<< . .

. 44
( : 71)

. . >>

matrices such that h(A) = F (AT A).
Proof. In dimension one, we deal with the well known assertion that each even
polynomial, i.e. h(x) = h(’x), is a polynomial in x2 . However in higher dimen-
sions, the proof is quite non trivial. We present only the main ideas and refer
the reader to our source, [Atiyah, Bott, Patodi, 73, p. 323], for more details.
First notice that it su¬ces to prove the lemma for non singular matrices, for
then the assertion follows by continuity. Next, if AT A = P with P non singular
and if there is a symmetric Q, Q2 = P , then A lies in the O(m)-orbit of Q.
Indeed, Q is also non singular and B = AQ’1 satis¬es B T B = Q’1 AT AQ’1 =
Im . So it su¬ces to restrict ourselves to symmetric matrices.
Hence we want to ¬nd a polynomial map g satisfying h(Q) = g(Q2 ) for all
symmetric matrices. For every symmetric matrix P , there is the square root

P = Q if we extend the ¬eld of scalars to its algebraic closure. This can be
computed easily if we express √ = B T DB with an√
P orthogonal matrix B and

diagonal matrix D, since then P = B DB and D is the diagonal matrix
with the square roots of the eigen values of P on its diagonal. But we should
express Q as a universal polynomial in the elements pij of the matrix P . Let us
assume that all eigenvalues »i of P are di¬erent. Then we can write
P ’ »j
Q= »i .
»i ’ » j
i=1 j=i

Notice that the eigen values »i are given by rational functions of the elements
pij of P . Thus, in order to make this to a polynomial expression, we have ¬rst to
extend the ¬eld of complex numbers to the ¬eld K of rational functions (i.e. the
elements are ratios of polynomials in pij ™s). So for matrices with entries from K,
all eigen values depend polynomially on pij ™s. We also need their square roots

to express Q, but next we shall prove that after inserting Q = P into h(Q) all
square roots will factor out. For any ¬xed P , let us consider the splitting ¬eld

L over K with respect to the roots of the equation det(P ’ »2 ) = 0. So P is
polynomial over L. As a polynomial map, h extends to gl(m, L) and the next
sublemma shows that it is in fact O(m, L)-invariant.
Sublemma. Let L be any algebraic extension of R and let f : O(m, L) ’ L be
a rational function. If f vanishes on O(m, R) then f is zero.

Electronic edition of: Natural Operations in Differential Geometry, Springer-Verlag, 1993
33. Topics from Riemannian geometry 267

Proof. The Cayley map C : o(m, R) ’ O(m, R) is a birational isomorphism of
the orthogonal group with an a¬ne space. Hence there are ˜enough real points™
to make zero all coe¬cients of the rational map. For more details see [Atiyah,
Bott, Patodi, 73]
Now the basic fact is, that for any automorphism σ : L ’ L of the Galois
group of L over K we have (σQ)2 = σP = P and since both Q and σQ are sym-
metric, B = σQQ’1 is orthogonal. Hence we get σh(Q) = h(σQ) = h(BQ) =
h(Q). Since this holds for all σ, h(Q) lies in K and so h(Q) = g(Q2 ) for a
rational function g.
The latter equality remains true if P is a real symmetric matrix such that all
its eigen values are distinct and the denominator of g(P ) is non zero. If g = F/G
for two polynomials F and G, we get F (AT A) = h(A)G(AT A). If we choose A
so that G(AT A) = 0, we get F (AT A) = 0. Hence g is a globally de¬ned rational
function without poles and so a polynomial.
Thus, we have found a polynomial F on the space of symmetric matrices
such that h(A) = F (AT A) holds for a Zariski open set in gl(m). This proves
our lemma.
Let us continue in the proof of the Weyl™s theorem. By the lemma, every
fx satis¬es fx (A) = gx (AT A) for certain polynomial gx and so we get a poly-
nomial mapping g : S 2 Rm— — V ’ R linear in V . For all B, A ∈ GL(m, C) we
have g((B ’1 )T AT AB ’1 , Bx) = f (AB ’1 , Bx) = f (A, x) = g(AT A, x) and so
g : S 2 Rm— — V ’ R is GL(m)-invariant. Then the composition of g with the
symmetrization yields a polynomial GL(m)-invariant map —2 Rm— ——k Rm ’ R,
linear in the second entry. Each multi homogeneous component of degree s+1 in
the sense of 24.11 is also GL(m)-invariant and so its total polarization is a linear
GL(m)-invariant map H : —2s Rm— ——k Rm ’ R. Hence, by the Invariant tensor
theorem, k = 2s and H is a sum of complete contractions over possible permu-
tations of indices. Since the original mapping • is given by •(x) = g(Im , x),
Weyl™s theorem follows.
33.3. To explain the coordinate form of 33.2, it is useful to consider an ar-
bitrary metric G = (gij ) ∈ S+ Rm— . Let O(G) ‚ GL(m) be the subgroup

of all linear isomorphisms preserving G, so that O(m) = O(Im ). Clearly,
theorem 33.2 holds for O(G)-invariant tensors as well. Every O(G)-invariant
i1 ...i
tensor B = (Bj1 ...jp ) ∈ —p Rm — —q Rm— induces an O(G)-invariant tensor
k ...k
gi1 k1 . . . gip kp Bj11...jqp ∈ —p+q Rm— . Hence theorem 33.2 implies that all O(G)-
invariant tensors in —p Rm — —q Rm— with p + q even are linearly generated by

g i1 k1 . . . g ip kp gσ(k1 )σ(k2 ) . . . gσ(jq’1 )σ(jq )

where g ik gjk = δj , g ij = g ji , for all permutations σ of p + q letters.

Consequently, all O(G)-equivariant tensor operations are generated by: ten-
sorizing by the metric tensor G : Rm ’ Rm— or by its inverse G : Rm— ’ Rm ,
applying contractions and permutations of indices, and taking linear combina-

Electronic edition of: Natural Operations in Differential Geometry, Springer-Verlag, 1993
268 Chapter VII. Further applications

33.4. Our next main goal is to prove the famous Gilkey theorem on natural
exterior forms on Riemannian metrics, i.e. to determine all natural operators
S+ T — Λp T — . This will be based on 33.2 and on the reduction theorems from

section 28. But since the resulting forms come from the Levi-Civit` connection
via the Chern-Weil construction, we ¬rst determine all natural operators trans-
forming linear symmetric connections into exterior forms. This will help us to
describe easily the metric operators later on.
Let us start with a description of natural tensors depending on symmetric
linear connections, i.e. natural operators Q„ P 1 T (p,q) , where T (p,q) Rm =
Rm — —p Rm — —q Rm— . Each covariant derivative of the curvature R(“) ∈
C ∞ (T M — T — M — Λ2 T — M ) of the connection “ on M is natural. Further every
tensor multiplication of two natural tensors and every contraction on one covari-
ant and one contravariant entry of a natural tensor give new natural tensors.
Finally we can tensorize any natural tensor with a GL(m)-invariant tensor, we
can permute any number of entries in the tensor products and we can repeat
each of these steps and take linear combinations.
Lemma. All natural operators Q„ P 1 T (p,q) are obtained by this procedure.
In particular, there are no non zero operators if q ’ p = 1 or q ’ p < 0.
Proof. By 23.5, every such operator has some ¬nite order r and so it is deter-
mined by a smooth Gr+2 -equivariant map f : Tm Q ’ V , where Q is the standard
¬ber of the connection bundle and V = —p Rm ——q Rm— . By the proof of the the-
orem 28.6, there is a G1 -equivariant map g : W r’1 ’ V such that f = g —¦ C r’1 .
Here W r’1 = W — . . . — Wr’1 , W = Rm — Rm— — Λ2 Rm— , Wi = W — —i Rm— ,
i = 1, . . . , r ’ 1. Therefore the coordinate expression of a natural tensor is given
by smooth maps
i ...i i i
ωj1 ...jp (Wjkl , . . . , Wjklm1 ...mr’1 ).
1 q

Hence we can apply the Homogeneous function theorem (see 24.1). The action
of the homotheties c’1 δj ∈ G1 gives
i ...i i ...i
cq’p ωj1 ...jp (Wjkl , . . . , Wjklm1 ...mr’1 ) = ωj1 ...jp (c2 Wjkl , . . . , cr+1 Wjklm1 ...mr’1 ).
i i i i
1 q 1 q

Hence the ω™s must be sums of homogeneous polynomials of degrees ds in the
variables Wjklm1 ...ms satisfying
2d0 + · · · + (r + 1)dr’1 = q ’ p.
Now we can consider the total polarization of each multi homogeneous compo-
nent and we obtain linear mappings
S d0 W — · · · — S dr’1 Wr’1 ’ V.
According to the Invariant tensor theorem, all the polynomials in question are
linearly generated by monomials obtained by multiplying an appropriate number
of variables Wjkl± and applying some of the GL(m)-equivariant operations.
If q = p, then the polynomials would be of degree zero, and so only the
GL(m)-invariant tensors can appear. If q ’ p = 1 or q ’ p < 0, there are no non
negative integers solving (1).

Electronic edition of: Natural Operations in Differential Geometry, Springer-Verlag, 1993
33. Topics from Riemannian geometry 269

33.5. Natural forms depending on linear connections. To determine
Λq T — , we have to consider the case p = 0 and
the natural operators Q„ P 1
apply the alternation to the subscripts. It is well known that the Chern-Weil
construction associates a natural form to every polynomial P which is de¬ned
on Rm — Rm— and invariant under the action of GL(m). This natural form is
obtained by substitution of the entries of the matrix valued curvature 2-form
R for the variables and taking the wedge product for multiplication. So if P
is homogeneous of degree j, then P (R) is a natural 2j-form. Let us denote by
ωq the form obtained from the tensor product of q copies of the curvature R
by taking its trace and alternating over the remaining entries. In coordinates,
kq kq’1
ωq = (Rk1 ab Rk2 cd . . . Rkq ef ), where we alternate over all indices a, . . . , f . One
¬nds easily that the polynomials Pq depending on the entries of the matrix 2-
form R correspond to the homogeneous components of degree q in det(Im + R)
and so the forms ωq equal the Chern forms cq up to the constant factor (i/(2π))q .
The wedge product on the linear space of all natural forms depending on
connections de¬nes the structure of a graded algebra.
•m Λp T — is gener-
Theorem. The algebra of all natural operators Q„ P 1 p=0
ated by the Chern forms cq .
In particular, there are no natural forms with odd degrees and consequently
all natural forms are closed.
Proof. We have to continue our discussion from the proof of the lemma 33.4.
However, we need some relations on the absolute derivatives Rjklm1 ...ms of the
curvature tensor. First recall the antisymmetry, the ¬rst and the second Bianchi
identity, cf. 28.5

i i
Rjkl = ’Rjlk
i i i
(2) Rjkl + Rklj + Rljk = 0
i i i
(3) Rjklm + Rjlmk + Rjmkl = 0

Lemma. The alternation of Rjklm1 ...ms over any 3 indices among the ¬rst four
subscripts is zero.
Proof. Since the covariant derivative commutes with the tensor operations like
i i
the permutation of indices, it su¬ces to discuss the variables Rjkl and Rjklm .
By (2), the alternation over the subscripts in Rjkl is zero and (3) yields the same
for the alternation over k, l, m in Rjklm . In view of (1), it remains to discuss
i i i
the alternation of Rjklm over j, l, m. (1) implies Rjkml = ’Rjmkl and so we
can rewrite this alternation as follows

i i i i
Rjklm + Rjmkl + Rjlmk ’ Rjlmk
i i i i
+Rmkjl + Rmlkj + Rmjlk ’ Rmjlk
i i i i
+Rlkmj + Rljkm + Rlmjk ’ Rlmjk .

Electronic edition of: Natural Operations in Differential Geometry, Springer-Verlag, 1993
270 Chapter VII. Further applications

The ¬rst three entries on each row form a cyclic permutation and hence give
zero. The same applies to the last column.
Now it is easy to complete the proof of the theorem. Consider ¬rst a monomial
containing at least one quantity Rjklm1 ...ms with s > 0. Then there exists
one term of the product with three free subscripts among the ¬rst four ones
or one term Rjkl with all free subscripts, so that the monomial vanishes after
i i i
alternation. Further, (1) and (2) imply Rjkl ’ Rlkj = ’Rklj . Hence we can
restrict ourselves to contractions with the ¬rst subscripts and so all the possible
kq kq’1
natural forms are generated by the expressions Rk1 ab Rk2 cd . . . Rkq ef where the
indices a, . . . , f remain free for alternation. But these are coordinate expressions
of the forms ωq .
33.6. Characteristic classes. The dimension of the homogeneous component
of the algebra of natural forms of degree 2s equals the number π(s) of the parti-
tions of s into sums of positive integers. Since all natural forms are closed, they
determine cohomology classes in the De Rham cohomologies of the underlying
manifolds. It is well known from the Chern-Weil theory that these classes do
not depend on the connection. This can be deduced as follows.
¯ ¯
Consider two linear connections “, “ expressed locally by “i , “i ∈ (T — M —
j j
i ¯i
— — 2—
T M )—T M , and their curvatures Rj , Rj ∈ (T M —T M )—Λ T M . Write “t =
t“ + (1 ’ t)“ and analogously Rt for the curvatures. Let Pq be the polynomial
de¬ning the form ωq and Q be its total polarization. We de¬ne „q (“, “) =
1 ¯ d d
q 0 Q(“ ’ “, Rt , . . . , Rt )dt. The structure equation yields dt Rt = dt (d“t ) ’
d d
dt “t § “t ’ “t § dt “t = d(“ ’ “) and we calculate easily in normal coordinates
1 1
d d
ωq (“) ’ ωq (“) = Q(Rt , . . . , Rt )dt = q Q( Rt , Rt , . . . , Rt )dt
dt dt
0 0
¯ ¯
dQ(“ ’ “, Rt , . . . , Rt )dt = d„q (“, “).

Λ2q’1 T — and the
In fact, „q is one of many natural operators Q„ P 1 — Q„ P 1
integration helps us to ¬nd the proper linear combination of more elementary
operators which are obtained by a procedure similar to that from 33.4“33.5. The
form „q (“, “) is called the transgression.
33.7. Natural forms on Riemannian manifolds. Since there is the natural
Levi-Civit` connection, we can evaluate the natural forms from 33.5 using the
curvature of this connection. In this case 28.14.(3) holds, i.e.
n n
gin Wjklm1 ...mr = ’gjn Wiklm1 ...mr .
For gij = δij , r = 0, this implies
Rjkl = ’Rikl
k k
q q’1
and so the contractions in a monomial Rk1 ab Rk2 cd . . . Rkq ef yield zero if q is
odd. The natural forms pj = (2π)’2j ω2j are called the Pontryagin forms. The

Electronic edition of: Natural Operations in Differential Geometry, Springer-Verlag, 1993
33. Topics from Riemannian geometry 271

dimension of the homogeneous component of degree 4s of the algebra of forms
generated by the Pontryagin forms is π(s), cf. 33.6.
If we assume the dependence of the natural operators on the metric, then
every two indices of any tensor can be contracted. In particular, the complete
contractions of covariant derivatives of the curvature of the Levi-Civit` connec-
tion give rise to natural functions of all even orders grater then one. Composing
k natural functions with any ¬xed smooth function Rk ’ R, we get a new natu-
ral function. Since every natural form can be multiplied by any natural function
without loosing naturality, we see that there is no hope to describe all natural
forms in a way similar to 33.5. However, in Riemannian geometry we often meet
operations with a sort of homogeneity with respect to the change of the scale of
the metric and these can be described in more details.
Our operators will have several arguments as a rule and we shall use the
following brief notation in this section: Given several natural bundles Fa , . . . , Fb ,
we write Fa — . . . — Fb for the natural bundle associating to each m-manifold M
the ¬bered product Fa M —M . . .—M Fb M and similarly on morphisms. (Actually,
this is the product in the category of functors, cf. 14.11.) Hence D : F1 —F2 G

means a natural operator transforming couples of sections from C (F1 M ) and
C ∞ (F2 M ) to sections from C ∞ (GM ) (which is also denoted by D : F1 •F2 G
in this book). Analogously, given natural operators D1 : F1 G1 and D2 : F2
G2 , we use the symbol D1 — D2 : F1 — F2 G1 — G2 .
De¬nition. Let E and F be natural bundles over m-manifolds. We say that a
natural operator D : S+ T — — E
F is conformal, if D(c2 g, s) = D(g, s) for all
metrics g, sections s, and all positive c ∈ R. If F is a natural vector bundle and
D satis¬es D(c2 g) = c» D(g), then » is called the weight of D.
Let us notice that the weight of the metric gij is 2 (we consider the inclusion
g : S+ T — S 2 T — ), that of its inverse g ij is ’2, while the curvature and all its

covariant derivatives are conformal.
33.8. Gilkey theorem. There are no non zero natural forms on Riemannian
manifolds with a positive weight. The algebra of all conformal natural forms on
Riemannian manifolds is generated by the Pontryagin forms.
33.9. Let us start the proof with a discussion on the reduction procedure de-
veloped in section 28. Even if we have no estimate on the order, we can get
an analogous result. Consider an arbitrary natural operator Q„ P 1 — E F.
By the non-linear Peetre theorem, D is of order in¬nity and so it is determined
by the restriction D of its associated mapping J ∞ ((Q„ P 1 — E)Rm ) ’ F Rm
to the ¬ber over the origin. Moreover, we obtain an open ¬ltration of the
whole ¬ber J0 ((Q„ P 1 — E)Rm ) consisting of maximal G∞ -invariant open sub-


sets Uk where the associated mapping D factorizes through Dk : πk (Uk ) ‚
J0 ((Q„ P 1 — E)Rm ) ’ F0 Rm . Now, we can apply the same procedure as in

the section 28 to this invariant open submanifolds πk (Uk ).
Let F be a ¬rst order bundle functor on Mfm , E be an open natural sub
bundle of a vector bundle functor E on Mfm . The curvature and its covariant
derivatives are natural operators ρk : Q„ P 1 Rk , with values in tensor bundles

Electronic edition of: Natural Operations in Differential Geometry, Springer-Verlag, 1993
272 Chapter VII. Further applications

Rk , Rk Rm = Rm —Wk , W0 = Rm —Rm— —Λ2 Rm— , Wk+1 = Wk —Rm— . Similarly,
the covariant di¬erentiation of sections of E forms natural operators dk : Q„ P 1 —
Ek , where E0 = E, E0 Rm =: Rm —V0 , d0 is the inclusion, Ek Rm = Rm —Vk ,
Vk+1 = Vk — Rm— . Let us write Dk = (ρ0 , . . . , ρk’2 , d0 , . . . , dk ) : Q„ P 1 — E
Rk’2 — E k , where Rl = R0 — . . . — Rl , E l = E0 — . . . — El . All Dk are natural
operators. In 28.8 we de¬ned the Ricci sub bundles Z k ‚ Rk’2 — E k and we
know Dk : Q„ P 1 — E Zk.
Let us further de¬ne the functor Z ∞ as the inverse limit of Z k , k ∈ N, with
respect to the obvious natural transformations (projections) pk : Z k ’ Z , k > ,
and similarly D∞ : Q„ P 1 — E Z ∞ . As a corollary of 28.11 and the non linear
Peetre theorem we get
Proposition. For every natural operator D : Q„ P 1 — E F there is a unique
˜ ˜
∞ ∞
natural transformation D : Z ’ F such that D = D—¦D . Furthermore, for ev-
ery m-dimensional compact manifold M and every section s ∈ C ∞ (Q„ P 1 M —M
EM ), there is a ¬nite order k and a neighborhood V of s in the C k -topology
˜ ˜ ˜
such that DM |(D∞ )M (V ) = (πk )— (Dk )M , for some (Dk )M : (Dk )M (V ) ’

C ∞ (Z k M ), and DM |V = (Dk )M —¦ (Dk )M |V .
In words, a natural operator D : Q„ — E F is determined in all coordinate
charts of an arbitrary m-dimensional manifold M by a universal smooth mapping
de¬ned on the curvatures and all their covariant derivatives and on the sections
of EM and all their covariant derivatives, which depends ˜locally™ only on ¬nite
number of these arguments.
33.10. The Riemannian case. In section 28, we also applied the reduction
procedure to operators depending on Riemannian metrics and general vector
¬elds. In fact we have viewed the operators D : S+ T — — E
F as operators
¯ 2—
D : Q„ P — (S+ T — E) F independent of the ¬rst argument and we have
used the Levi-Civit` connection “ : S+ T —

<< . .

. 44
( : 71)

. . >>