nLab Poincaré duality algebra



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Generally, a Poincaré duality dg-algebra is a dg-algebra with structure mimicking Poincaré duality in ordinary cohomology.

On the other hand a Poincaré duality C *C^\ast-algebra is a C*-algebra which represents a space in noncommutative topology for which there is a sensible notion of Poincaré duality in K-theory (operator K-theory/K-homology).


For graded-commutative algebras


The structure of a Poincaré duality algebra in dimension nn on a graded-commutative graded associative algebra AA is a linear function ϵ:A nk\epsilon \colon A_n \to k to the ground field such that all the induced bilinear forms

A kA nkA nϵk A_k \otimes A_{n-k} \stackrel{\otimes}{\to} A_n \stackrel{\epsilon}{\to} k

are non-degenerate.

e.g. (Lambrechst-Stanley 07)

For C *C^\ast-algebras

For C*-algebras hence in noncommutative topology there is the following notion of Poincaré duality, which is really Poincaré with respect not to ordinary cohomology but K-theory (operator K-theory).

We start with the definition of Poincaré self-duality and then generalize to Poincaré dual pairs.


A separable C*-algebra AA \in C*Alg is a Poincaré duality algebra (or PD algebra, for short ) if it is dualizable object when regarded as an object of the KK-theory-category, with dual object its opposite algebra.

The element Δ\Delta in def. is called a fundamental class of AA.

This appears as (BMRS 07, def. 2.1, following Connes, p. 601) following (Connes).


Explicitly def. says that AA is a PD algebra if there exists ΔKK(AA op,)\Delta \in KK(A \otimes A^{op}, \mathbb{C}) and Δ KK(,AA op)\Delta^\vee \in KK(\mathbb{C}, A \otimes A^{op}) such that

Δ A opΔ=id AKK(A,A) \Delta^\vee \otimes_{A^{op}} \Delta = id_A \in KK(A,A)


Δ AΔ=id A opKK(A op,A op). \Delta^\vee \otimes_A \Delta = id_{A^{op}} \in KK(A^{op}, A^{op}) \,.

For AA BB two Poincaré duality algebras, def. , and for f:ABf \colon A \to B a homomorphism between them, regarded as a morphism f *:BAf^\ast \colon B \to A in KK-theory, the correspondung dual morphism f!:ABf! \colon A \to B is the one such that postcomposition in KKKK with this corresponds to the Umkehr map/push forward in generalized cohomology? in KK-theory.

For more on this see below at Properties – K-Orientation and Umkehr mpas.


For C *C^\ast-algebras which are groupoid convolution algebras C *(𝒢)C^\ast(\mathcal{G}) the opposite algebra is Morita equivlant (since a groupoid 𝒢\mathcal{G} is equivalent to its opposite groupoid 𝒢 op\mathcal{G}^{op}, the equivalence being induced by the functor which sends a morphism to its inverse). But given a circle 2-bundle χ:𝒢B 2U(1)\chi \colon \mathcal{G} \to \mathbf{B}^2 U(1) the corresponding twisted groupoid convolution algebra is such that passing to the opposite corresponds to passing to the inverse twist χ-\chi.

Therefore it makes sense to consider more generally


For AA a C*-algebra a Poincaré dual for AA is a dual object BC *AlgKKB \in C^\ast Alg \to KK in KK-theory.

Below in the Proposition-Section is discussed how under Poincaré-duality the twist changes.


For dg-Algebras

For C *C^\ast-algebras

Duals and twists


Let XX be a closed manifold with spin^c-structure. Then there is a Poincaré duality isomorphism

K (X)K (X). K^\bullet(X) \simeq K_\bullet(X) \,.

For instance (Connes, chapter 2.7, prop. 5).

(…) The relaton between Poincaré duality on algebras of functions and spin^c-structure is discussed in (Connes, around p. 603). (…)

Notice that the obstruction to spin^c structure is the third integral Stiefel-Whitney class W 3:BSOB 2U(1)W_3 \colon B SO \to B^2 U(1). If this does not vanish on a manifold, then a Poincaré dual/dual object in KK-theory still exists, but is the same manifold equipped with a twist shifted by W 3(τ X)W_3(\tau_X), where τ X\tau_X denotes the (co)tangent bundle of XX.


For XX a (compact) manifold and cH 3(X,)c \in H^3(X,\mathbb{Z}) the class of a circle 2-bundle/bundle gerbe 𝒢\mathcal{G} on XX, write

C c(X)C *AlgKK C_c(X) \in C^\ast Alg \to KK

for the corresponding twisted groupoid convolution algebra, the one whose operator K-theory is the cc-twisted K-theory of XX:

KK (,C c(X))K +c(X). KK_\bullet(\mathbb{C}, C_c(X)) \simeq K_{\bullet + c}(X) \,.

Let XX be a compact manifold with tangent bundle τ X\tau_X and let cH 3(X,)c \in H^3(X,\mathbb{Z}) be a twist. Then the C*-algebra C c(X)C_{c}(X) of def. has a dual object in the full subcategory of KK-theory on separable C*-algebras, given by

(C c(X)) C 1cW 3(τ X)(X), (C_c(X))^\vee \simeq C_{\frac{1}{c\otimes W_3(\tau_X)}}(X) \,,

hence by the same manifold but with twist the inverse of the third integral Stiefel-Whitney class and the original twist.

The same remains true in GG-equivariant KK-theory, for GG a locally compact topological group.

The non-equivariant case is in (Brodzki-Mathai-Rosenberg-Szabo 06, section 7.3) and the generalization to the equivariant case in (Tu 06, theorem 3.1) (where we use remark in order to interpret the opposite twisted convolution algebra up to equivalence as inducing the inverse twist).

K-Orientation and Umkehr maps

We discuss Umkehr maps/fiber integration in generalized cohomology in K-theory using Poincaré duality algebras / dual objects in KK-theory.


Every homomorphism f:ABf \colon A \to B between PD C *C^\ast-algebras is K-orientable in KK-theory. The K-orientation is given by the corresponding dual morphism, hence the element f!:BAf! \colon B \to A given as the composite

f!Δ A A opf op B opΔ B. f! \coloneqq \Delta^\vee_A \otimes_{A^{op}} f^{op} \otimes_{B^{op}} \Delta_B \,.

(BMRS 07, 3.3)

More generally we have the following.


Let i:QXi \colon Q \to X be a map of compact manifolds and let χ:XB 2U(1)\chi \colon X \to B^2 U(1) modulate a circle 2-bundle regarded as a twist for K-theory. Then forming twisted groupoid convolution algebras yields a KK-theory morphism of the form

C i *χ(Q)i *C χ(X), C_{i^\ast \chi}(Q) \stackrel{i^\ast}{\longleftarrow} C_{\chi}(X) \,,

with notation as in def. . By prop. the dual morphism is of the form

C 1i *χW 3(TQ)(Q)i !C 1χW 3(TX)(X). C_{\frac{1}{i^\ast \chi \otimes W_3(T Q)}}(Q) \stackrel{i_!}{\longrightarrow} C_{\frac{1}{\chi \otimes W_3(T X)}}(X) \,.

If we redefine the twist on XX to absorb this “quantum correction” as χ1χW 3(TX)\chi \mapsto \frac{1}{\chi \otimes W_3(T X)} then this is

C i *χW 3(i *TX)W 3(TQ)(Q)i !C χ(X), C_{i^\ast \chi\frac{W_3(i^\ast T X)}{W_3(T Q)}}(Q) \stackrel{i_!}{\longrightarrow} C_{\chi}(X) \,,

Postcomposition with this map in KK-theory now yields a map from the i *χW 3(NQ)i^\ast \chi \otimes W_3(N Q)-twisted K-theory of QQ to the χ\chi-twisted K-theory of XX:

i !:K +W 3(NQ)+i *χ(Q)K +χ. i_! \colon K_{\bullet + W_3(N Q) + i^\ast \chi}(Q) \to K_{\bullet +\chi} \,.

This is the twisted Umkehr map in this context.

If we here think of i:QXi \colon Q \hookrightarrow X as being the inclusion of a D-brane worldvolume, then χ\chi would be the class of the background B-field and an element

[ξ]K +W 3(NQ)+i *χ(Q) [\xi] \in K_{\bullet + W_3(N Q) + i^\ast \chi}(Q)

is called (the K-class of) a Chan-Paton gauge field on the D-brane satisfying the Freed-Witten-Kapustin anomaly cancellation mechanism. (The orginal Freed-Witten anomaly cancellation assumes ξ\xi given by a twisted line bundle in which case it exhibits a twisted spin^c structure on QQ.) Finally its push-forward

[i !ξ]K χ(X) [i_! \xi] \in K_{\bullet- \chi}(X)

is called the corresponding D-brane charge.

See (Nuiten 13).



For A=C 0(X)A = C_0(X) the algebra of functions on a compact complex manifold XX, then AA is a PD algebra with fundamental class Δ\Delta in K-homology given by the Dolbeault operator on X×XX \times X.

(BMRS 07, example 3.2)


For A=C 0(X)A = C_0(X) the algebra of functions vanishing at infinity of a manifold XX with spin^c structure. Take B=C 0(T *X) KKA opAB = C_0(T^\ast X) \simeq_{KK} A^{op} \simeq A. Then Δ\Delta constructed from the Dirac operator on the Clifford algebra bundle over T *XT^\ast X is a fundamental class.

(BMRS 07, proof of theorem 2.9)


For graded associative algebras

  • Pascal Lambrechts, Don Stanley, Poincaré duality and commutative differential graded algebras (arXiv:math/0701309)

For C *C^\ast-algebras

For C*-algebras/in noncommutative topology:

  • Henri Moscovici, Eigenvalue inequalities and Poincaré duality in noncommutative geometry, Commun. Math. Phys. 184 , 3 (1997) 619

Chapter 6.4 β\beta (starting p. 601) in

Def. 2.1 in

Duality including the twisted K-theory induced by twisted spin^c structure over manifolds is discussed in section 7 of

and generalized to equivariant KK-theory in

More on dual objects in KK is in

Discussion of the twisted Umkehr map and the Freed-Witten-Kapustin anomaly in this context is in

Last revised on May 19, 2023 at 20:41:01. See the history of this page for a list of all contributions to it.