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Revista de la Unión Matemática Argentina
versión impresa ISSN 0041-6932versión On-line ISSN 1669-9637
Rev. Unión Mat. Argent. v.47 n.1 Bahía Blanca ene./jun. 2006
Spectral properties of elliptic operators on bundles of -manifolds
Ricardo A. Podestá
Abstract: We present some results on the spectral geometry of compact Riemannian manifolds having holonomy group isomorphic to ,
, for the Laplacian on mixed forms and for twisted Dirac operators.
2000 Mathematics Subject Classification. Primary 58J53; Secondary 58C22, 20H15.
Key words and phrases. compact flat manifolds, spectrum, Laplacian, spin structures, Dirac operator, isospectrality, spectral asymmetry, eta invariants
Supported by Conicet
This expository article is based on a homonymous talk I gave during the "II Encuentro de Geometría" which took place in La Falda, Sierras de Córdoba, from June 6th to 11th of 2005. It summarizes previous results from [MP], [MP2], [MPR], and [Po], answering standard questions in spectral geometry by using a special class of compact Riemannian manifolds.
Spectral Geometry. It is a kind of mixture between Spectral Theory and Riemannian Geometry. The general situation is to consider (pseudo) differential operators acting on sections of bundles of Riemannian manifolds. However, one usually considers a compact Riemannian manifold , a vector bundle
and an elliptic self-adjoint differential operator
acting on smooth sections of
, i.e.
. Since
is compact,
has a discrete spectrum, denoted by
, consisting of real eigenvalues of finite multiplicity which accumulate only at infinity. In symbols, we have
,
.
,
-eigenspace.
We can also think of as being the set
.
Two manifolds are called isospectral with respect to
, or simply
-isospectral, if
. That is, if
have the same set of eigenvalues with the same corresponding multiplicities. It is a general fact that the spectrum determines the dimension and the volume of
. In other words, if
are
-isospectral, then
and
. The spectrum is said to be asymmetric if
for some
.
Main Aim. The goal of Spectral Geometry is to study the spectrum of and the interrelations between this object and the geometry or topology of
. That is, knowing the spectrum, what can be said geometrically about
? Conversely, which spectral data can be deduced provided that we know the geometry of
? This can be summarize in the following diagram
Incidentally, by using the diagonal "maps" one could say something about the horizontal "map".
Main Problems. There are several different ways of studying the spectrum. In my opinion, the following are the three most important and interesting ones. In this paper we shall collect results concerning all of them. Computation of
. The problem is to determine the eigenvalues
and their multiplicities
. This is in general a difficult task in the sense that this cannot always be done. Indeed, there are few classes of manifolds with explicitly known spectrum for some given operator. The simplest case is the Laplacian
acting on smooth functions on the torus
.
Isospectrality. Physically, it is a problem with more than a century old and inquires about the possibility of changing shape while sounding the same. Mathematically, it begun in 1964 with the famous Kac's question Can one hear the shape of a drum? ([Ka]) and the negative answer given by Milnor to a related question ([Mi]). There are basically two antagonistic approaches to this problem: criteria vs. counterexamples. In the first case, one seeks sufficient conditions ensuring that two manifolds are isospectral. This is what some people have called Optimistic Spectral Geometry. On the contrary, in the second case, one tries to produce examples of pairs of isospectral manifolds which are very similar to each other but differing in some geometrical or topological property
. In this case, we say that this particular property
cannot be heard or that we cannot hear property
. This has been fairly called Pessimistic Spectral Geometry, but I would faintly call it Deafferential Geometry. One interesting challenge here is to construct big families (the bigger the best) with respect to the dimension
, of isospectral
-manifolds which are topologically very different (the more different the best) to each other. One purpose of this might be to tightly highlight the fact that if some property cannot be heard, it is not merely an isolated casualty but a concrete reality we cannot ignore.
Spectral asymmetry: following the acoustic jargon before, one studies now when our drum (the manifold) is out of tune. That is, when the positive and the negative spectra differ. The usual devices designed to detect this phenomenon are the eta series and the eta invariant. One wants to compute them explicitly.
Summary of results. Here we give a list of the principal results obtained for -manifolds (that is, compact flat manifolds having holonomy group isomorphic to
) relative to the problems mentioned before. The results will be properly stated and explained in the body of the paper.
A. Full Laplacian : (1) all
-manifolds covered by the same torus (or by isospectral tori) are isospectral on differential forms of mixed degree; (2) There are big families of
-isospectral manifolds.
B. Spin structures: (3) we give necessary and sufficient conditions for their existence; (4) there are families of -manifolds which are spin while there are others which are not spin; (5) we answer Webb's question: "Can one hear the property of being spin on a compact Riemannian manifold?".
C. Dirac spectrum: (6) we compute the multiplicities of the eigenvalues of twisted Dirac operators for an arbitrary spin
-manifold.
D. Dirac isospectrality: (7) we obtain several examples of pairs of
-isospectral manifolds having different topological, geometrical or spectral properties; (8) there are big families of
-isospectral manifolds.
E. Spectral asymmetry. (9) we give a characterization of those manifolds having asymmetric Dirac spectrum; (10) explicit expressions for the eta series and the -invariant are given; (11) we answer Schueth's question: "Can one hear the
-invariants of a compact Riemannian manifold?".
What are we talking about?. A Bieberbach group is a crystallographic group without torsion. That is, a discrete, cocompact, torsion-free subgroup of the isometries of
. Such
acts properly discontinuously on
, thus
is a compact flat Riemannian manifold having fundamental group
. Any element
decomposes uniquely as
, where
and
denotes translation by
.
By the classical Bieberbach's theorems we have the following two basic results: (i) the translations in form a normal maximal abelian subgroup
of finite index, with
a lattice in
which is
-stable for each
(as usual, one identifies
with
) and (ii) the restriction to
of the canonical projection
given by
is a group homomorphism with kernel
and
is a finite subgroup of
. It turns out that
satisfies the exact sequence of groups
The group is called the holonomy group of
and it is isomorphic to the linear holonomy group of the Riemannian manifold
. The action of
on
by conjugation defines an integral representation
which is usually called the integral holonomy representation of
. Note that this representation does not determine the group
, i.e. there may be many non-isomorphic Bieberbach groups with the same holonomy representation.
A compact Riemannian manifold with holonomy group isomorphic to will be called an
-manifold (see [Ch]). We shall only be concerned with
-manifolds which, by the Cartan-Ambrose-Singer theorem, are necessarily flat.
Compact flat manifolds. A flat manifold is a closed, connected, Riemannian manifold , whose curvature identically vanishes. Notably, by the Killing-Hopf theorem any compact flat manifold
is isometric to a quotient
, with
a Bieberbach group. Putting the Bieberbach theorem's into Riemannian language (see [Wo] or [Ch]) we get that: (i)
is covered by the associated flat torus
and the covering
is a local isometry, (ii)
is affinely equivalent to
if and only if
and (iii) there is a finite number of classes of affine equivalence of compact flat manifolds, in each dimension.
Up to equivalence, in dimension 1 there is only one compact flat manifold, the circle , while in dimension 2 there are two, the torus
and the Klein bottle
:
The number of compact flat manifolds grows rapidly with the dimension and a classification is unfortunately known only up to dimension 6.
There are two nice results concerning compact flat manifolds. One says that there are a plethora of them while the other says that all these manifolds bound: (1) Every finite group can be realized as the holonomy group of a compact flat manifold ([AK]) and (2) If
is a compact flat
-manifold then there is a compact
-manifold
such that
([HR]).
Into the jungle. A -manifold is just a compact flat
-manifold whose holonomy group is isomorphic to
, with
. Thus, it is of the form
where
, with
and
satisfying
,
,
and
,
for
.
Some friendly tribes. We now introduce some particularly interesting classes of -manifolds that will be used in the rest of the paper.
-manifolds. They generalize the Klein Bottle in the sense that they are quotients of tori divided by a
-action. They are determined by the integral holonomy representation which can be parametrized by the block matrices
![2j + h + l = n](/img/revistas/ruma/v47n1/1a13142x.png)
![j + h ⁄= 0](/img/revistas/ruma/v47n1/1a13143x.png)
![n Mj,h = ⟨Bj,hL e2n,Λ⟩\ℝ](/img/revistas/ruma/v47n1/1a13144x.png)
![Λ](/img/revistas/ruma/v47n1/1a13145x.png)
![H1 (Mj,h,ℤ ) ≃ ℤj+l ⊕ ℤh2](/img/revistas/ruma/v47n1/1a13146x.png)
![∑ ( )( ) βp(Mj,h) = [ip=∕20] j+2hi pj+-l2i](/img/revistas/ruma/v47n1/1a13147x.png)
![0 ≤ p ≤ n](/img/revistas/ruma/v47n1/1a13148x.png)
Primitive
-manifolds. We recall that primitive means that
, that is
has trivial center. By a construction due to Calabi (see [Ca], [Wo]), any compact flat manifold can be obtained from a primitive one. Primitive
-manifolds are also determined by the integral holonomy representation, which decomposes as a sum of integral representations of rank
([Ti]).
Diagonal type. A compact flat manifold
is of diagonal type if there is an orthonormal
-basis
of
satisfying
,
, for every
. In this case we say that
have diagonal holonomy representation. One can assume that
is the canonical lattice and that
. They necessarily have holonomy group
. (See [MR3]).
Hantzsche-Wendt manifolds. (Or, HW-manifolds, for short). They are the orientable
-manifolds in odd dimension
. Any such manifold
is given by
where
fixes
,
for
, if
, and
(note that
). They generalize the only orientable
-manifold existing in dimension 3, historically called the Hantzsche-Wendt manifold, and were studied in [MR]. They are primitive, of diagonal type and, furthermore, they are rational homology spheres, i.e.
for every HW-manifold
. Also, one can associate certain directed graphs to them.
Generalized Hantzsche-Wendt manifolds. (Or GHW-manifolds). They are simply the
-manifolds in dimension
. They share many properties with HW-manifolds but they are not primitive in general. There are
different integral holonomy representations, all of diagonal type. (See [RS]).
A little bit of numerology. As we have seen, we have the following natural inclusions . In the table below we compare the cardinality of these families. We see that, at least in low dimensions, the class of
-manifolds represents more than half of the compact flat manifolds.
Consider the Laplacian on -forms
. It is a first order elliptic differential operator acting on smooth sections of the
-exterior bundle
of
. The spectrum of this operator on compact flat manifolds was studied in [MR2] (see also [MR3], [MR4]). The multiplicity of the eigenvalue
of
has the expression
![]() |
where , with
, and
is the character of the
-exterior representation. For
of diagonal type, this character is given by integer values of certain polynomials. In fact, for
, we have
![n B nB = dim (ℝ )](/img/revistas/ruma/v47n1/1a13204x.png)
![n K p(x)](/img/revistas/ruma/v47n1/1a13205x.png)
![n](/img/revistas/ruma/v47n1/1a13206x.png)
![p](/img/revistas/ruma/v47n1/1a13207x.png)
![Kn0(x) = 1](/img/revistas/ruma/v47n1/1a13208x.png)
![Kn1(x) = - 2x + n](/img/revistas/ruma/v47n1/1a13209x.png)
![( ) Kn2 (x) = 2x2 - 2nx + n2](/img/revistas/ruma/v47n1/1a13210x.png)
![( ) Kn3(x ) = - 4x3 + 2nx2 - (n2 - n + 2)x + n 3 3 3](/img/revistas/ruma/v47n1/1a13211x.png)
From now on in this section we refer to [MPR]. One can simply define a Laplacian on arbitrary forms by considering the -Laplacians altogether, that is, we can take
![⊕ Λ(T M ) = n Λp(T M ) p=0](/img/revistas/ruma/v47n1/1a13214x.png)
![M](/img/revistas/ruma/v47n1/1a13215x.png)
![2 4π μ](/img/revistas/ruma/v47n1/1a13216x.png)
![∑n dF,μ(Γ ) = p=0 dp,μ(Γ )](/img/revistas/ruma/v47n1/1a13217x.png)
![p](/img/revistas/ruma/v47n1/1a13218x.png)
![Δp](/img/revistas/ruma/v47n1/1a13219x.png)
![p](/img/revistas/ruma/v47n1/1a13220x.png)
![ΔF](/img/revistas/ruma/v47n1/1a13221x.png)
We have the following curious "optimistic" result from [MPR]:
Theorem 2.1. Let be a Bieberbach group of dimension
with translation lattice
and holonomy group
. Then, the eigenvalue
of
has multiplicity
where
.
Thus, two -manifolds
are
-isospectral if and only if the translation lattices
are isospectral. In particular, for fixed
and
, all
-manifolds with covering torus
are
-isospectral.
Sketch of proof. Let . Then, the
's diagonalize simultaneously with eigenvalues
. Thus, every
is conjugate in
to the diagonal matrix
where
is the identity matrix in
. Thus
. Hence, we have
and, adding over
![]() |
Now, for , one can show that
. Since
if and only if
, we finally get that
, as asserted. □
Note 1. The proof seems to be of an entirely combinatorial nature since it only depends on (), and the Krawtchouk polynomials at integer values
have some combinatorial interpretations in the literature.
Example 2.2 (-manifolds of dim 3). We illustrate the theorem in the simplest non-trivial case, i.e. when
and
. Up to diffeomorphism, there are only three
-manifolds in dimension 3 (see [Wo]). They are
and
, in the notation of page 138.
In the tables below we give the multiplicities for , with
, and also for
, of the 2 lowest non trivial eigenvalues.
![]() | ![]() |
The values in the tables show that the manifolds are not -isospectral to each other for any
. However, we can see how all these multiplicities balance, that is how they manage to distribute themselves in order to have equal sums for each eigenvalue, in each case.
We really need the hypothesis in the theorem. The "magical" averaging phenomenon, present when considering all the
-Laplacians
simultaneously, only seems to work in the case considered. The result does not hold, in general, for holonomy groups different from
. There is a pair of 6-dimensional orientable
-manifolds,
, which are not
-isospectral ([MPR, Ex. 3.5]), even though they are isospectral on functions. In fact, let
and take
and
where
,
,
,
with
Following [MR2] one can prove that
are
-isospectral (and hence
-isospectral, by orientability) but they are not
-isospectral for
. Since
we have that
. By [Hi], the Betti numbers
,
, for
and
are respectively given by 1,2,5,8,5,2,1 and 1,2,3,4,3,2,1. In this way we get that
while
, hence
are not isospectral on forms.
Note 2. One can also consider the Laplacian on even/odd forms given by and
. The results in Theorem 2.1 hold mutatis mutandis but now with
. In particular,
.
Big families of -isospectral manifolds. As a straight consequence of the result in the previous theorem, we can exhibit several big families of
-isospectral
-manifolds in arbitrary dimension
. Here, big alludes to the fact that the cardinality of the family grows polynomially —or even exponentially— with respect to
and, also, that all the manifolds in each family are not homeomorphic to each other. It is worth noting that for the Laplacian on functions, or on
-forms, there are not known examples of such exponential families. In the famous isospectral deformations of Gordon and Wilson (see [GW]), the manifolds have different metrics but they are all homeomorphic to each other.
Consider the following families: ,
primitive
-manifolds
and
. For simplicity, we assume that all the groups have the canonical lattice of translations
. Thus, by Theorem 2.1, all manifolds belonging to each family are mutually isospectral on forms. We now indicate the order of the cardinality of each family. We have that
and
(see [MP], [Ti]). By using a small subfamily of HW-manifolds it is proved in [MR] that
. Furthermore, based on an example given in [LS], Rossetti constructed a very big family
of GHW-manifolds with
(see [MPR]).
Spin structures play a role in geometry and physics. One relevant fact is that they allow to define Dirac operators. On an arbitrary Riemannian manifold , the Laplacian on functions and the Laplacian on
-forms are always defined. On the other hand, for the Dirac operator to be defined,
needs to have an extra geometric structure. More precisely, to each spin structure on
one can construct the so called spinor bundle and a Dirac operator acting on sections of it. However, one needs to have some care here, since not every Riemannian manifold admits a spin structure (see [LS]).
Let be an oriented Riemannian
-manifold and let
be the
-principal bundle of oriented frames. Inside the group of units of the Clifford algebra
of
lies the compact connected Lie group
![1 n n 𝕊 (ℝ ) = {x ∈ ℝ : ||x || = 1}](/img/revistas/ruma/v47n1/1a13333x.png)
![μ(g)(x) = gxg -1](/img/revistas/ruma/v47n1/1a13335x.png)
![μ](/img/revistas/ruma/v47n1/1a13336x.png)
![Spin (n)](/img/revistas/ruma/v47n1/1a13337x.png)
![n ≥ 3](/img/revistas/ruma/v47n1/1a13338x.png)
![SO (n )](/img/revistas/ruma/v47n1/1a13339x.png)
A spin structure on an orientable manifold is an equivariant 2-fold covering
such that
, where
is a
-principal bundle. A manifold endowed with a spin structure is called a spin manifold.
Fortunately, for compact flat manifolds we can get rid of this complicated geometrical-topological definition by using the following result. The spin structures of are in a 1-1 correspondence (see [LM] or [Fr2]) with the group homomorphisms
commuting the diagram
![]() | (3.1) |
This gives a purely algebraic alternative definition, simpler than the original one, which is in fact a criterion to decide the existence of spin structures. It can be used not only to construct such structures, but also to count them.
Spin structures on -manifolds. In this subsection we refer to [MP]. Let
be a
-manifold and
a spin structure as in (3.1). Since
, then
for any
. Let
be a
-basis of
and put
. For
, with
, we have
. For any
we fix a distinguish (though arbitrary) element
. Then,
![]() | (3.2) |
where depends on
and on the choice of
.
The map is determined by its action on the generators of
, and so we can identify it with the
-tuple
. Since
is a group homomorphism it must satisfy, for every
, the following conditions
![B2 = Id](/img/revistas/ruma/v47n1/1a13375x.png)
![ɛ](/img/revistas/ruma/v47n1/1a13376x.png)
![Λ](/img/revistas/ruma/v47n1/1a13377x.png)
![δɛ = ɛ|Λ ∈ Hom (Λ, {±1 })](/img/revistas/ruma/v47n1/1a13378x.png)
![]() |
The following theorem says that the above necessary conditions 's for the existence of spin structures on
-manifolds, are also sufficient .
Theorem 3.1. Let be an
-dimensional Bieberbach group with holonomy group
,
, and let
be as in (3.2). Then, the map
![M Γ](/img/revistas/ruma/v47n1/1a13388x.png)
![k ˆΛ (Γ ) × {±1 }](/img/revistas/ruma/v47n1/1a13389x.png)
![M Γ](/img/revistas/ruma/v47n1/1a13390x.png)
![0](/img/revistas/ruma/v47n1/1a13391x.png)
![r 2](/img/revistas/ruma/v47n1/1a13392x.png)
![r ≥ k](/img/revistas/ruma/v47n1/1a13393x.png)
Applications. By applying Theorem 3.1 we can: (1) study the existence of spin structures in particular families of -manifolds, (2) give a simple method to obtain spin manifolds and (3) determine the audibility of the spin structures, that is, whether spin structures can be heard or not.
Spin structures in families. (i) Every orientable -manifold is spin and orientable
-manifolds of diagonal type have
spin structures (see [MP]), the same as for any
-torus (see [Fr]). (ii) Orientable primitive
-manifolds are spin. (iii) The 3-dimensional HW-manifold is spin. HW-manifolds of dimension
,
, are not spin. See [Po] for the case
. The general case, i.e.
odd, was proved independently by J. P. Rossetti ([Ro], by using a criterion in [MP]) and by S. Console ([Co], by computing the second Stiefel-Whitney classes
).
How to get spin manifolds easily?. By using the doubling procedure in [JR] or [BDM]. Let be an
-dimensional Bieberbach group with translation lattice
and holonomy group
. The double of
is the Bieberbach group defined by
, with
. It follows that d
has translation lattice
and holonomy group
. The associated manifold
has dimension
and is Kähler (see [DM]). Now, doubling an orientable manifold of diagonal type gives a spin manifold (see [MP2]). If the manifold is not orientable, then one has to double twice.
Spin structures are not audible. Take and consider the manifolds
and
where
and
are Bieberbach groups of diagonal type given, in diagonal notation (see [MR2], [MP]), in the following table. For example, the
in the first column means that
. Also,
and
,
.
It is easy to see that are orientable
-manifolds of dimension 6 and that they are
-isospectral for every
,
. Note that they are not primitive since
. Using Theorem 3.1 we can check that
has no spin structures while
has
spin structures of the form
, with
. Thus, we cannot hear the spin structures of Riemannian manifolds!
We begin with the ingredients necessary to define twisted Dirac operators on Riemannian manifolds. Let be an orientable compact flat manifold endowed with a spin structure
as in (3.1), denoted by
from now on. Let
be the spin representation, that is the restriction to
of any irreducible complex representation of the complexified Clifford algebra
. It is wellknown that
and that
is irreducible if
is odd while, if
is even,
splits into two inequivalent irreducible representations
of the same dimension, called the half-spin representations. Let
be a unitary representation such that
. As usual, we take
and
.
Now, the morphism allows to construct the spinor bundle twisted by
![]() |
with action given by . One can identify the space of smooth sections of this bundle,
, with the set
.
With the above identification, the Dirac operator twisted by on compact flat manifolds is
given by
![ei](/img/revistas/ruma/v47n1/1a13464x.png)
![Ln (ei) ⊗ Id](/img/revistas/ruma/v47n1/1a13465x.png)
![Sn ⊗ V](/img/revistas/ruma/v47n1/1a13466x.png)
![e1,...,en](/img/revistas/ruma/v47n1/1a13467x.png)
![ℝn](/img/revistas/ruma/v47n1/1a13468x.png)
![(ρ,V ) = (1,ℂ )](/img/revistas/ruma/v47n1/1a13469x.png)
![D](/img/revistas/ruma/v47n1/1a13470x.png)
![Dρ](/img/revistas/ruma/v47n1/1a13471x.png)
![n ℝ](/img/revistas/ruma/v47n1/1a13472x.png)
![2 D ρ = - Δs,ρ](/img/revistas/ruma/v47n1/1a13473x.png)
![f ∈ ker Dρ](/img/revistas/ruma/v47n1/1a13474x.png)
![f](/img/revistas/ruma/v47n1/1a13475x.png)
has a discrete spectrum consisting of real eigenvalues
,
, of finite multiplicity
. Explicit expressions for
for an arbitrary pair
were obtained in [MP2]. We now recall this result.
Let where
. Put
, with
the dual lattice of
, and
![]() |
Now, for each , let
denotes the set of elements fixed by
in
, that is
. Furthermore, for
, let
be a fixed (though arbitrary) element in the maximal torus of
, conjugate in
to
. Finally, define a sign
, depending on
and on the conjugacy class of
in
, in the following way. If
and
, let
such that
. Hence,
. Take
if
is conjugate to
in
and
otherwise. As a consequence,
and
for every
(see Definition 2.3, Remark 2.4 and Lemma 6.2 in [MP2] for details).
For odd, the multiplicity of the eigenvalue
, for
, is given by
![]() | (4.1) |
while for even, it is given by the first term in (4.1), where the sum is taken over all
, with
replaced by
. For
, with
even or odd, we have that
, if
, and
, otherwise.
Dirac spectrum of -manifolds. In the particular case when
, the formula (4.1) becomes more tractable and allows one to give shorter expressions for the multiplicities. Also, one can characterize all the spin
-manifolds
having asymmetric twisted Dirac spectrum. To wit
Theorem 4.1. Let be an
-dimensional spin
-manifold.
(i) If , then the spectrum
is symmetric and the non-zero eigenvalue
of
has multiplicity
![]() | (4.2) |
(ii) If , then
is asymmetric if and only if:
and there exists
with
and
such that
. In this case, the asymmetric spectrum is the set
![]() |
where and, if we put
, we have:
![]() |
Also, by (iii), has no non-trivial harmonic spinors.
If is symmetric then
is given by (4.2).
(iii) The number of independent harmonic spinors is given by
![ɛ|Λ = 1](/img/revistas/ruma/v47n1/1a13555x.png)
![d ρ,0(Γ ,ɛ) = 0](/img/revistas/ruma/v47n1/1a13556x.png)
![k > m](/img/revistas/ruma/v47n1/1a13557x.png)
![M Γ](/img/revistas/ruma/v47n1/1a13558x.png)
![M Γ](/img/revistas/ruma/v47n1/1a13559x.png)
![M Γ](/img/revistas/ruma/v47n1/1a13560x.png)
![2md ρ](/img/revistas/ruma/v47n1/1a13561x.png)
![M = T Γ Λ](/img/revistas/ruma/v47n1/1a13562x.png)
![ɛ = 1](/img/revistas/ruma/v47n1/1a13563x.png)
We now deal with the isospectral problem for twisted Dirac operators . We claim the existence of twisted Dirac isospectral manifolds having different spectral, geometrical or topological properties. We will compare
-isospectrality with other notions of isospectrality such as isospectrality with respect to the spinor Laplacian
and the
-Laplacian
. We will also look at the spectrum of closed geodesic with and without multiplicities, that is the so called
-spectrum and
-spectrum, respectively.
By using -manifolds one can obtain the following results from [MP2].
Theorem 5.1. There are families of Riemannian
-manifolds, pairwise non homeomorphic, which are mutually
-isospectral for each
, but they are neither isospectral on functions nor
-isospectral. Furthermore,
can be chosen satisfying any of the following extra properties:
(i) Every has (or has no) harmonic spinors.
(ii) All 's in
have the same
-Betti numbers for
and they are
-isospectral to each other for any
odd.
Theorem 5.2. There are pairs of non-homeomorphic spin manifolds which, for each , are:
(i) -isospectral but not
-isospectral; or
(ii) -isospectral for
and
-isospectral such that they are
-isospectral, or not, depending on the choice of the spin structure; or
(iii) -isospectral and
-isospectral for
, which are
-isospectral but not
-isospectral.
We can summarize the previous results in the table below
D-isospectrality vs. other types of isospectrality
Theorem 5.3. There are big families of -isospectral manifolds. More precisely, there exists a family
of pairwise non-homeomorphic Riemannian
-manifolds that are all mutually
-isospectral, for many different choices of spin structures, with the cardinality of
depending exponentially on
or, even better, on
.
6. Eta series and eta invariants
Let be a self-adjoint elliptic differential operator of order
on a compact
-manifold
. To study the spectral asymmetry of
, Atiyah, Patodi and Singer introduced in [APS] the so called eta series defined by
This series converges for and defines a holomorphic function
which has a meromorphic continuation to
with simple poles (possibly) at
,
. It is a non trivial fact that this function is really finite at
(See [APS2] for
odd, [Gi] for
even, and [Wod] using different methods). The number
is a spectral invariant, called the eta invariant, which does not depend on the metric, although
does. It gives a measure of the spectral asymmetry of
and it is important because it appears in the "correction term" of some Index Theorems for manifolds with boundary. For example, if
, the classical Dirac operator, and
is a compact spin manifold with
, under certain global boundary conditions, the index of
is given by
where is the Hirzebruch
-polynomial in the Pontrjagin forms
,
is the eta-invariant associated to
, and
(see [APS2]). Note the beauty of the above expression relating topological, geometrical and spectral data!
For , the twisted Dirac operator, we have the following result:
Theorem 6.1 ([MP2]). Let be a spin
-manifold of dimension
. If
is asymmetric then:
![]() |
where , with
and
, denotes the Hurwitz zeta function and
and
are as defined in Theorem 4.1. In particular,
has an analytic continuation to
that is everywhere holomorphic.
Furthermore, the eta invariant is given by .
Eta invariants are not audible. There are 7-dimensional -manifolds
which are
-isospectral for
such that
but
(see [Po]). The trick is to pick one manifold having symmetric spectrum while the other not. It turns out that 7 is the minimum dimension in which this can be done. The moral is that we cannot hear the
-invariant of compact Riemannian manifolds!
Acknowledgements. I wish to thank the organizers of the "II Encuentro de Geometría" for having invited me to participate in the event, where so many good mathematicians were present. It was also a pleasure for me to write this extended version of the talk given there.
[AK] Auslander, L., Kuranishi, M., On the holonomy group of locally euclidean spaces, Ann. of Math. 65, (411-415) 1957. [ Links ]
[APS] Atiyah, M.F., Patodi V.K., Singer, I.M., Spectral asymmetry and Riemannian geometry, Bull. Lond. Math. Soc. 5, (229-234) 1973. [ Links ]
[APS2] Atiyah, M. F., Patodi V. K., Singer, I. M., Spectral asymmetry and Riemannian geometry I, II, III, Math. Proc. Cambridge Philos. Soc. 77 (43-69), (1975); 78 (405-432), (1975); 79 (71-99), (1976). [ Links ]
[BDM] Barberis, M. L., Dotti, I. G., Miatello, R. J., Clifford structures on certain locally homogeneous manifolds. AGAG 13, (289-301), 1995. [ Links ]
[Ca] Calabi, E., Closed, locally euclidean, 4-dimensional manifolds, Bull. Amer. Math. Soc., 63, 2, (135), 1957. [ Links ]
[Ch] Charlap, L., Bieberbach groups and flat manifolds, Springer, Universitext, 1988. [ Links ]
[Co] Console, S., private communication 2005. [ Links ]
[DM] Dotti, I., Miatello, R., Quaternion Kähler flat manifolds, Diff. Geom. Appl. 15 (59-77), 2001. [ Links ]
[Fr] Friedrich, T., Die Abhängigkeit des Dirac-Operators von der Spin-Struktur, Coll. Math. XLVII (57-62), 1984. [ Links ]
[Fr2] Friedrich, T., Dirac operator in Riemannian geometry, Amer. Math. Soc. GSM 25, 1997. [ Links ]
[Gi] Gilkey P. B., The Residue of the Global Function at the Origin, Adv. in Math. 40, (290-307) 1981. [ Links ]
[GW] Gordon, C., Wilson, E., Isospectral deformations of compact solvmanifolds, J. Diff. Geom. 19, (241-256) 1984. [ Links ]
[Hi] Hiller H., Cohomology of Bieberbach groups, Mathematika 32, (55-59) 1985. [ Links ]
[HR] Hamrick, G. C., Royster, D. C., Flat Riemannian manifolds are boundaries, Invent. Math. 66, (405-413) 1982. [ Links ]
[JR] Johnson, F. E. A., Rees, E. G., Kähler groups and rigidity phenomena, Math. Proc. Cambridge Philos. Soc. 109, (31-44), 1991. [ Links ]
[Ka] Kac, M., Can one hear the shape of a drum?, Amer. Math. M. 73, (1-23) 1964. [ Links ]
[KL] Krasikov, I., Litsyn, S., On integral zeros of Krawtchouk polynomials, J. Combin. Theory A 74 (71-99), 1996. [ Links ]
[LM] Lawson, H. B., Michelsohn, M. L., Spin geometry, Princeton U. P., NJ, 1989. [ Links ]
[LS] Lee, R., Szczarba R. H., On the integral Pontrjagin classes of a Riemannian flat manifold, Geom. Dedicata 3 (1-9), 1974. [ Links ]
[Mi] Milnor, J., Eigenvalues of the Laplace operator on certain manifolds, Proc. Nat. Acad. Sci. USA 51, 542 1964. [ Links ]
[MP] Miatello, R. J., Podestá, R. A., Spin structures and spectra of -manifolds, Math. Zeitschrift 247, (319-335) 2004. arXiv:math.DG/0311354. [ Links ]
[MP2] Miatello, R. J., Podestá, R. A., The spectrum of twisted Dirac operators on compact flat manifolds, Trans. Amer. Math. Soc. 358 (2006) 4569-4603. arXiv:math.DG/0312004. [ Links ]
[MPR] Miatello, R. J., Podestá, R. A., Rossetti, J. P., -manifolds are isopectral on forms. arXiv:math.DG/0408285. [ Links ]
[MR] Miatello, R. J., Rossetti, J. P., Isospectral Hantzsche-Wendt manifolds, J. Reine Angew. Math. 515 (1-23), 1999. [ Links ]
[MR2] Miatello, R. J., Rossetti, J. P., Flat manifolds isospectral on p-forms, Jour. Geom. Anal. 11 (647-665), 2001. [ Links ]
[MR3] Miatello, R. J., Rossetti, J. P., Comparison of twisted Laplace p-spectra for flat manifolds with diagonal holonomy, Ann. Global Anal. Geom. 21 (341-376), 2002. [ Links ]
[MR4] Miatello, R. J., Rossetti, J. P., P-spectrum and length spectrum of compact flat manifolds, Jour. Geom. Anal. 13, 4, (631-657), 2003. [ Links ]
[Po] Podestá, R. A., El operador de Dirac en variedades compactas planas, Tesis Doctoral, FaMAF-UNC, 2004. [ Links ]
[Ro] Rossetti, J. P., private communication, 2004. [ Links ]
[RS] Rossetti, J. P., Szczepanski, A., Generalized Hantzsche-Wendt manifolds, Rev. Matemática Iberoamericana 21, 3, 2005, to appear. [ Links ]
[Ti] Tirao, P., Primitive compact flat manifolds with holonomy group , Pacific J. of Math. 198, 1, (207-233) 2001. [ Links ]
[Wo] Wolf, J., Spaces of constant curvature, Mc Graw-Hill, NY, 1967. [ Links ]
[Wod] Wodzicki, M., Spectral asymmetry and zeta functions, Inventiones mathematicae 66, (115-135) 1982. [ Links ]
Ricardo A. Podestá
FaMAF-CIEM
Universidad Nacional de Córdoba
Córdoba, Argentina.
podesta@mate.uncor.edu
Recibido: 10 de agosto de 2005
Aceptado: 22 de septiembre de 2006