(A) =∞.(2) Our graded algebras are all N-graded: A =
⊕∞n=0 An.
Two candidates:
1 Artin-Schelter regular algebras
2 Graded twisted Calabi-Yau algebras
Q: How do these compare?
Same if A is connected: A0 = k .
Today’s talk: What happens when A is not connected?
Manny Reyes Twisted CY and AS regular properties September 15, 2016 3 / 32
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Noncommutative polynomial algebras
What kind of noncommutative graded algebras A deserve to be viewed as“noncommutative polynomials”? (k = an arbitrary field.)
Notes: (1) We allow GKdim(A) =∞.(2) Our graded algebras are all N-graded: A =
⊕∞n=0 An.
Two candidates:
1 Artin-Schelter regular algebras
2 Graded twisted Calabi-Yau algebras
Q: How do these compare?
Same if A is connected: A0 = k .
Today’s talk: What happens when A is not connected?
Manny Reyes Twisted CY and AS regular properties September 15, 2016 3 / 32
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Non-connected algebras: an apology
Why should we care about non-connected algebras?
“Intrinsic” examples: Quivers algebras with relations kQ/I havenontrivial idempotents. (And their associated derived categories can beuseful.)
“Extrinsic” examples: Twisted group algebras (or smash products)constructed from A can contain idempotents, even if A does not.
While nontrivial idempotents make these much less “like polynomials,” it’sstill useful to understand when they are “homologically nice.”
Manny Reyes Twisted CY and AS regular properties September 15, 2016 4 / 32
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1 Noncommutative polynomial algebras: two candidates
2 Basics of twisted Calabi-Yau algebras
3 Locally finite algebras
4 “Generalized AS regular” versus twisted CY algebras
5 Twisted CY algebras in dimensions 1 and 2
Manny Reyes Twisted CY and AS regular properties September 15, 2016 5 / 32
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Preliminaries: the enveloping algebra
The enveloping algebra of A is Ae = A⊗ Aop. A left/right Ae-module Mis the same as a k-central (A,A)-bimodule:
(a⊗ bop) ·m = a ·m · b = m · (b ⊗ aop)
Provides a convenient way to discuss homological algebra for bimodules:
Projective/injective bimodules ! Projective/injective Ae-modules
Resolutions of (A,A)-bimodules ! resolutions of Ae-modules
Def: A is homologically smooth if A has a projective resolution in Ae-Modof finite length whose terms are finitely generated over Ae . (A is a perfectAe-module.)
This implies finite global dimension.
Manny Reyes Twisted CY and AS regular properties September 15, 2016 6 / 32
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Preliminaries: the enveloping algebra
The enveloping algebra of A is Ae = A⊗ Aop. A left/right Ae-module Mis the same as a k-central (A,A)-bimodule:
(a⊗ bop) ·m = a ·m · b = m · (b ⊗ aop)
Provides a convenient way to discuss homological algebra for bimodules:
Projective/injective bimodules ! Projective/injective Ae-modules
Resolutions of (A,A)-bimodules ! resolutions of Ae-modules
Def: A is homologically smooth if A has a projective resolution in Ae-Modof finite length whose terms are finitely generated over Ae . (A is a perfectAe-module.)
This implies finite global dimension.
Manny Reyes Twisted CY and AS regular properties September 15, 2016 6 / 32
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Preliminaries: the enveloping algebra
The enveloping algebra of A is Ae = A⊗ Aop. A left/right Ae-module Mis the same as a k-central (A,A)-bimodule:
(a⊗ bop) ·m = a ·m · b = m · (b ⊗ aop)
Provides a convenient way to discuss homological algebra for bimodules:
Projective/injective bimodules ! Projective/injective Ae-modules
Resolutions of (A,A)-bimodules ! resolutions of Ae-modules
Def: A is homologically smooth if A has a projective resolution in Ae-Modof finite length whose terms are finitely generated over Ae . (A is a perfectAe-module.)
This implies finite global dimension.
Manny Reyes Twisted CY and AS regular properties September 15, 2016 6 / 32
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Calabi-Yau and twisted CY algebras
Definition
(i) A is twisted Calabi-Yau of dimension d if it is homologically smoothand there is an invertible (A,A)-bimodule U such that, as Ae-modules,
ExtiAe (A,Ae) ∼=
{0 if i 6= d ,
U if i = d .
(ii) [Ginzburg] A is Calabi-Yau of dimension d if it twisted CY ofdimension d with U = A.
The CY condition is “self-duality” of sorts: if P• → A→ 0 is a projectiveAe-resolution, then HomAe (P•,A
e) is also a resolution of A.
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Calabi-Yau and twisted CY algebras
Definition
(i) A is twisted Calabi-Yau of dimension d if it is homologically smoothand there is an invertible (A,A)-bimodule U such that, as Ae-modules,
ExtiAe (A,Ae) ∼=
{0 if i 6= d ,
U if i = d .
(ii) [Ginzburg] A is Calabi-Yau of dimension d if it twisted CY ofdimension d with U = A.
The CY condition is “self-duality” of sorts: if P• → A→ 0 is a projectiveAe-resolution, then HomAe (P•,A
e) is also a resolution of A.
Manny Reyes Twisted CY and AS regular properties September 15, 2016 7 / 32
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Commutative examples of Calabi-Yau algebras
(1) Calabi-Yau varieties: Coordinate rings of smooth affine Calabi-Yauvarieties are CY algebras [Ginzburg]
We can also consider graded Calabi-Yau algebras: take the projectiveAe-resolution and Ext isomorphism to be in the graded category.
(2) Graded commutative examples: just direct sums of k[x1, . . . , xn].
We emphasize (2): So graded Calabi-Yau algebras are “noncommutativepolynomial rings.”
But so are the Artin-Schelter regular algebras. How do these compare?
Manny Reyes Twisted CY and AS regular properties September 15, 2016 8 / 32
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Commutative examples of Calabi-Yau algebras
(1) Calabi-Yau varieties: Coordinate rings of smooth affine Calabi-Yauvarieties are CY algebras [Ginzburg]
We can also consider graded Calabi-Yau algebras: take the projectiveAe-resolution and Ext isomorphism to be in the graded category.
(2) Graded commutative examples: just direct sums of k[x1, . . . , xn].
We emphasize (2): So graded Calabi-Yau algebras are “noncommutativepolynomial rings.”
But so are the Artin-Schelter regular algebras. How do these compare?
Manny Reyes Twisted CY and AS regular properties September 15, 2016 8 / 32
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Artin-Schelter regular algebras
The more standard notion of “noncommutative polynomial algebra.”
Def: A connected graded algebra A is Artin-Schelter (AS) regular ofdimension d if A has global dimension d <∞ and
ExtiA(k,A) ∼=
{0, i 6= d ,
k(`), i = d
in Mod -A, and similarly for ExtiAop(k,A). (We allow GKdim(A) =∞.)
Many examples already discussed at this conference!
How does this compare with the CY condition?
Theorem [Yekutieli & Zhang], [R., Rogalski, Zhang]: A connected gradedalgebra is twisted CY-d if and only if it is AS regular of dimension d .
So twisted CY yields the expected “noncommutative polynomial algebras.”
Manny Reyes Twisted CY and AS regular properties September 15, 2016 9 / 32
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Artin-Schelter regular algebras
The more standard notion of “noncommutative polynomial algebra.”
Def: A connected graded algebra A is Artin-Schelter (AS) regular ofdimension d if A has global dimension d <∞ and
ExtiA(k,A) ∼=
{0, i 6= d ,
k(`), i = d
in Mod -A, and similarly for ExtiAop(k,A). (We allow GKdim(A) =∞.)
Many examples already discussed at this conference!
How does this compare with the CY condition?
Theorem [Yekutieli & Zhang], [R., Rogalski, Zhang]: A connected gradedalgebra is twisted CY-d if and only if it is AS regular of dimension d .
So twisted CY yields the expected “noncommutative polynomial algebras.”
Manny Reyes Twisted CY and AS regular properties September 15, 2016 9 / 32
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Algebras from quivers and potentials
Quiver algebras: Quiver algebras with (twisted) superpotentials tend togive rise to (twisted) CY algebras.
Ex: [Bocklandt] For Q =a1,a2
!)
a3,a4
ai and the superpotential
W =∑ (a1a3a2a4 + a1a4a2a3), the Jacobi algebra B = CQ/(∂aW ) is
Calabi-Yau of dimension 3.
Relations:
∂a1W = 0 a3a2a4 = −a4a2a3
∂a2W = 0 a4a1a3 = −a2a3a1
∂a3W = 0 a2a4a1 = −a1a4a2
∂a4W = 0 a1a3a2 = −a2a3a1
Q: When is a superpotential “nice”? Hard in general, but answered forconnected CY-3 algebras by [Mori & Smith], [Mori & Ueyama].
Manny Reyes Twisted CY and AS regular properties September 15, 2016 10 / 32
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Algebras from quivers and potentials
Quiver algebras: Quiver algebras with (twisted) superpotentials tend togive rise to (twisted) CY algebras.
Ex: [Bocklandt] For Q =a1,a2
!)
a3,a4
ai and the superpotential
W =∑ (a1a3a2a4 + a1a4a2a3), the Jacobi algebra B = CQ/(∂aW ) is
Calabi-Yau of dimension 3. Relations:
∂a1W = 0 a3a2a4 = −a4a2a3
∂a2W = 0 a4a1a3 = −a2a3a1
∂a3W = 0 a2a4a1 = −a1a4a2
∂a4W = 0 a1a3a2 = −a2a3a1
Q: When is a superpotential “nice”? Hard in general, but answered forconnected CY-3 algebras by [Mori & Smith], [Mori & Ueyama].
Manny Reyes Twisted CY and AS regular properties September 15, 2016 10 / 32
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Algebras from quivers and potentials
Quiver algebras: Quiver algebras with (twisted) superpotentials tend togive rise to (twisted) CY algebras.
Ex: [Bocklandt] For Q =a1,a2
!)
a3,a4
ai and the superpotential
W =∑ (a1a3a2a4 + a1a4a2a3), the Jacobi algebra B = CQ/(∂aW ) is
Calabi-Yau of dimension 3. Relations:
∂a1W = 0 a3a2a4 = −a4a2a3
∂a2W = 0 a4a1a3 = −a2a3a1
∂a3W = 0 a2a4a1 = −a1a4a2
∂a4W = 0 a1a3a2 = −a2a3a1
Q: When is a superpotential “nice”? Hard in general, but answered forconnected CY-3 algebras by [Mori & Smith], [Mori & Ueyama].
Manny Reyes Twisted CY and AS regular properties September 15, 2016 10 / 32
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Constructions preserving twisted CY property
Direct and tensor products:
Theorem
Thm: Let A1 and A2 be twisted Calabi-Yau algebras of dimension d1 andd2, respectively.
If d1 = d2 = d, then A1 × A2 is twisted CY of dimension d.
A1 ⊗ A2 is twisted CY of dimension d1 + d2.
Extension of scalars and Morita equivalence:
Theorem
Thm: Let A be twisted CY of dimension d.
A⊗ K is twisted CY-d for every field extension K/k.
Every algebra Morita equivalent to A is twisted CY-d.
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Constructions preserving twisted CY property
Direct and tensor products:
Theorem
Thm: Let A1 and A2 be twisted Calabi-Yau algebras of dimension d1 andd2, respectively.
If d1 = d2 = d, then A1 × A2 is twisted CY of dimension d.
A1 ⊗ A2 is twisted CY of dimension d1 + d2.
Extension of scalars and Morita equivalence:
Theorem
Thm: Let A be twisted CY of dimension d.
A⊗ K is twisted CY-d for every field extension K/k.
Every algebra Morita equivalent to A is twisted CY-d.
Manny Reyes Twisted CY and AS regular properties September 15, 2016 11 / 32
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An example with U nontrivial
How do we find examples with U 6= 1Aµ(l)?
Ex: Set B = k[x , y ]o Z2: twisted CY-2 with a Nakayama automorphismand B0 = ke1 ⊕ ke2 (here char(k) 6= 2).
M2(B) has 1 = f1 + f2 + f3 + f4 for primitive idempotents
f1 =
(e1 00 0
), f2 =
(e2 00 0
), f3 =
(0 00 e1
), f4 =
(0 00 e2
)Set e = f1 + f2 + f3 (full idempotent), then A = eM2(B)e is Moritaequivalent to A and thus is twisted CY-2.
Have indecomposable decomposition as projective right modules
AA∼= P ⊕ P ⊕ Q but UA
∼= P ⊕ Q ⊕ Q.
So UA not free =⇒ U � 1Aµ(`).
Manny Reyes Twisted CY and AS regular properties September 15, 2016 12 / 32
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An example with U nontrivial
How do we find examples with U 6= 1Aµ(l)?
Ex: Set B = k[x , y ]o Z2: twisted CY-2 with a Nakayama automorphismand B0 = ke1 ⊕ ke2 (here char(k) 6= 2).
M2(B) has 1 = f1 + f2 + f3 + f4 for primitive idempotents
f1 =
(e1 00 0
), f2 =
(e2 00 0
), f3 =
(0 00 e1
), f4 =
(0 00 e2
)Set e = f1 + f2 + f3 (full idempotent), then A = eM2(B)e is Moritaequivalent to A and thus is twisted CY-2.
Have indecomposable decomposition as projective right modules
AA∼= P ⊕ P ⊕ Q but UA
∼= P ⊕ Q ⊕ Q.
So UA not free =⇒ U � 1Aµ(`).
Manny Reyes Twisted CY and AS regular properties September 15, 2016 12 / 32
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An example with U nontrivial
How do we find examples with U 6= 1Aµ(l)?
Ex: Set B = k[x , y ]o Z2: twisted CY-2 with a Nakayama automorphismand B0 = ke1 ⊕ ke2 (here char(k) 6= 2).
M2(B) has 1 = f1 + f2 + f3 + f4 for primitive idempotents
f1 =
(e1 00 0
), f2 =
(e2 00 0
), f3 =
(0 00 e1
), f4 =
(0 00 e2
)Set e = f1 + f2 + f3 (full idempotent), then A = eM2(B)e is Moritaequivalent to A and thus is twisted CY-2.
Have indecomposable decomposition as projective right modules
AA∼= P ⊕ P ⊕ Q but UA
∼= P ⊕ Q ⊕ Q.
So UA not free =⇒ U � 1Aµ(`).
Manny Reyes Twisted CY and AS regular properties September 15, 2016 12 / 32
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1 Noncommutative polynomial algebras: two candidates
2 Basics of twisted Calabi-Yau algebras
3 Locally finite algebras
4 “Generalized AS regular” versus twisted CY algebras
5 Twisted CY algebras in dimensions 1 and 2
Manny Reyes Twisted CY and AS regular properties September 15, 2016 13 / 32
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Working with locally finite algebras
We work in the setting of locally finite algebras: A =⊕
An with alldimk(An) <∞. So A0 = (arbitrary!) finite-dimensional algebra
The “good” choice for graded Nakayama’s Lemma & minimal gradedprojective resolutions (after Minamoto & Mori):
Graded Jacobson radical: J(A) = J(A0) + A≥1.
We obtain a f.d. semisimple algebra S = A/J(A) = A0/J(A0).
First problem: We’d like the f.d. algebra B := A0 to be “well behaved.”
Recall that twisted CY algebras must be homologically smooth.
Lemma: If A is homologically smooth, then so is A0.
Manny Reyes Twisted CY and AS regular properties September 15, 2016 14 / 32
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Working with locally finite algebras
We work in the setting of locally finite algebras: A =⊕
An with alldimk(An) <∞. So A0 = (arbitrary!) finite-dimensional algebra
The “good” choice for graded Nakayama’s Lemma & minimal gradedprojective resolutions (after Minamoto & Mori):
Graded Jacobson radical: J(A) = J(A0) + A≥1.
We obtain a f.d. semisimple algebra S = A/J(A) = A0/J(A0).
First problem: We’d like the f.d. algebra B := A0 to be “well behaved.”
Recall that twisted CY algebras must be homologically smooth.
Lemma: If A is homologically smooth, then so is A0.
Manny Reyes Twisted CY and AS regular properties September 15, 2016 14 / 32
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Working with locally finite algebras
We work in the setting of locally finite algebras: A =⊕
An with alldimk(An) <∞. So A0 = (arbitrary!) finite-dimensional algebra
The “good” choice for graded Nakayama’s Lemma & minimal gradedprojective resolutions (after Minamoto & Mori):
Graded Jacobson radical: J(A) = J(A0) + A≥1.
We obtain a f.d. semisimple algebra S = A/J(A) = A0/J(A0).
First problem: We’d like the f.d. algebra B := A0 to be “well behaved.”
Recall that twisted CY algebras must be homologically smooth.
Lemma: If A is homologically smooth, then so is A0.
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Graded homologically smooth algebras
Lemma: If A is homologically smooth, then so is B = A0.
How should we think about f.d. homologically smooth algebras?
Fact: If B is a f.d. algebra, then TFAE:
1 B is homologically smooth
2 pdim(Be B) <∞3 Be has finite global dimension
4 B ⊗ K has finite global dimension for every field extension K/k
Reminiscent of separable algebras S , defined by the equivalent conditions:
1 S is projective as a left Se-module
2 Se is semisimple
3 S ⊗ K is semisimple for all field extensions K/k
4 S ∼=∏n
i=1Mni (Di ) with all Z (Di )/k separable
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Graded homologically smooth algebras
Lemma: If A is homologically smooth, then so is B = A0.
How should we think about f.d. homologically smooth algebras?
Fact: If B is a f.d. algebra, then TFAE:
1 B is homologically smooth
2 pdim(Be B) <∞3 Be has finite global dimension
4 B ⊗ K has finite global dimension for every field extension K/k
Reminiscent of separable algebras S , defined by the equivalent conditions:
1 S is projective as a left Se-module
2 Se is semisimple
3 S ⊗ K is semisimple for all field extensions K/k
4 S ∼=∏n
i=1Mni (Di ) with all Z (Di )/k separable
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Graded homolgically smooth algebras
For passage between A-modules and Ae-modules, it’s important that Ahave S = A/J(A) = A0/J(A0) separable. One example:
Lemma
If A is locally finite with S separable, then gl. dim(A) is equal to
pdim(AS) = pdim(Ae A) = pdim(SA).
Fortunately, this holds in our case. (Special thanks to MathOverflow!)
Theorem (Rickard)
If B is a finite-dimensional homologically smooth algebra, thenS = B/J(B) is separable.
In particular, A twisted CY =⇒ S separable.
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Graded homolgically smooth algebras
For passage between A-modules and Ae-modules, it’s important that Ahave S = A/J(A) = A0/J(A0) separable. One example:
Lemma
If A is locally finite with S separable, then gl. dim(A) is equal to
pdim(AS) = pdim(Ae A) = pdim(SA).
Fortunately, this holds in our case. (Special thanks to MathOverflow!)
Theorem (Rickard)
If B is a finite-dimensional homologically smooth algebra, thenS = B/J(B) is separable.
In particular, A twisted CY =⇒ S separable.
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Dualities and graded socles
It’s well known that twisted CY-d algebras satisfy Van den Bergh duality:
ExtiAe (A,M) ∼= TorAe
d−i (A,U ⊗A M)
for left Ae-modules M.
We can use this to deduce further dualities for “1-sided” modules. . .
One important consequence allows us to “homologically compute” thesocle of a module:
Proposition: For A locally finite twisted CY-d and AM a graded module,there is an isomorphism of graded left S-modules
TorAd (S ,M) ∼= U−1 ⊗A soc(M).
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Dualities and graded socles
It’s well known that twisted CY-d algebras satisfy Van den Bergh duality:
ExtiAe (A,M) ∼= TorAe
d−i (A,U ⊗A M)
for left Ae-modules M.
We can use this to deduce further dualities for “1-sided” modules. . .
One important consequence allows us to “homologically compute” thesocle of a module:
Proposition: For A locally finite twisted CY-d and AM a graded module,there is an isomorphism of graded left S-modules
TorAd (S ,M) ∼= U−1 ⊗A soc(M).
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Twisted CY algebras of dimension 0
Socle formula: TorAd (S ,M) ∼= U−1 ⊗A soc(M)
Taking the case M = A yields:
Cor: If A is locally finite twisted CY-d , if d > 1 then soc(A) = 0.
For the d = 0 case we have:
Cor: For a (not necessarily graded) algebra A, TFAE:
1 A is (twisted) CY-0
2 A is twisted CY and is a finite-dimensional k-algebra
3 A is a separable k-algebra.
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Twisted CY algebras of dimension 0
Socle formula: TorAd (S ,M) ∼= U−1 ⊗A soc(M)
Taking the case M = A yields:
Cor: If A is locally finite twisted CY-d , if d > 1 then soc(A) = 0.
For the d = 0 case we have:
Cor: For a (not necessarily graded) algebra A, TFAE:
1 A is (twisted) CY-0
2 A is twisted CY and is a finite-dimensional k-algebra
3 A is a separable k-algebra.
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1 Noncommutative polynomial algebras: two candidates
2 Basics of twisted Calabi-Yau algebras
3 Locally finite algebras
4 “Generalized AS regular” versus twisted CY algebras
5 Twisted CY algebras in dimensions 1 and 2
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Existing notions of generalized regularity
What should play the role of the AS regular property for non-connectedalgebras? There is precedent in the work of:
Martinez-Villa: gl. dim(A) = d , the functors ExtdA(−,A) andExtdAop(−,A) interchange graded simple modules, and otherExtiA(S ,A) = 0 = ExtiAop(T ,A).
Minamoto & Mori: gl. dim(A) = d and there is a bimodule isomorphism:
ExtiA(A0,A) ∼=
{0, i 6= d ,
(A∗0)σ(`), i = d .
Note: The Ext condition can be written as RHomA(A0,A) ∼= (A∗0)σ(`)[d ].
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Generalized regularity properties
For our purposes, we wish to allow a “twist” by a general invertible AUA.
Definitions: Let A be a locally finite graded algebra of (graded) globaldimension d <∞.
(a) A is MV-regular if there is a bijection π from the iso-classes of gradedsimple left modules to the graded simple right modules withRHomA(M,A) ∼= π(M)[d ] for all graded simple AM.
(b) A is MM-regular if RHomA(A0,A) ∼= A∗0 ⊗A U[d ] as Ae-complexes forsome invertible U.
(c) A is J-regular if RHomA(S ,A) ∼= S ⊗A U[d ] as Ae-complexes for someinvertible U, where S = A/J(A).
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Equivalence of twisted CY and AS regular properties
These properties are exactly what we need to characterize twisted CYalgebras:
Theorem
Let A be a locally finite graded algebra, and set S = A/J(A). Then TFAE:
1 A is twisted Calabi-Yau of dimension d.
2 A is MV-regular of dimension d and S is separable.
3 A is MM-regular of dimension d and S is separable.
4 A is J-regular of dimension d and S is separable.
So the twisted CY property (involving bimodules) can be verified using oneof these AS regular properties (involving one-sided modules).
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AS regular properties for quivers with relations
Q: Suppose A = kQ/I is a graded quotient for a connected quiver Q.How do these regularity properties translate?
Here S = A0 = ke1 ⊕ · · · ⊕ ken, with non-iso. simple modules Si = kei .
Lemma: For such A, every invertible AUA is “boring”: U = 1Aµ(`).
Such µ permutes the vertices {1, . . . , n}; call this permutation µ also.
The regularity conditions (and equivalently, the twisted CY condition)amount to:
ExtiA(Sj ,A) ∼=
{Sµ(j)(`), i = d ,
0, i 6= d ,
for all the graded simple Sj , and similar condition as simple right modules.
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AS regular properties for quivers with relations
Q: Suppose A = kQ/I is a graded quotient for a connected quiver Q.How do these regularity properties translate?
Here S = A0 = ke1 ⊕ · · · ⊕ ken, with non-iso. simple modules Si = kei .
Lemma: For such A, every invertible AUA is “boring”: U = 1Aµ(`).
Such µ permutes the vertices {1, . . . , n}; call this permutation µ also.
The regularity conditions (and equivalently, the twisted CY condition)amount to:
ExtiA(Sj ,A) ∼=
{Sµ(j)(`), i = d ,
0, i 6= d ,
for all the graded simple Sj , and similar condition as simple right modules.
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1 Noncommutative polynomial algebras: two candidates
2 Basics of twisted Calabi-Yau algebras
3 Locally finite algebras
4 “Generalized AS regular” versus twisted CY algebras
5 Twisted CY algebras in dimensions 1 and 2
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The noetherian property for low-dimensional algebras
If A is twisted CY-d of finite GK dimension, then is A noetherian?
We can answer yes when d ≤ 2.
How to show an algebra is noetherian? The following is essentially part of“Cohen-type” arguments to establish that a ring is (left) noetherian.
Lemma: A graded algebra A is left noetherian if (and only if) everygraded left noetherian A-module is finitely presented.
Proof: Suppose A weren’t left noetherian. “Zornify” to obtain left ideal Imaximal w.r.t. not being finitely generated. Then A/I is not finitelypresented (Schanuel’s Lemma). But every J ) I finitely generated impliesA/I is noetherian, a contradiction.
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The noetherian property for low-dimensional algebras
If A is twisted CY-d of finite GK dimension, then is A noetherian?
We can answer yes when d ≤ 2.
How to show an algebra is noetherian? The following is essentially part of“Cohen-type” arguments to establish that a ring is (left) noetherian.
Lemma: A graded algebra A is left noetherian if (and only if) everygraded left noetherian A-module is finitely presented.
Proof: Suppose A weren’t left noetherian. “Zornify” to obtain left ideal Imaximal w.r.t. not being finitely generated. Then A/I is not finitelypresented (Schanuel’s Lemma). But every J ) I finitely generated impliesA/I is noetherian, a contradiction.
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The noetherian property for low-dimensional algebras
If A is twisted CY-d of finite GK dimension, then is A noetherian?
We can answer yes when d ≤ 2.
How to show an algebra is noetherian? The following is essentially part of“Cohen-type” arguments to establish that a ring is (left) noetherian.
Lemma: A graded algebra A is left noetherian if (and only if) everygraded left noetherian A-module is finitely presented.
Proof: Suppose A weren’t left noetherian. “Zornify” to obtain left ideal Imaximal w.r.t. not being finitely generated.
Then A/I is not finitelypresented (Schanuel’s Lemma). But every J ) I finitely generated impliesA/I is noetherian, a contradiction.
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The noetherian property for low-dimensional algebras
If A is twisted CY-d of finite GK dimension, then is A noetherian?
We can answer yes when d ≤ 2.
How to show an algebra is noetherian? The following is essentially part of“Cohen-type” arguments to establish that a ring is (left) noetherian.
Lemma: A graded algebra A is left noetherian if (and only if) everygraded left noetherian A-module is finitely presented.
Proof: Suppose A weren’t left noetherian. “Zornify” to obtain left ideal Imaximal w.r.t. not being finitely generated. Then A/I is not finitelypresented (Schanuel’s Lemma).
But every J ) I finitely generated impliesA/I is noetherian, a contradiction.
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The noetherian property for low-dimensional algebras
If A is twisted CY-d of finite GK dimension, then is A noetherian?
We can answer yes when d ≤ 2.
How to show an algebra is noetherian? The following is essentially part of“Cohen-type” arguments to establish that a ring is (left) noetherian.
Lemma: A graded algebra A is left noetherian if (and only if) everygraded left noetherian A-module is finitely presented.
Proof: Suppose A weren’t left noetherian. “Zornify” to obtain left ideal Imaximal w.r.t. not being finitely generated. Then A/I is not finitelypresented (Schanuel’s Lemma). But every J ) I finitely generated impliesA/I is noetherian, a contradiction.
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The noetherian property
Original Goal: Show A left noetherian.
New Goal: Show noetherian A-modules are finitely presented.
But how? The “socle formula” TorAd (S ,M) ∼= U−1 ⊗ soc(M) is handierthan it might seem. . .
Fact: If M has minimal resolution · · · → P2 → P1 → P0 → M → 0, theneach Pi is f.g. (or zero) if and only if TorAi (S ,M) is f.d. (or zero).
Cor: If A is twisted CY-d and AM is graded noetherian, then the term Pd
in the resolution above is finitely generated.
Proof: M is noetherian ⇒ soc(M) is f.d. ⇒ TorAd (S ,M) is f.d.
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The noetherian property
Original Goal: Show A left noetherian.
New Goal: Show noetherian A-modules are finitely presented.
But how? The “socle formula” TorAd (S ,M) ∼= U−1 ⊗ soc(M) is handierthan it might seem. . .
Fact: If M has minimal resolution · · · → P2 → P1 → P0 → M → 0, theneach Pi is f.g. (or zero) if and only if TorAi (S ,M) is f.d. (or zero).
Cor: If A is twisted CY-d and AM is graded noetherian, then the term Pd
in the resolution above is finitely generated.
Proof: M is noetherian ⇒ soc(M) is f.d. ⇒ TorAd (S ,M) is f.d.
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The noetherian property
Original Goal: Show A left noetherian.
New Goal: Show noetherian A-modules are finitely presented.
But how? The “socle formula” TorAd (S ,M) ∼= U−1 ⊗ soc(M) is handierthan it might seem. . .
Fact: If M has minimal resolution · · · → P2 → P1 → P0 → M → 0, theneach Pi is f.g. (or zero) if and only if TorAi (S ,M) is f.d. (or zero).
Cor: If A is twisted CY-d and AM is graded noetherian, then the term Pd
in the resolution above is finitely generated.
Proof: M is noetherian ⇒ soc(M) is f.d. ⇒ TorAd (S ,M) is f.d.
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Twisted CY-1 algebras
This is already enough for algebras of dimension 1:
Theorem: If A is locally finite twisted Calabi-Yau of dimension 1, then Ais noetherian.
Proof: If M is noetherian with minimal resolution 0→ P1 → P0 → M, wehave P1 f.g. by the “socle argument.” So M is finitely presented.
By the way, what do twisted CY-1 algebras look like?
Theorem: A locally finite graded algebra A is twisted Calabi-Yau ofdimension 1 if and only if A ∼= TS(V ) is a tensor algebra, where S is aseparable algebra and V is an invertible positively graded Se-module.
But note that the noetherian result is proved without the structuretheorem!
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Twisted CY-1 algebras
This is already enough for algebras of dimension 1:
Theorem: If A is locally finite twisted Calabi-Yau of dimension 1, then Ais noetherian.
Proof: If M is noetherian with minimal resolution 0→ P1 → P0 → M, wehave P1 f.g. by the “socle argument.” So M is finitely presented.
By the way, what do twisted CY-1 algebras look like?
Theorem: A locally finite graded algebra A is twisted Calabi-Yau ofdimension 1 if and only if A ∼= TS(V ) is a tensor algebra, where S is aseparable algebra and V is an invertible positively graded Se-module.
But note that the noetherian result is proved without the structuretheorem!
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Twisted CY-1 algebras
This is already enough for algebras of dimension 1:
Theorem: If A is locally finite twisted Calabi-Yau of dimension 1, then Ais noetherian.
Proof: If M is noetherian with minimal resolution 0→ P1 → P0 → M, wehave P1 f.g. by the “socle argument.” So M is finitely presented.
By the way, what do twisted CY-1 algebras look like?
Theorem: A locally finite graded algebra A is twisted Calabi-Yau ofdimension 1 if and only if A ∼= TS(V ) is a tensor algebra, where S is aseparable algebra and V is an invertible positively graded Se-module.
But note that the noetherian result is proved without the structuretheorem!
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The noetherian argument in dimension 2
We don’t expect all twisted CY-2 algebras to be noetherian:
Zhang: studied non-noetherian AS regular algebras A of dimension 2. Hefound that such A is noetherian ⇐⇒ GKdim(A) <∞
We found that a similar result holds for twisted CY algebras:
Theorem
Let A be a locally finite twisted Calabi-Yau algebra of dimension 2. ThenA is noetherian if and only if A has finite GK dimension.
Again, this is proved without first classifying the iso-types of A.
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The noetherian argument in dimension 2
We don’t expect all twisted CY-2 algebras to be noetherian:
Zhang: studied non-noetherian AS regular algebras A of dimension 2. Hefound that such A is noetherian ⇐⇒ GKdim(A) <∞
We found that a similar result holds for twisted CY algebras:
Theorem
Let A be a locally finite twisted Calabi-Yau algebra of dimension 2. ThenA is noetherian if and only if A has finite GK dimension.
Again, this is proved without first classifying the iso-types of A.
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The noetherian argument in dimension 2
Theorem
Let A be a locally finite twisted Calabi-Yau algebra of dimension 2. ThenA is noetherian if and only if A has finite GK dimension.
Idea of Proof: Suppose M is a graded noetherian A-module.
Projective resolution: 0→ P2 → P1 → P0 → M → 0.
Clearly P0 is f.g.
“Socle argument” =⇒ P2 is also f.g.
Exactness above with P2,P0 f.g. and GKdim(A) <∞ impliesGKdim(P1) <∞.
Deduce that P1 is finitely generated.
Open Q: If A above is not graded, must A still be noetherian?
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The noetherian argument in dimension 2
Theorem
Let A be a locally finite twisted Calabi-Yau algebra of dimension 2. ThenA is noetherian if and only if A has finite GK dimension.
Idea of Proof: Suppose M is a graded noetherian A-module.
Projective resolution: 0→ P2 → P1 → P0 → M → 0.
Clearly P0 is f.g. “Socle argument” =⇒ P2 is also f.g.
Exactness above with P2,P0 f.g. and GKdim(A) <∞ impliesGKdim(P1) <∞.
Deduce that P1 is finitely generated.
Open Q: If A above is not graded, must A still be noetherian?
Manny Reyes Twisted CY and AS regular properties September 15, 2016 29 / 32
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The noetherian argument in dimension 2
Theorem
Let A be a locally finite twisted Calabi-Yau algebra of dimension 2. ThenA is noetherian if and only if A has finite GK dimension.
Idea of Proof: Suppose M is a graded noetherian A-module.
Projective resolution: 0→ P2 → P1 → P0 → M → 0.
Clearly P0 is f.g. “Socle argument” =⇒ P2 is also f.g.
Exactness above with P2,P0 f.g. and GKdim(A) <∞ impliesGKdim(P1) <∞.
Deduce that P1 is finitely generated.
Open Q: If A above is not graded, must A still be noetherian?
Manny Reyes Twisted CY and AS regular properties September 15, 2016 29 / 32
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The noetherian argument in dimension 2
Theorem
Let A be a locally finite twisted Calabi-Yau algebra of dimension 2. ThenA is noetherian if and only if A has finite GK dimension.
Idea of Proof: Suppose M is a graded noetherian A-module.
Projective resolution: 0→ P2 → P1 → P0 → M → 0.
Clearly P0 is f.g. “Socle argument” =⇒ P2 is also f.g.
Exactness above with P2,P0 f.g. and GKdim(A) <∞ impliesGKdim(P1) <∞.
Deduce that P1 is finitely generated.
Open Q: If A above is not graded, must A still be noetherian?
Manny Reyes Twisted CY and AS regular properties September 15, 2016 29 / 32
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Presentation of twisted CY-2 algebras
So what do twisted CY-2 algebras actually look like?
Zhang: AS regular algebras of dimension 2 are “free algebras in n ≥ 2indeterminates modulo twisted potentials.”
If A is a graded quotient of a quiver algebra kQ, obtain a similardescription as follows.
Denote:
Vertex space: (kQ)0 = ke1 ⊕ · · · ⊕ ken
Arrow space: V = (kQ)1; space of arrows j → i is eiVej
Required data:
Permutation µ of {1, . . . , n}Linear automorphism τ of V such that τ(ejVei ) = eµ(i)Vej
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Presentation of twisted CY-2 algebras
So what do twisted CY-2 algebras actually look like?
Zhang: AS regular algebras of dimension 2 are “free algebras in n ≥ 2indeterminates modulo twisted potentials.”
If A is a graded quotient of a quiver algebra kQ, obtain a similardescription as follows.
Denote:
Vertex space: (kQ)0 = ke1 ⊕ · · · ⊕ ken
Arrow space: V = (kQ)1; space of arrows j → i is eiVej
Required data:
Permutation µ of {1, . . . , n}Linear automorphism τ of V such that τ(ejVei ) = eµ(i)Vej
Manny Reyes Twisted CY and AS regular properties September 15, 2016 30 / 32
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Presentation of twisted CY-2 algebras
Required data:
Permutation µ of {1, . . . , n}Linear automorphism τ of V such that τ(ejVei ) = eµ(i)Vej
Def: A(Q, τ) = kQ/(∑
a τ(a)a). (Algebra with “mesh relations.”)
This construction generalizes preprojective algebras (where µ = id)
Theorem: Every twisted CY-2 algebra that is a graded quotient of kQ isof the form A = A(Q, τ), such that the incidence matrix M of Q hasspectral radius ρ(M) ≥ 2. (GKdim(A) <∞ ⇐⇒ ρ(M) = 2.)
(The converse should hold, too.)
Manny Reyes Twisted CY and AS regular properties September 15, 2016 31 / 32
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Presentation of twisted CY-2 algebras
Required data:
Permutation µ of {1, . . . , n}Linear automorphism τ of V such that τ(ejVei ) = eµ(i)Vej
Def: A(Q, τ) = kQ/(∑
a τ(a)a). (Algebra with “mesh relations.”)
This construction generalizes preprojective algebras (where µ = id)
Theorem: Every twisted CY-2 algebra that is a graded quotient of kQ isof the form A = A(Q, τ), such that the incidence matrix M of Q hasspectral radius ρ(M) ≥ 2. (GKdim(A) <∞ ⇐⇒ ρ(M) = 2.)
(The converse should hold, too.)
Manny Reyes Twisted CY and AS regular properties September 15, 2016 31 / 32
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Thank you!
Manny Reyes Twisted CY and AS regular properties September 15, 2016 32 / 32