arXiv:math.OA/0002125 v1 15 Feb 2000 - Alain Connes

35 Pages·2003·0.28 MB·English

CYCLIC COHOMOLOGY AND HOPF SYMMETRY
Alain CONNES1 and Henri MOSCOVICI2∗
1 Coll`ege de France, 3, rue Ulm, 75005 Paris
and
I.H.E.S., 35, route de Chartres, 91440 Bures-sur-Yvette
0 2 Department of Mathematics, The Ohio State University
0
0 231 W. 18th Avenue, Columbus, OH 43210, USA
2
Abstract
b
e
F Cyclic cohomology has been recently adapted to the treatment of
5 Hopf symmetryin noncommutative geometry. Theresulting theory of
1 characteristic classes for Hopf algebras and their actions on algebras

allows to expand the range of applications of cyclic cohomology. It is

1 the goal of the present paper to illustrate these recent developments,
v
with special emphasis on the application to transverse index theory,

5
andpointtowards futuredirections. Inparticular,wehighlightthere-
2
1 markableaccordbetweenourframeworkforcycliccohomologyofHopf
2 algebras on one hand and both the algebraic as well as the analytic
0
theory of quantum groups on the other, manifest in the construction
0
0 of the modular square.
/
A
O
.
h
Introduction
t
a
m Cyclic cohomology of noncommutative algebras is playing in noncom-
: mutative geometry a similar rˆole to that of de Rham cohomology in differ-
v
i ential topology [11]. In [14] and [15], cyclic cohomology has been adapted to
X
Hopf algebras and their actions on algebras, which are analogous to the Lie
r
a group/algebra actions on manifolds and embody a natural notion of sym-
metry in noncommutative geometry. The resulting theory of characteristic
classes for Hopf actions allows in turn to widen the scope of applications of
∗ThesecondauthorhasconductedthisworkduringemploymentbytheClayMathematics
Institute, as a CMI Scholar in residence at Harvard University. This material is based
in part upon research supported by the National Science Foundation under award no.
DMS-9706886.
1
cyclic cohomology to index theory. It is the goal of the present paper to
review these recent developments and point towards future directions.
The contents of the paper are as follows. In 1 we recall the basic
§
notationpertainingtothecyclictheory. Theadaptationofcycliccohomology
to Hopf algebrasandHopf actionsis reviewed in 2, where we also discuss the
§
relationship with Lie group/algebra cohomology. 3 deals with the geometric
§
Hopfalgebrasarising intransverse differentialgeometryandtheirapplication
to transverse index theory. Finally, 4 illustrates the remarkable agreement
§
between our framework for cyclic cohomology of Hopf algebras and both the
algebraic as well as the analytic theory of quantum groups.
1 Cyclic cohomology
Cyclic cohomology has first appeared as a cohomology theory for alge-
bras ([5], [7], [26]). In its simplest form, the cyclic cohomology HC∗( ) of
A
an algebra (over R or C in what follows) is the cohomology of the cochain
A
complex C∗( ), b , where Cn( ), n 0, consists of the (n+1)-linear forms
{ λ A } λ A ≥
ϕ on satisfying the cyclicity condition
A
ϕ(a0,a1,...,an) = ( 1)nϕ(a1,a2,...,a0), a0,a1,...,an (1.1)
− ∈ A
and the coboundary operator is given by
n
(bϕ)(a0,...,an+1) = ( 1)j ϕ(a0,...,ajaj+1,...,an+1)
−
Xj=0
+( 1)n+1ϕ(an+1a0,a1,...,an). (1.2)
−
Whenthealgebra comes equipped withalocallyconvex topologyforwhich
A
the product is continuous, the above complex is replaced by its topological
version: Cn( ) then consists of all continuous (n + 1)-linear form on
λ A A
satisfying (1.1).
Cyclic cohomology provides numerical invariants of K-theory classes as
follows ([8]). Given an n-dimensional cyclic cocycle ϕ on , n even, the
A
scalar
ϕ Tr(E,E,...,E) (1.3)
⊗
2
is invariant under homotopy for idempotents
E2 = E M ( ) = M (C).
N N
∈ A A⊗
In the above formula, ϕ Tr is the extension of ϕ to M ( ), using the
N
⊗ A
standard trace Tr on M (C):
N
ϕ Tr(a0 µ0,a1 µ1,...,an µn) = ϕ(a0,a1,...,an)Tr(µ0µ1 µn).
⊗ ⊗ ⊗ ⊗ ···
This defines a pairing [ϕ],[E] between cyclic cohomology and K–theory,
h i
which extends to the general noncommutative framework the Chern-Weil
construction of characteristic classes of vector bundles.
Indeed, if = C∞(M) for a closed manifold M and
A
ϕ(f0,f1,...,fn) = Φ, f0df1 df2 ... dfn , f0,f1,...,fn ,
h ∧ ∧ ∧ i ∈ A
where Φ is an n-dimensional closed de Rham current on M, then up to
normalization the invariant defined by (1.3) is equal to
Φ, ch∗( ) ;
h E i
here ch∗( ) denotes the Chern character of the rank N vector bundle on
E E
M whose fiber at x M is the range of E(x) M (C).
N
∈ ∈
Note that, in the above example,
ϕ(fσ(0),fσ(1),...,fσ(n)) = ε(σ)ϕ(f0,f1,...,fn),
for any permutation σ of the set [n] = 0,1,...,n , with signature ε(σ).
{ }
However, the extension ϕ Tr to M ( ), used in the pairing formula (1.3),
N
⊗ A
retains only the cyclic invariance.
A simple but very useful class of examples of cyclic cocycles on a non-
commutative algebra is obtained from group cohomology ([10], [3], [12]), as
follows. Let Γ be an arbitrary group and let = CΓ be its group ring.
A
Then any normalized group cocycle c Zn(Γ,C) , representing an arbitrary
∈
cohomology class [c] H∗(BΓ) = H∗(Γ), gives rise to a cyclic cocycle ϕ on
c
∈
the algebra by means of the formula
A
0 if g ...g = 1
ϕ (g ,g ,...,g ) = 0 n 6 (1.4)
c 0 1 n (cid:26)c(g ,...,g ) if g ...g = 1
1 n 0 n
3
extended by linearity to CΓ.
In a dual fashion, one defines the cyclic homology HC ( ) of an alge-
∗
A
bra as the homology of the chain complex Cλ( ), b consisting of the
A { ∗ A }
coinvariants under cyclic permutations of the tensor powers of , and with
A
the boundary operator b obtained by transposing the coboundary formula
(1.2). Then the pairing between cyclic cohomology and K–theory (1.3) fac-
tors through the natural pairing between cohomology and homology, i.e.
ϕ Tr(E,E,...,E) = ϕ, ch(E) , (1.5)
⊗ h i
where, again up to normalization,
ch (E) = E ... E (n+1 times) (1.6)
n
⊗ ⊗
represents the Chern character in HC ( ), for n even, of the K–theory class
n
A
[E] K ( ).
0
∈ A
The cyclic cohomology of an (unital) algebra has an equivalent de-
A
scription, in terms of the bicomplex (CC∗,∗( ), b, B), defined as follows.
A
With Cn( ) denoting the linear space of (n+1)-linear forms on A, set
A
CCp,q( ) = Cq−p( ), q p,
A A ≥
(1.7)
CCp,q( ) = 0, q < p.
A
The vertical operator b : Cn( ) Cn+1( ) is defined as
A → A
(bϕ)(a0,...,an+1) =
n( 1)jϕ(a0,...,ajaj+1,...,an+1)
0 −
P+( 1)n+1ϕ(an+1a0,a1,...,an). (1.8)
−
The horizontal operator B : Cn( ) Cn−1( ) is defined by the formula
A → A
B = NB ,
0
where
B, ϕ(a0,...,an−1) = ϕ(1,a0,...,an−1) ( 1)nϕ(a0,...,an−1,1)
0
− −
(Nψ)(a0,...,an) = n( 1)njψ(aj,aj+1,...,aj−1). (1.9)
0 −
P
4
ThenHC∗( )isthecohomologyofthefirstquadranttotalcomplex(TC∗( ), b+
A A
B), formed as follows:
n
TCn( ) = CCp,n−p( ). (1.10)
A A
Xp=0
On the other hand, the cohomology of the full direct sum total complex
(TCΣ ( ), b+B), formed by taking direct sums as follows:
∗
A
TCΣ ( ) = CCp,n−p( ), (1.11)
n
A A
Xs
gives the ( Z/2–graded) periodic cyclic cohomology groups HC∗ ( ).
per
A
There is a dual description for the cyclic homology of , in terms of the
A
dual bicomplex (CC ( ), b, B), with C ( ) = ⊗n+1 and the boundary
∗,∗ n
A A A
operators b, B obtained by transposing the corresponding coboundaries. The
periodiccyclichomologygroupsHC per( )areobtainedfromthefull product
∗
A
total complex (TC ∗( ), b+B), formed by taking direct products as follows:
Π
A
TC n( ) = Π CC ( ). (1.12)
Π p p,n−p
A A
The Chern character of an idempotent e2 = e is given in this picture by
∈ A
the periodic cycle (ch (e)) , with components:
n n=2,4,...
(2k)! 1
ch (e) = e, ch (e) = ( 1)k (e⊗2k+1 e⊗2k), k 1. (1.13)
0 2k
− k! − 2 ⊗ ≥
ThefunctorsHC0 andHC fromthecategoryofalgebrastothecategory
0
of vector spaces have clear intrinsic meaning: the first assigns to an algebra
the vector space of traces on , while the second associates to its
A A A
abelianization /[ , ]. From a conceptual viewpoint, it is important to
A A A
realize the higher co/homologies HC∗, resp. HC , as derived functors. The
∗
obviousobstructiontosuchaninterpretationisthenon-additivenatureofthe
category of algebras and algebra homomorphisms. This has been remedied
in [9], by replacing it with the category of Λ-modules over the cyclic category
Λ.
The cyclic category Λ is a small category, obtained by enriching with
cyclic morphisms the familiar simplicial category ∆ of totally ordered finite
5
sets and increasing maps. We recall the presentation of ∆ by generators and
relations. It has one object [n] = 0 < 1 < ... < n for each integer n 0,
{ } ≥
and is generated by faces δ : [n 1] [n] (the injection that misses i), and
i
− →
degeneracies σ : [n+1] [n] (the surjection which identifies j with j +1),
j
→
with the following relations:
δ δ = δ δ for i < j, σ σ = σ σ i j (1.14)
j i i j−1 j i i j+1
≤
δ σ i < j
i j−1
σ δ = 1 if i = j or i = j +1
j i n
δ σ i > j +1.
i−1 j

To obtain Λ one adds for each n a new morphism τ : [n] [n] such that,
n
→
τ δ = δ τ 1 i n,
n i i−1 n−1
≤ ≤
τ σ = σ τ 1 i n, (1.15)
n i i−1 n+1
≤ ≤
τn+1 = 1 .
n n
Note that the above relations also imply:
τ δ = δ , τ σ = σ τ2 . (1.16)
n 0 n n 0 n n+1
Alternatively, Λ can be defined by means of its “cyclic covering”, the
category EΛ. The latter has one object (Z,n) for each n 0 and the
≥
morphisms f : (Z,n) (Z,m) are given by non decreasing maps f : Z Z ,
→ →
such that f(x+n) = f(x)+m, x Z. One has Λ = EΛ/Z, with respect
∀ ∈
to the obvious action of Z by translation.
To any algebra A one associates a module ♮ over the category Λ by
A
assigning to each integer n 0 the vector space Cn( ) of (n + 1)-linear
≥ A
forms ϕ(a0,...,an) on A, and to the generating morphisms the operators
6
δ : Cn−1 Cn, σ : Cn+1 Cn defined as follows:
i i
→ →
(δ ϕ)(a0,...,an) = ϕ(a0,...,aiai+1,...,an), i = 0,1,...,n 1,
i
−
(δ ϕ)(a0,...,an) = ϕ(ana0,a1,...,an−1);
n
(σ ϕ)(a0,...,an) = ϕ(a0,1,a1,...,an),
0
(σ ϕ)(a0,...,an) = ϕ(a0,...,aj,1,aj+1,...,an), j = 1,...,n 1,
j
−
(σ ϕ)(a0,...,an) = ϕ(a0,...,an,1);
n
(τ ϕ)(a0,...,an) = ϕ(an,a0,...,an−1).
n
(1.17)
These operations satisfy the relations (1.14) and (1.15), which shows that ♮
A
is indeed a Λ-module.
One thus obtains the desired interpretation of the cyclic co/homology
groups of a k-algebra over a ground ring k in terms of derived functors
A
over the cyclic category ([9]):
HCn( ) Extn(k♮, ♮) and HC ( ) TorΛ( ♮,k♮).
A ≃ Λ A n A ≃ n A
Moreover, all of the fundamental properties of the cyclic co/homology of al-
gebras, suchasthelongexact sequence relatingittoHochschild co/homology
([8], [23]), are shared by the functors Ext∗/TorΛ-functors and, in this gener-
Λ ∗
ality, can be attributed to the coincidence between the classifying space BΛ
of the small category Λ and the classifying space BS1 P (C) of the circle
∞
≃
group.
Let us finally mention that, from the very definition of Ext∗(k♮,F) and
Λ
the existence of a canonical projective biresolution for k♮ ([9]), it follows
that the cyclic cohomology groups HC∗(F) of a Λ-module F, as well as
the periodic ones HC ∗(F), can be computed by means of a bicomplex
per
analogous to (1.7). A similar statement holds for the cyclic homology groups.
7
2 Cyclic theory for Hopf algebras
The familiar antiequivalence between suitable categories of spaces and
matching categories of associative algebras, effected by the passage to coor-
dinates, is of great significance in both the purely algebraic context (affine
schemes versus commutative algebras) as well as the topological one (locally
compact spaces versus commutative C∗-algebras). By extension, it has been
adopted as a fundamental principle of noncommutative geometry. When ap-
plied to the realm of symmetry, it leads to promoting the notion of a group,
whose coordinates form a commutative Hopf algebra, to that of a general
Hopf algebra. The cyclic categorical formulation recalled above allows to
adapt cyclic co/homology in a natural way to the treatment of symmetry
in noncommutative geometry. This has been done in [14], [15] and will be
reviewed below.
We consider a Hopf algebra over k = R or C, with unit η : k ,
H → H
counit ε : k and antipode S : . We use the standard definitions
H → H → H
([25]) together the usual convention for denoting the coproduct:
∆(h) = h h , h . (2.1)
(1) (2)
⊗ ∈ H
X
Althoughweworkinthealgebraiccontext, weshallincludeadatumintended
to play the rˆole of the modular function of a locally compact group. For
reasons of consistency with the Hopf algebra context, this datum has a self-
dual nature: it comprises both a character δ ∗,
∈ H
δ(ab) = δ(a)δ(b), a,b , (2.2)
∀ ∈ H
and a group-like element σ ,
∈ H
∆(σ) = σ σ, ε(σ) = 1, (2.3)
⊗
related by the condition
δ(σ) = 1. (2.4)
Such a pair (δ,σ) will be called a modular pair.
The character δ gives rise to a δ-twisted antipode S = S : , defined
δ
H → H
by
e
S(h) = δ(h ) S(h ) , h . (2.5)
(1) (2)
∈ H
X
(h)
e
8
Like the untwisted antipode, S is an algebra antihomomorphism
S(h1h2) =eS(h2)S(h1) , h1,h2
∀ ∈ H
(2.6)
e e e
S(1) = 1,
a coalgebra twistedeantimorphism
∆S(h) = S(h ) S(h ) , h ; (2.7)
(2) (1)
⊗ ∀ ∈ H
X
(h)
e e
and it also satisfies the identities
ε S = δ, δ S = ε. (2.8)
◦ ◦
We start by associatingeto , viewedeonly as a coalgebra, the stan-
H
dard cosimplicial module known as the cobar resolution ([1], [4]), twisted
by the insertion of the group-like element σ . Specifically, we set
∈ H
Cn( ) = ⊗n, n 1 and C0( ) = k, then define the face operators
H H ∀ ≥ H
δ : Cn−1( ) Cn( ), 0 i n, as follows: if n > 1,
i
H → H ≤ ≤
δ (h1 ... hn−1) = 1 h1 ... hn−1,
0
⊗ ⊗ ⊗ ⊗ ⊗
δ (h1 ... hn−1) = h1 ... ∆hj ... hn−1
j
⊗ ⊗ ⊗ ⊗ ⊗ ⊗
= h1 ... hj hj ... hn−1, 1 j n 1,
⊗ ⊗ (1) ⊗ (2) ⊗ ⊗ ≤ ≤ −
X
(hj)
(2.9)
δ (h1 ... hn−1) = h1 ... hn−1 σ,
n
⊗ ⊗ ⊗ ⊗ ⊗
while if n = 1
δ (1) = 1, δ (1) = σ.
0 1
Next, the degeneracy operators σ : Cn+1( ) Cn( ), 0 i n, are
i
H → H ≤ ≤
defined by:
σ (h1 ... hn+1) = h1 ... ε(hi+1) ... hn+1
i
⊗ ⊗ ⊗ ⊗ ⊗ ⊗
= ε(hi+1)h1 ... hi hi+2 ... hn+1 (2.10)
⊗ ⊗ ⊗ ⊗ ⊗
and for n = 0
σ (h) = ε(h), h .
0
∈ H
9
Theremainingfeaturesofthegivendata,namelytheproductandtheantipode
of together with the character δ ∗, are used to define the candidate
H ∈ H
for the cyclic operator, τ : Cn( ) Cn( ), as follows:
n
H → H
τ (h1 ... hn) = (∆n−1S(h1)) h2 ... hn σ
n
⊗ ⊗ · ⊗ ⊗ ⊗
= S(h1 )h2 ... Se(h1 )hn S(h1 )σ. (2.11)
(n) ⊗ ⊗ (2) ⊗ (1)
X(h1)
e
Note that
τ2(h) = τ (S(h)σ) = σ−1S2(h)σ,
1 1
therefore the following is a necesseary conditionefor cyclicity:
(σ−1 S)2 = I. (2.12)
◦
The remarkable fact is that this conditeion is also sufficient for the implemen-
tation of the sought-for Λ-module.
A modular pair (δ,σ) satisfying (2.12) is called a modular pair in invo-
lution.
Theorem 1 ([14], [15]) Let be a Hopf algebra endowed with a modu-
H
lar pair (δ,σ) in involution. Then ♮ = Cn( ) equipped with the
H(δ,σ) { H }n≥0
operators given by (2.9) – (2.11) is a module over the cyclic category Λ.
The cyclic cohomology groups corresponding to the Λ-module ♮ ,
H(δ,σ)
denoted HC∗ ( ), can be computed from the bicomplex (CC∗,∗( ), b, B),
(δ,σ) H H
analogous to (1.7), defined as follows:
CCp,q( ) = Cq−p( ), q p,
H H ≥
(2.13)
CCp,q( ) = 0, q < p;
H
the operator
n
b : Cn−1( ) Cn( ), b = ( 1)iδ . (2.14)
i
H → H −
Xi=0
is explicitly given, if n 1, by
≥
b(h1 ... hn−1) = 1 h1 ... hn−1
⊗ ⊗ ⊗ ⊗ ⊗
10