Quillen Cohomology of Operadic Algebras andObstruction Theory
Michael A. Mandell
Indiana University
Banff Workshop on Functor Calculus and OperadsMarch 16, 2011
M.A.Mandell (IU) Obstruction Theory Mar 16 1 / 24
Introduction
Outline
1 Introduction to Quillen homology and cohomologyof operadic algebras
2 Structure of Quillen homology3 Postnikov towers and obstructions4 Application: BP
M.A.Mandell (IU) Obstruction Theory Mar 16 2 / 24
Introduction
Outline
1 Introduction to Quillen homology and cohomologyof operadic algebras
2 Structure of Quillen homology3 Postnikov towers and obstructions4 Application: BP
M.A.Mandell (IU) Obstruction Theory Mar 16 2 / 24
Introduction
Outline
1 Introduction to Quillen homology and cohomologyof operadic algebras
2 Structure of Quillen homology3 Postnikov towers and obstructions4 Application: BP
M.A.Mandell (IU) Obstruction Theory Mar 16 2 / 24
Introduction
Outline
1 Introduction to Quillen homology and cohomologyof operadic algebras
2 Structure of Quillen homology3 Postnikov towers and obstructions4 Application: BP
M.A.Mandell (IU) Obstruction Theory Mar 16 2 / 24
Introduction
Quillen Homology and Cohomology
General ContextA closed model category M
Assume that subcategory of abelian objects A also forms amodel categoryWith fibrations and weak equivalences as in M
And assume that the forgetful functor has a left adjoint Ab(“abelianization”)
DefinitionQuillen homology is the left derived functor of abelianization. Quillencohomology with coefficients in N ∈ A is [−,N].
M.A.Mandell (IU) Obstruction Theory Mar 16 3 / 24
Introduction
Quillen Homology and Cohomology
General ContextA closed model category M
Assume that subcategory of abelian objects A also forms amodel categoryWith fibrations and weak equivalences as in M
And assume that the forgetful functor has a left adjoint Ab(“abelianization”)
DefinitionQuillen homology is the left derived functor of abelianization. Quillencohomology with coefficients in N ∈ A is [−,N].
M.A.Mandell (IU) Obstruction Theory Mar 16 3 / 24
Introduction
Quillen Homology and Cohomology
General ContextA closed model category M
Assume that subcategory of abelian objects A also forms amodel categoryWith fibrations and weak equivalences as in M
And assume that the forgetful functor has a left adjoint Ab(“abelianization”)
DefinitionQuillen homology is the left derived functor of abelianization. Quillencohomology with coefficients in N ∈ A is [−,N].
M.A.Mandell (IU) Obstruction Theory Mar 16 3 / 24
Introduction
Quillen Homology and Cohomology
General ContextA closed model category M
Assume that subcategory of abelian objects A also forms amodel categoryWith fibrations and weak equivalences as in M
And assume that the forgetful functor has a left adjoint Ab(“abelianization”)
DefinitionQuillen homology is the left derived functor of abelianization. Quillencohomology with coefficients in N ∈ A is [−,N].
M.A.Mandell (IU) Obstruction Theory Mar 16 3 / 24
Introduction
Quillen Homology and Cohomology
General ContextA closed model category M
Assume that subcategory of abelian objects A also forms amodel categoryWith fibrations and weak equivalences as in M
And assume that the forgetful functor has a left adjoint Ab(“abelianization”)
DefinitionQuillen homology is the left derived functor of abelianization. Quillencohomology with coefficients in N ∈ A is [−,N].
M.A.Mandell (IU) Obstruction Theory Mar 16 3 / 24
Introduction
Abelian Objects in Operadic Algebras
Let O be an operad in chain cxs of abelian groups or modules.Assume O(0) = 0.
QuestionWhat is an abelian object in the category of O-algebras?
Must have structure map O(m)⊗ N⊗m → N be zero for m > 1.Just has the structure of an R-module for R = O(1).Equivalently, the structure of an algebra over IR.
AnswerAn IR-algebra = R-module.
M.A.Mandell (IU) Obstruction Theory Mar 16 4 / 24
Introduction
Abelian Objects in Operadic Algebras
Let O be an operad in chain cxs of abelian groups or modules.Assume O(0) = 0.
QuestionWhat is an abelian object in the category of O-algebras?
Must have structure map O(m)⊗ N⊗m → N be zero for m > 1.Just has the structure of an R-module for R = O(1).Equivalently, the structure of an algebra over IR.
AnswerAn IR-algebra = R-module.
M.A.Mandell (IU) Obstruction Theory Mar 16 4 / 24
Introduction
Abelian Objects in Operadic Algebras
Let O be an operad in chain cxs of abelian groups or modules.Assume O(0) = 0.
QuestionWhat is an abelian object in the category of O-algebras?
Must have structure map O(m)⊗ N⊗m → N be zero for m > 1.Just has the structure of an R-module for R = O(1).Equivalently, the structure of an algebra over IR.
AnswerAn IR-algebra = R-module.
M.A.Mandell (IU) Obstruction Theory Mar 16 4 / 24
Introduction
Abelian Objects in Operadic Algebras
Let O be an operad in chain cxs of abelian groups or modules.Assume O(0) = 0.
QuestionWhat is an abelian object in the category of O-algebras?
Must have structure map O(m)⊗ N⊗m → N be zero for m > 1.Just has the structure of an R-module for R = O(1).Equivalently, the structure of an algebra over IR.
AnswerAn IR-algebra = R-module.
M.A.Mandell (IU) Obstruction Theory Mar 16 4 / 24
Introduction
Abelian Objects in Operadic Algebras
Let O be an operad in chain cxs of abelian groups or modules.Assume O(0) = 0.
QuestionWhat is an abelian object in the category of O-algebras?
Must have structure map O(m)⊗ N⊗m → N be zero for m > 1.Just has the structure of an R-module for R = O(1).Equivalently, the structure of an algebra over IR.
AnswerAn IR-algebra = R-module.
M.A.Mandell (IU) Obstruction Theory Mar 16 4 / 24
Introduction
Abelian Objects in Operadic Algebras
Let O be an operad in chain cxs of abelian groups or modules.Assume O(0) = 0.
QuestionWhat is an abelian object in the category of O-algebras?
Must have structure map O(m)⊗ N⊗m → N be zero for m > 1.Just has the structure of an R-module for R = O(1).Equivalently, the structure of an algebra over IR.
AnswerAn IR-algebra = R-module.
M.A.Mandell (IU) Obstruction Theory Mar 16 4 / 24
Introduction
Abelianization = Indecomposables
Forgetful functor from abelian O-algebras to O-algebras is “restrictionof scalars” from IR-algebras to O-algebras along O → IR.
Left adjoint is “extension of scalars”
IR ◦O (−)
which is indecomposables:
IR ◦ O ◦ A //// IR ◦ A // QA
R ⊗(⊕O(n)⊗Σn A⊗n) //
// R ⊗ A // QA
M.A.Mandell (IU) Obstruction Theory Mar 16 5 / 24
Introduction
Abelianization = Indecomposables
Forgetful functor from abelian O-algebras to O-algebras is “restrictionof scalars” from IR-algebras to O-algebras along O → IR.
Left adjoint is “extension of scalars”
IR ◦O (−)
which is indecomposables:
IR ◦ O ◦ A //// IR ◦ A // QA
R ⊗(⊕O(n)⊗Σn A⊗n) //
// R ⊗ A // QA
M.A.Mandell (IU) Obstruction Theory Mar 16 5 / 24
Introduction
Abelianization = Indecomposables
Forgetful functor from abelian O-algebras to O-algebras is “restrictionof scalars” from IR-algebras to O-algebras along O → IR.
Left adjoint is “extension of scalars”
IR ◦O (−)
which is indecomposables:
IR ◦ O ◦ A //// IR ◦ A // QA
R ⊗(⊕O(n)⊗Σn A⊗n) //
// R ⊗ A // QA
M.A.Mandell (IU) Obstruction Theory Mar 16 5 / 24
Introduction
Abelianization = Indecomposables
Forgetful functor from abelian O-algebras to O-algebras is “restrictionof scalars” from IR-algebras to O-algebras along O → IR.
Left adjoint is “extension of scalars”
IR ◦O (−)
which is indecomposables:
IR ◦ O ◦ A //// IR ◦ A // QA
R ⊗(⊕O(n)⊗Σn A⊗n) //
// R ⊗ A // QA
M.A.Mandell (IU) Obstruction Theory Mar 16 5 / 24
Introduction
Abelianization = Indecomposables
Forgetful functor from abelian O-algebras to O-algebras is “restrictionof scalars” from IR-algebras to O-algebras along O → IR.
Left adjoint is “extension of scalars”
IR ◦O (−)
which is indecomposables:
IR ◦ O ◦ A //// IR ◦ A // QA
R ⊗(⊕O(n)⊗Σn A⊗n) //
// R ⊗ A // QA
M.A.Mandell (IU) Obstruction Theory Mar 16 5 / 24
Structure of Quillen Homology
Quillen Homology = Koszul Dual Coalgebra
Quillen homology = Left derived functor of IR ◦O (−)
We know how to do this left derived functor much more generally thanwe know that O-algebras are a closed model category.
Choose a flat right O-module approximation E of IR and look atE ◦O (−).
If R is commutative and O nice, can take E = (DO ◦ O,d).
Then E ◦O A = BOA is the “bar dual” or “Koszul dual” DO-coalgebra ofA.
M.A.Mandell (IU) Obstruction Theory Mar 16 6 / 24
Structure of Quillen Homology
Quillen Homology = Koszul Dual Coalgebra
Quillen homology = Left derived functor of IR ◦O (−)
We know how to do this left derived functor much more generally thanwe know that O-algebras are a closed model category.
Choose a flat right O-module approximation E of IR and look atE ◦O (−).
If R is commutative and O nice, can take E = (DO ◦ O,d).
Then E ◦O A = BOA is the “bar dual” or “Koszul dual” DO-coalgebra ofA.
M.A.Mandell (IU) Obstruction Theory Mar 16 6 / 24
Structure of Quillen Homology
Quillen Homology = Koszul Dual Coalgebra
Quillen homology = Left derived functor of IR ◦O (−)
We know how to do this left derived functor much more generally thanwe know that O-algebras are a closed model category.
Choose a flat right O-module approximation E of IR and look atE ◦O (−).
If R is commutative and O nice, can take E = (DO ◦ O,d).
Then E ◦O A = BOA is the “bar dual” or “Koszul dual” DO-coalgebra ofA.
M.A.Mandell (IU) Obstruction Theory Mar 16 6 / 24
Structure of Quillen Homology
Quillen Homology = Koszul Dual Coalgebra
Quillen homology = Left derived functor of IR ◦O (−)
We know how to do this left derived functor much more generally thanwe know that O-algebras are a closed model category.
Choose a flat right O-module approximation E of IR and look atE ◦O (−).
If R is commutative and O nice, can take E = (DO ◦ O,d).
Then E ◦O A = BOA is the “bar dual” or “Koszul dual” DO-coalgebra ofA.
M.A.Mandell (IU) Obstruction Theory Mar 16 6 / 24
Structure of Quillen Homology
Homotopical Origin of Coalgebra Structure
Now don’t assume that R is commutative but do assume thatO-algebras form a closed model category.
Then we have a Quillen adjunction
Q : O-Alg // IR-Alg :Zoo
and a derived adjunction
QL : Ho(O-Alg)// Ho(IR-Alg) :Z Roo
QLA is a coalgebra over the comonad QLZ R.
Goodwillie Calculus: QLZ R(X ) =⊕
(D(n)⊗ X⊗n)hΣn
M.A.Mandell (IU) Obstruction Theory Mar 16 7 / 24
Structure of Quillen Homology
Homotopical Origin of Coalgebra Structure
Now don’t assume that R is commutative but do assume thatO-algebras form a closed model category.
Then we have a Quillen adjunction
Q : O-Alg // IR-Alg :Zoo
and a derived adjunction
QL : Ho(O-Alg)// Ho(IR-Alg) :Z Roo
QLA is a coalgebra over the comonad QLZ R.
Goodwillie Calculus: QLZ R(X ) =⊕
(D(n)⊗ X⊗n)hΣn
M.A.Mandell (IU) Obstruction Theory Mar 16 7 / 24
Structure of Quillen Homology
Homotopical Origin of Coalgebra Structure
Now don’t assume that R is commutative but do assume thatO-algebras form a closed model category.
Then we have a Quillen adjunction
Q : O-Alg // IR-Alg :Zoo
and a derived adjunction
QL : Ho(O-Alg)// Ho(IR-Alg) :Z Roo
QLA is a coalgebra over the comonad QLZ R.
Goodwillie Calculus: QLZ R(X ) =⊕
(D(n)⊗ X⊗n)hΣn
M.A.Mandell (IU) Obstruction Theory Mar 16 7 / 24
Structure of Quillen Homology
Homotopical Origin of Coalgebra Structure
Now don’t assume that R is commutative but do assume thatO-algebras form a closed model category.
Then we have a Quillen adjunction
Q : O-Alg // IR-Alg :Zoo
and a derived adjunction
QL : Ho(O-Alg)// Ho(IR-Alg) :Z Roo
QLA is a coalgebra over the comonad QLZ R.
Goodwillie Calculus: QLZ R(X ) =⊕
(D(n)⊗ X⊗n)hΣn
M.A.Mandell (IU) Obstruction Theory Mar 16 7 / 24
Structure of Quillen Homology
Homotopical Origin of Coalgebra Structure
Now don’t assume that R is commutative but do assume thatO-algebras form a closed model category.
Then we have a Quillen adjunction
Q : O-Alg // IR-Alg :Zoo
and a derived adjunction
QL : Ho(O-Alg)// Ho(IR-Alg) :Z Roo
QLA is a coalgebra over the comonad QLZ R.
Goodwillie Calculus: QLZ R(X ) =⊕
(D(n)⊗ X⊗n)hΣn
M.A.Mandell (IU) Obstruction Theory Mar 16 7 / 24
Structure of Quillen Homology
Koszul Duality
(R is commutative, probably a field.)(O is Σ∗ projective.)
If A is connected, you can recover A from DO structure on BOA(= QLA):
COBOA '−→ A
[Getzler-Jones], [Fresse]
M.A.Mandell (IU) Obstruction Theory Mar 16 8 / 24
Postnikov Towers and Obstructions
Eckmann-Hilton Duality
A1 // A2 // A3 // · · · // A
KA1 KA2oo KA3oo · · ·oo KAoo
M.A.Mandell (IU) Obstruction Theory Mar 16 9 / 24
Postnikov Towers and Obstructions
Eckmann-Hilton Duality
A1 A2oo A3oo · · ·oo Aoo
KA1 // KA2 // KA3 // · · · // KA
=⇒ Postnikov Tower
M.A.Mandell (IU) Obstruction Theory Mar 16 10 / 24
Postnikov Towers and Obstructions
Eckmann-Hilton Duality
A1 A2oo A3oo · · ·oo Aoo
KA1 // KA2 // KA3 // · · · // KA
=⇒ Postnikov Tower
M.A.Mandell (IU) Obstruction Theory Mar 16 10 / 24
Postnikov Towers and Obstructions
Postnikov Towers for Operadic Algebras
Now possibly working in spectra with O an operad of spaces.No longer assume O(0) = ∗. Might want unit.
For any O-algebra A, tower of O-modules
A→ · · · → An+1 → An → · · · → A0
withπiA→ πiAn iso for i ≤ nπiAn = 0 for i > n
ProblemBuild as a tower of principal fibrations of O-modules.
M.A.Mandell (IU) Obstruction Theory Mar 16 11 / 24
Postnikov Towers and Obstructions
Postnikov Towers for Operadic Algebras
Now possibly working in spectra with O an operad of spaces.No longer assume O(0) = ∗. Might want unit.
For any O-algebra A, tower of O-modules
A→ · · · → An+1 → An → · · · → A0
withπiA→ πiAn iso for i ≤ nπiAn = 0 for i > n
ProblemBuild as a tower of principal fibrations of O-modules.
M.A.Mandell (IU) Obstruction Theory Mar 16 11 / 24
Postnikov Towers and Obstructions
Postnikov Towers for Operadic Algebras
Now possibly working in spectra with O an operad of spaces.No longer assume O(0) = ∗. Might want unit.
For any O-algebra A, tower of O-modules
A→ · · · → An+1 → An → · · · → A0
withπiA→ πiAn iso for i ≤ nπiAn = 0 for i > n
ProblemBuild as a tower of principal fibrations of O-modules.
M.A.Mandell (IU) Obstruction Theory Mar 16 11 / 24
Postnikov Towers and Obstructions
Postnikov Towers for Operadic Algebras
Now possibly working in spectra with O an operad of spaces.No longer assume O(0) = ∗. Might want unit.
For any O-algebra A, tower of O-modules
A→ · · · → An+1 → An → · · · → A0
withπiA→ πiAn iso for i ≤ nπiAn = 0 for i > n
ProblemBuild as a tower of principal fibrations of O-modules.
M.A.Mandell (IU) Obstruction Theory Mar 16 11 / 24
Postnikov Towers and Obstructions
Postnikov Towers for Operadic Algebras
Now possibly working in spectra with O an operad of spaces.No longer assume O(0) = ∗. Might want unit.
For any O-algebra A, tower of O-modules
A→ · · · → An+1 → An → · · · → A0
withπiA→ πiAn iso for i ≤ nπiAn = 0 for i > n
ProblemBuild as a tower of principal fibrations of O-modules.
M.A.Mandell (IU) Obstruction Theory Mar 16 11 / 24
Postnikov Towers and Obstructions
Principal Fibrations
A→ · · · → An+1 → An → · · · → A0
(For simplicity, assume O augments to comm. operad.)
Work in the category of O-algebras lying over A0 = Hπ0A.
For an A0-module M, have the square-zero O-algebraA0 n M = A0 ∨M.
We will construct the Postnikov tower with
An+1 //
��
A0
��
An // A0 n Σn+2Hπn+1A
homotopy fiber squares.M.A.Mandell (IU) Obstruction Theory Mar 16 12 / 24
Postnikov Towers and Obstructions
Principal Fibrations
A→ · · · → An+1 → An → · · · → A0
(For simplicity, assume O augments to comm. operad.)
Work in the category of O-algebras lying over A0 = Hπ0A.
For an A0-module M, have the square-zero O-algebraA0 n M = A0 ∨M.
We will construct the Postnikov tower with
An+1 //
��
A0
��
An // A0 n Σn+2Hπn+1A
homotopy fiber squares.M.A.Mandell (IU) Obstruction Theory Mar 16 12 / 24
Postnikov Towers and Obstructions
Principal Fibrations
A→ · · · → An+1 → An → · · · → A0
(For simplicity, assume O augments to comm. operad.)
Work in the category of O-algebras lying over A0 = Hπ0A.
For an A0-module M, have the square-zero O-algebraA0 n M = A0 ∨M.
We will construct the Postnikov tower with
An+1 //
��
A0
��
An // A0 n Σn+2Hπn+1A
homotopy fiber squares.M.A.Mandell (IU) Obstruction Theory Mar 16 12 / 24
Postnikov Towers and Obstructions
Principal Fibrations
A→ · · · → An+1 → An → · · · → A0
(For simplicity, assume O augments to comm. operad.)
Work in the category of O-algebras lying over A0 = Hπ0A.
For an A0-module M, have the square-zero O-algebraA0 n M = A0 ∨M.
We will construct the Postnikov tower with
An+1 //
��
A0
��
An // A0 n Σn+2Hπn+1A
homotopy fiber squares.M.A.Mandell (IU) Obstruction Theory Mar 16 12 / 24
Postnikov Towers and Obstructions
Topological (André-)Quillen Cohomology
The mapAn → A0 n Σn+2Hπn+1A
is an element of topological Quillen cohomology
kn+1O ∈ Dn+2(An; Hπn+1A)
Step 1. Have extension of scalars isomorphism
Ho(O-Alg/A0)(C,A0 n M) ∼= Ho(O-A0-Alg/A0)(A0 ∧ C,A0 n M)
Now in context of augmented algebras.
M.A.Mandell (IU) Obstruction Theory Mar 16 13 / 24
Postnikov Towers and Obstructions
Topological (André-)Quillen Cohomology
The mapAn → A0 n Σn+2Hπn+1A
is an element of topological Quillen cohomology
kn+1O ∈ Dn+2(An; Hπn+1A)
Step 1. Have extension of scalars isomorphism
Ho(O-Alg/A0)(C,A0 n M) ∼= Ho(O-A0-Alg/A0)(A0 ∧ C,A0 n M)
Now in context of augmented algebras.
M.A.Mandell (IU) Obstruction Theory Mar 16 13 / 24
Postnikov Towers and Obstructions
Topological (André-)Quillen Cohomology
The mapAn → A0 n Σn+2Hπn+1A
is an element of topological Quillen cohomology
kn+1O ∈ Dn+2(An; Hπn+1A)
Step 1. Have extension of scalars isomorphism
Ho(O-Alg/A0)(C,A0 n M) ∼= Ho(O-A0-Alg/A0)(A0 ∧ C,A0 n M)
Now in context of augmented algebras.
M.A.Mandell (IU) Obstruction Theory Mar 16 13 / 24
Postnikov Towers and Obstructions
Step 2. The augmented/non-unital equivalence
Ho(O-A0-Alg/A0)(A0 ∧ C,A0 n M) ∼= Ho(O-A0-Alg)(IR(A0 ∧ C),ZM)
for O the non-unital version of O,
O(n) =
{O(n) n > 0∗ n = 0
Step 3. The indecomposables/zero-multiplication adjunction
Ho(O-A0-Alg)(IR(A0 ∧ C),ZM) ∼= Ho(A0-Mod)(QLIR(A0 ∧ C),M)
∼= π0FA0(QLIR(A0 ∧ C),M).
M.A.Mandell (IU) Obstruction Theory Mar 16 14 / 24
Postnikov Towers and Obstructions
Step 2. The augmented/non-unital equivalence
Ho(O-A0-Alg/A0)(A0 ∧ C,A0 n M) ∼= Ho(O-A0-Alg)(IR(A0 ∧ C),ZM)
for O the non-unital version of O,
O(n) =
{O(n) n > 0∗ n = 0
Step 3. The indecomposables/zero-multiplication adjunction
Ho(O-A0-Alg)(IR(A0 ∧ C),ZM) ∼= Ho(A0-Mod)(QLIR(A0 ∧ C),M)
∼= π0FA0(QLIR(A0 ∧ C),M).
M.A.Mandell (IU) Obstruction Theory Mar 16 14 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower?
Theorem (Hurewicz Theorem)Suppose A→ B is n-connected and M is connected. ThenDq(B,A; M) = 0 for q ≤ n and Dn+1(B,A; M) = Hn+1(B,A; M).
Theorem (Universal Coefficient Theorem)There is a natural spectral sequence
Ep,q2 = Extp,qπ∗A0
(D∗(B,A; A0), π∗M)
converging conditionally to Dp+q(B,A; M).
Apply to the (n + 1)-connected map A→ An+1
M.A.Mandell (IU) Obstruction Theory Mar 16 15 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower?
Theorem (Hurewicz Theorem)Suppose A→ B is n-connected and M is connected. ThenDq(B,A; M) = 0 for q ≤ n and Dn+1(B,A; M) = Hn+1(B,A; M).
Theorem (Universal Coefficient Theorem)There is a natural spectral sequence
Ep,q2 = Extp,qπ∗A0
(D∗(B,A; A0), π∗M)
converging conditionally to Dp+q(B,A; M).
Apply to the (n + 1)-connected map A→ An+1
M.A.Mandell (IU) Obstruction Theory Mar 16 15 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower?
Theorem (Hurewicz Theorem)Suppose A→ B is n-connected and M is connected. ThenDq(B,A; M) = 0 for q ≤ n and Dn+1(B,A; M) = Hn+1(B,A; M).
Theorem (Universal Coefficient Theorem)There is a natural spectral sequence
Ep,q2 = Extp,qπ0A(D∗(B,A; A0), π∗M)
converging conditionally to Dp+q(B,A; M).
Apply to the (n + 1)-connected map A→ An+1
M.A.Mandell (IU) Obstruction Theory Mar 16 15 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower?
Theorem (Hurewicz Theorem)Suppose A→ B is n-connected and M is connected. ThenDq(B,A; M) = 0 for q ≤ n and Dn+1(B,A; M) = Hn+1(B,A; M).
Theorem (Universal Coefficient Theorem)There is a natural spectral sequence
Ep,q2 = Extp,qπ0A(D∗(B,A; A0), π∗M)
converging conditionally to Dp+q(B,A; M).
Apply to the (n + 1)-connected map A→ An+1
M.A.Mandell (IU) Obstruction Theory Mar 16 15 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower? (cont.)
Applying Hurewicz to the (n + 1)-connected map A→ An+1, we get
Dq(An+1,A; A0) = 0, q ≤ n + 1Dn+2(An+1,A; A0) = Hn+2(An+1,A; A0) = Hn+2(An+1,A;π0A)
= π0A⊗ πn+1A
Applying Universal Coefficient, we get
Dn+2(An,A; Hπn+1A) = Homπ0A(π0A⊗ πn+1A, πn+1A)
= Hom(πn+1A, πn+1A).
M.A.Mandell (IU) Obstruction Theory Mar 16 16 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower? (cont.)
Applying Hurewicz to the (n + 1)-connected map A→ An+1, we get
Dq(An+1,A; A0) = 0, q ≤ n + 1Dn+2(An+1,A; A0) = Hn+2(An+1,A; A0) = Hn+2(An+1,A;π0A)
= π0A⊗ πn+1A
Applying Universal Coefficient, we get
Dn+2(An,A; Hπn+1A) = Homπ0A(π0A⊗ πn+1A, πn+1A)
= Hom(πn+1A, πn+1A).
M.A.Mandell (IU) Obstruction Theory Mar 16 16 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower? (cont.)
Applying Hurewicz to the (n + 1)-connected map A→ An+1, we get
Dq(An+1,A; A0) = 0, q ≤ n + 1Dn+2(An+1,A; A0) = Hn+2(An+1,A; A0) = Hn+2(An+1,A;π0A)
= π0A⊗ πn+1A
Applying Universal Coefficient, we get
Dn+2(An,A; Hπn+1A) = Homπ0A(π0A⊗ πn+1A, πn+1A)
= Hom(πn+1A, πn+1A).
M.A.Mandell (IU) Obstruction Theory Mar 16 16 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower? (cont.)
Applying Hurewicz to the (n + 1)-connected map A→ An+1, we get
Dq(An+1,A; A0) = 0, q ≤ n + 1Dn+2(An+1,A; A0) = Hn+2(An+1,A; A0) = Hn+2(An+1,A;π0A)
= π0A⊗ πn+1A
Applying Universal Coefficient, we get
Dn+2(An,A; Hπn+1A) = Homπ0A(π0A⊗ πn+1A, πn+1A)
= Hom(πn+1A, πn+1A).
M.A.Mandell (IU) Obstruction Theory Mar 16 16 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower? (cont.)
Applying Hurewicz to the (n + 1)-connected map A→ An+1, we get
Dq(An+1,A; A0) = 0, q ≤ n + 1Dn+2(An+1,A; A0) = Hn+2(An+1,A; A0) = Hn+2(An+1,A;π0A)
= π0A⊗ πn+1A
Applying Universal Coefficient, we get
Dn+2(An,A; Hπn+1A) = Homπ0A(π0A⊗ πn+1A, πn+1A)
= Hom(πn+1A, πn+1A).
M.A.Mandell (IU) Obstruction Theory Mar 16 16 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower? (cont.)
Dn+2(An,A; Hπn+1A) = Hom(πn+1A, πn+1A).
Choosing identity element, we get a (homotopy class of) diagram
A //
��
A0
��
An // A0 n Σn+2Hπn+1A
Construct An+1 as homotopy pullback.Get A→ An+1 → An.
M.A.Mandell (IU) Obstruction Theory Mar 16 17 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower? (cont.)
Dn+2(An,A; Hπn+1A) = Hom(πn+1A, πn+1A).
Choosing identity element, we get a (homotopy class of) diagram
A //
��
A0
��
An // A0 n Σn+2Hπn+1A
Construct An+1 as homotopy pullback.Get A→ An+1 → An.
M.A.Mandell (IU) Obstruction Theory Mar 16 17 / 24
Postnikov Towers and Obstructions
How do you construct the Postnikov tower? (cont.)
Dn+2(An,A; Hπn+1A) = Hom(πn+1A, πn+1A).
Choosing identity element, we get a (homotopy class of) diagram
A
++WWWWWWWWWWWWWWWWWWW
��22
2222
2222
""
An+1 //
��
A0
��
An // A0 n Σn+2Hπn+1A
Construct An+1 as homotopy pullback.Get A→ An+1 → An.
M.A.Mandell (IU) Obstruction Theory Mar 16 17 / 24
Postnikov Towers and Obstructions
Obstruction Theory
A→ · · · → An+1 → An → · · · → A0
TheoremA map of O-algebras f : B → An lifts (in the homotopy category) to amap of O-algebras B → An+1 if and only if f ∗kn+1
O = 0 inDn+2(B;πn+1A). When a lift exists, the set of lifts has a free transitiveaction of Dn+1(B;πn+1A).
TheoremAn O-algebra structure on An lifts to an O-algebra structure on An+1 ifand only if the spectrum-level k-invariant kn+1 ∈ Hn+2(An, πn+1A) liftsto an element of Dn+2(An, πn+1A).
M.A.Mandell (IU) Obstruction Theory Mar 16 18 / 24
Postnikov Towers and Obstructions
Obstruction Theory
A→ · · · → An+1 → An → · · · → A0
TheoremA map of O-algebras f : B → An lifts (in the homotopy category) to amap of O-algebras B → An+1 if and only if f ∗kn+1
O = 0 inDn+2(B;πn+1A). When a lift exists, the set of lifts has a free transitiveaction of Dn+1(B;πn+1A).
TheoremAn O-algebra structure on An lifts to an O-algebra structure on An+1 ifand only if the spectrum-level k-invariant kn+1 ∈ Hn+2(An, πn+1A) liftsto an element of Dn+2(An, πn+1A).
M.A.Mandell (IU) Obstruction Theory Mar 16 18 / 24
Application: BP
Application: BP
Theorem (Basterra-Mandell)BP has an E4 ring spectrum structure. It is unique up to automorphismin the homotopy category of E4 ring spectra.
Existence:Compute topological Quillen (co)homology in a range forPostnikov section BPn.Play off of MU
Uniqueness:
Compute topological Quillen (co)homology.Obstructions for constructing an E4 map BP → BP ′ are zero.Any map of spectra BP → BP is either zero on an equivalence.
M.A.Mandell (IU) Obstruction Theory Mar 16 19 / 24
Application: BP
Application: BP
Theorem (Basterra-Mandell)BP has an E4 ring spectrum structure. It is unique up to automorphismin the homotopy category of E4 ring spectra.
Existence:Compute topological Quillen (co)homology in a range forPostnikov section BPn.Play off of MU
Uniqueness:
Compute topological Quillen (co)homology.Obstructions for constructing an E4 map BP → BP ′ are zero.Any map of spectra BP → BP is either zero on an equivalence.
M.A.Mandell (IU) Obstruction Theory Mar 16 19 / 24
Application: BP
Application: BP
Theorem (Basterra-Mandell)BP has an E4 ring spectrum structure. It is unique up to automorphismin the homotopy category of E4 ring spectra.
Existence:Compute topological Quillen (co)homology in a range forPostnikov section BPn.Play off of MU
Uniqueness:
Compute topological Quillen (co)homology.Obstructions for constructing an E4 map BP → BP ′ are zero.Any map of spectra BP → BP is either zero on an equivalence.
M.A.Mandell (IU) Obstruction Theory Mar 16 19 / 24
Application: BP
Application: BP
Theorem (Basterra-Mandell)BP has an E4 ring spectrum structure. It is unique up to automorphismin the homotopy category of E4 ring spectra.
Existence:Compute topological Quillen (co)homology in a range forPostnikov section BPn.Play off of MU
Uniqueness:
Compute topological Quillen (co)homology.Obstructions for constructing an E4 map BP → BP ′ are zero.Any map of spectra BP → BP is either zero on an equivalence.
M.A.Mandell (IU) Obstruction Theory Mar 16 19 / 24
Application: BP
Application: BP
Theorem (Basterra-Mandell)BP has an E4 ring spectrum structure. It is unique up to automorphismin the homotopy category of E4 ring spectra.
Existence:Compute topological Quillen (co)homology in a range forPostnikov section BPn.Play off of MU
Uniqueness:
Compute topological Quillen (co)homology.Obstructions for constructing an E4 map BP → BP ′ are zero.Any map of spectra BP → BP is either zero on an equivalence.
M.A.Mandell (IU) Obstruction Theory Mar 16 19 / 24
Application: BP
Application: BP
Theorem (Basterra-Mandell)BP has an E4 ring spectrum structure. It is unique up to automorphismin the homotopy category of E4 ring spectra.
Existence:Compute topological Quillen (co)homology in a range forPostnikov section BPn.Play off of MU
Uniqueness:
Compute topological Quillen (co)homology.Obstructions for constructing an E4 map BP → BP ′ are zero.Any map of spectra BP → BP is either zero on an equivalence.
M.A.Mandell (IU) Obstruction Theory Mar 16 19 / 24
Application: BP
Computing topological Quillen homology of BP
Need some facts:H∗BP = Z(p)[ξ1, ξ2, . . .]
For augmented/non-unital En HZ(p)-algebras, topological Quillenhomology can be computed as an iterated bar construction.
D∗(BP; HZ(p)) ∼= π∗+4B4(HZ(p) ∧ BP)
We compute that this is free and concentrated in even degrees.
It follows that D∗(BP; HZ(p)) is concentrated in even degrees.
Obstruction for lifting a map BP → BP ′n to BP ′n+1 is an element ofDn+2(BP; Hπn+1BP).
M.A.Mandell (IU) Obstruction Theory Mar 16 20 / 24
Application: BP
Computing topological Quillen homology of BP
Need some facts:H∗BP = Z(p)[ξ1, ξ2, . . .]
For augmented/non-unital En HZ(p)-algebras, topological Quillenhomology can be computed as an iterated bar construction.
D∗(BP; HZ(p)) ∼= π∗+4B4(HZ(p) ∧ BP)
We compute that this is free and concentrated in even degrees.
It follows that D∗(BP; HZ(p)) is concentrated in even degrees.
Obstruction for lifting a map BP → BP ′n to BP ′n+1 is an element ofDn+2(BP; Hπn+1BP).
M.A.Mandell (IU) Obstruction Theory Mar 16 20 / 24
Application: BP
Computing topological Quillen homology of BP
Need some facts:H∗BP = Z(p)[ξ1, ξ2, . . .]
For augmented/non-unital En HZ(p)-algebras, topological Quillenhomology can be computed as an iterated bar construction.
D∗(BP; HZ(p)) ∼= π∗+4B4(HZ(p) ∧ BP)
We compute that this is free and concentrated in even degrees.
It follows that D∗(BP; HZ(p)) is concentrated in even degrees.
Obstruction for lifting a map BP → BP ′n to BP ′n+1 is an element ofDn+2(BP; Hπn+1BP).
M.A.Mandell (IU) Obstruction Theory Mar 16 20 / 24
Application: BP
Computing topological Quillen homology of BP
Need some facts:H∗BP = Z(p)[ξ1, ξ2, . . .]
For augmented/non-unital En HZ(p)-algebras, topological Quillenhomology can be computed as an iterated bar construction.
D∗(BP; HZ(p)) ∼= π∗+4B4(HZ(p) ∧ BP)
We compute that this is free and concentrated in even degrees.
It follows that D∗(BP; HZ(p)) is concentrated in even degrees.
Obstruction for lifting a map BP → BP ′n to BP ′n+1 is an element ofDn+2(BP; Hπn+1BP).
M.A.Mandell (IU) Obstruction Theory Mar 16 20 / 24
Application: BP
Computing topological Quillen homology of BP
Need some facts:H∗BP = Z(p)[ξ1, ξ2, . . .]
For augmented/non-unital En HZ(p)-algebras, topological Quillenhomology can be computed as an iterated bar construction.
D∗(BP; HZ(p)) ∼= π∗+4B4(HZ(p) ∧ BP)
We compute that this is free and concentrated in even degrees.
It follows that D∗(BP; HZ(p)) is concentrated in even degrees.
Obstruction for lifting a map BP → BP ′n to BP ′n+1 is an element ofDn+2(BP; Hπn+1BP).
M.A.Mandell (IU) Obstruction Theory Mar 16 20 / 24
Application: BP
Computing topological Quillen homology of BP
Need some facts:H∗BP = Z(p)[ξ1, ξ2, . . .]
For augmented/non-unital En HZ(p)-algebras, topological Quillenhomology can be computed as an iterated bar construction.
D∗(BP; HZ(p)) ∼= π∗+4B4(HZ(p) ∧ BP)
We compute that this is free and concentrated in even degrees.
It follows that D∗(BP; HZ(p)) is concentrated in even degrees.
Obstruction for lifting a map BP → BP ′n to BP ′n+1 is an element ofDn+2(BP; Hπn+1BP).
M.A.Mandell (IU) Obstruction Theory Mar 16 20 / 24
Application: BP
Computing topological Quillen homology of BP
Need some facts:H∗BP = Z(p)[ξ1, ξ2, . . .]
For augmented/non-unital En HZ(p)-algebras, topological Quillenhomology can be computed as an iterated bar construction.
D∗(BP; HZ(p)) ∼= π∗+4B4(HZ(p) ∧ BP)
We compute that this is free and concentrated in even degrees.
It follows that D∗(BP; HZ(p)) is concentrated in even degrees.
Obstruction for lifting a map BP → BP ′n to BP ′n+1 is an element ofDn+2(BP; Hπn+1BP).
M.A.Mandell (IU) Obstruction Theory Mar 16 20 / 24
Application: BP
The Computation
Want to show π∗B4(HZ(p) ∧ BP) is free and concentrated in evendegrees.
Suffices to compute homotopy groups of
B4(HZp ∧ BP) ∧HZp HZ/p ∼= B4(HZ/p ∧ BP)
Start with
π∗(HZ/p ∧ BP) = H∗(BP;Z/p) = Z/p[ξ1, ξ2, . . .], |ξi | = 2pi − 2.
Spectral sequence to compute π∗ of B(HZ/p ∧ BP) collapses at E2and you get π∗B(Z/p ∧ BP) is exterior on odd degree classes σξi indegree 2pi − 1.
M.A.Mandell (IU) Obstruction Theory Mar 16 21 / 24
Application: BP
The Computation
Want to show π∗B4(HZ(p) ∧ BP) is free and concentrated in evendegrees.
Suffices to compute homotopy groups of
B4(HZp ∧ BP) ∧HZp HZ/p ∼= B4(HZ/p ∧ BP)
Start with
π∗(HZ/p ∧ BP) = H∗(BP;Z/p) = Z/p[ξ1, ξ2, . . .], |ξi | = 2pi − 2.
Spectral sequence to compute π∗ of B(HZ/p ∧ BP) collapses at E2and you get π∗B(Z/p ∧ BP) is exterior on odd degree classes σξi indegree 2pi − 1.
M.A.Mandell (IU) Obstruction Theory Mar 16 21 / 24
Application: BP
The Computation
Want to show π∗B4(HZ(p) ∧ BP) is free and concentrated in evendegrees.
Suffices to compute homotopy groups of
B4(HZp ∧ BP) ∧HZp HZ/p ∼= B4(HZ/p ∧ BP)
Start with
π∗(HZ/p ∧ BP) = H∗(BP;Z/p) = Z/p[ξ1, ξ2, . . .], |ξi | = 2pi − 2.
Spectral sequence to compute π∗ of B(HZ/p ∧ BP) collapses at E2and you get π∗B(Z/p ∧ BP) is exterior on odd degree classes σξi indegree 2pi − 1.
M.A.Mandell (IU) Obstruction Theory Mar 16 21 / 24
Application: BP
The Computation
Want to show π∗B4(HZ(p) ∧ BP) is free and concentrated in evendegrees.
Suffices to compute homotopy groups of
B4(HZp ∧ BP) ∧HZp HZ/p ∼= B4(HZ/p ∧ BP)
Start with
π∗(HZ/p ∧ BP) = H∗(BP;Z/p) = Z/p[ξ1, ξ2, . . .], |ξi | = 2pi − 2.
Spectral sequence to compute π∗ of B(HZ/p ∧ BP) collapses at E2and you get π∗B(Z/p ∧ BP) is exterior on odd degree classes σξi indegree 2pi − 1.
M.A.Mandell (IU) Obstruction Theory Mar 16 21 / 24
Application: BP
Spectral sequence to compute π∗ of B2(HZ/p ∧ BP) collapses at E2and you get E∞-term is truncated polynomial on classes γjσ
2ξi indegree 2pi+j , for i = 1,2, . . ., and j = 0,1, . . .
Need to figure out multiplicative extensions in order to get the fullcomputation for π∗B2(Z/p ∧ BP)
Map BP → HZ/p is an E4 map, so can read of the Dyer-Lashofoperations (that exist on the homology of E4 ring spectra) on H∗BPfrom the Dyer-Lashof operations for H∗ The Dyer-Lashof operation
Qpiξi = ξi+1 + decomposables
implies that
(γjσ2ξi)
p =
Qpi+jγjσ
2ξi = γjσ2ξi+1.
But this is now the p-th power operation.
M.A.Mandell (IU) Obstruction Theory Mar 16 22 / 24
Application: BP
Spectral sequence to compute π∗ of B2(HZ/p ∧ BP) collapses at E2and you get E∞-term is truncated polynomial on classes γjσ
2ξi indegree 2pi+j , for i = 1,2, . . ., and j = 0,1, . . .
Need to figure out multiplicative extensions in order to get the fullcomputation for π∗B2(Z/p ∧ BP)
Map BP → HZ/p is an E4 map, so can read of the Dyer-Lashofoperations (that exist on the homology of E4 ring spectra) on H∗BPfrom the Dyer-Lashof operations for H∗ The Dyer-Lashof operation
Qpiξi = ξi+1 + decomposables
implies that
(γjσ2ξi)
p =
Qpi+jγjσ
2ξi = γjσ2ξi+1.
But this is now the p-th power operation.
M.A.Mandell (IU) Obstruction Theory Mar 16 22 / 24
Application: BP
Spectral sequence to compute π∗ of B2(HZ/p ∧ BP) collapses at E2and you get E∞-term is truncated polynomial on classes γjσ
2ξi indegree 2pi+j , for i = 1,2, . . ., and j = 0,1, . . .
Need to figure out multiplicative extensions in order to get the fullcomputation for π∗B2(Z/p ∧ BP)
Map BP → HZ/p is an E4 map, so can read of the Dyer-Lashofoperations (that exist on the homology of E4 ring spectra) on H∗BPfrom the Dyer-Lashof operations for H∗ The Dyer-Lashof operation
Qpiξi = ξi+1 + decomposables
implies that
(γjσ2ξi)
p =
Qpi+jγjσ
2ξi = γjσ2ξi+1.
But this is now the p-th power operation.
M.A.Mandell (IU) Obstruction Theory Mar 16 22 / 24
Application: BP
Spectral sequence to compute π∗ of B2(HZ/p ∧ BP) collapses at E2and you get E∞-term is truncated polynomial on classes γjσ
2ξi indegree 2pi+j , for i = 1,2, . . ., and j = 0,1, . . .
Need to figure out multiplicative extensions in order to get the fullcomputation for π∗B2(Z/p ∧ BP)
Map BP → HZ/p is an E4 map, so can read of the Dyer-Lashofoperations (that exist on the homology of E4 ring spectra) on H∗BPfrom the Dyer-Lashof operations for H∗ The Dyer-Lashof operation
Qpiξi = ξi+1 + decomposables
implies that(γjσ
2ξi)p = Qpi+j
γjσ2ξi = γjσ
2ξi+1.
But this is now the p-th power operation.
M.A.Mandell (IU) Obstruction Theory Mar 16 22 / 24
Application: BP
Associated graded was truncated poly on γjσ2ξi ; now know
(γjσ2ξi)
p = γjσ2xi+1. So we get
π∗B2(HZ/p ∧ BP) = Z/p[γ0σ2x1, γ1σ
2x1, γ2σ2x1, . . .]
polynomial on classes in degrees 2pj+1 for j = 0,1, . . .
Looking at the spectral sequence, we get π∗B3(HZ/p ∧ BP) is exterioron odd degree classes in degrees 2pj+1 + 1.
Looking at the spectral sequence, we get π∗B4(HZ/p ∧ BP) isconcentrated in even degrees.
M.A.Mandell (IU) Obstruction Theory Mar 16 23 / 24
Application: BP
Associated graded was truncated poly on γjσ2ξi ; now know
(γjσ2ξi)
p = γjσ2xi+1. So we get
π∗B2(HZ/p ∧ BP) = Z/p[γ0σ2x1, γ1σ
2x1, γ2σ2x1, . . .]
polynomial on classes in degrees 2pj+1 for j = 0,1, . . .
Looking at the spectral sequence, we get π∗B3(HZ/p ∧ BP) is exterioron odd degree classes in degrees 2pj+1 + 1.
Looking at the spectral sequence, we get π∗B4(HZ/p ∧ BP) isconcentrated in even degrees.
M.A.Mandell (IU) Obstruction Theory Mar 16 23 / 24
Application: BP
Associated graded was truncated poly on γjσ2ξi ; now know
(γjσ2ξi)
p = γjσ2xi+1. So we get
π∗B2(HZ/p ∧ BP) = Z/p[γ0σ2x1, γ1σ
2x1, γ2σ2x1, . . .]
polynomial on classes in degrees 2pj+1 for j = 0,1, . . .
Looking at the spectral sequence, we get π∗B3(HZ/p ∧ BP) is exterioron odd degree classes in degrees 2pj+1 + 1.
Looking at the spectral sequence, we get π∗B4(HZ/p ∧ BP) isconcentrated in even degrees.
M.A.Mandell (IU) Obstruction Theory Mar 16 23 / 24
Application: BP
M.A.Mandell (IU) Obstruction Theory Mar 16 24 / 24