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Equivariant holomorphic torsion

5.6 An example of legitimate group actions

5.6.2 Equivariant holomorphic torsion

In this sub-subsection, we present our results for the equivariant holomorphic torsion for the legitimate action described above, i.e. ~γ = γG/T, γG, γL

withγL=L˜g0 for an arbitrary but fixed elementg˜0∈G˜.

We summarise the result for an unconditioned choice of˜g0 inG˜ in the following theorem.

5.6 An example of legitimate group actions

Theorem 5.2:

Let G be a compact, connected, real even-dimensional Lie group and let T ⊂ G be a maximal torus ofG. And equipGwith a holomorphic fibre bundle structure, as described in Subsection 5.1.

Let furthermore χ :π1(G) → U(1) be a character of π1(G) and let L = ˜G×χC be the Hermitian line bundle overG associated via this character.

Furthermore, equip L with a holomorphic structure ∂¯L, given by ∂¯L = ¯∂Lχ +ε(πG(ω)), where ∂¯Lχ denotes the natural holomorphic structure on Lχ = ˜G×χC, we described in Lemma5.10 and whereω is a∂-closed form in¯ A(0,1)(G/T).

At last, let g˜0 be an element of G˜ such that for g0 = π1,G˜(˜g0) the (0,1)-form ω is left invariant under the pullback withLG/Tg0 .

Let~γ = (γG/T, γG, γL) = (LG/Tg0 , Lg0, L˜g0) denote the induced legitimate action of ˜g0 on L, given by

γL :=Lg˜0 : L −→ L [˜g, z]χ 7−→ [˜g0g, z]˜ χ.

Then we obtain the following expression for the equivariant holomorphic ζ-function for sufficiently large Re(z):

ZˇγLL(z) =−X

λ6=0

λ−zX

t

t(−1)tind(γW(λ;t),W(λ;t)).

Proof.

We start using Theorem 5.1 to get the following expression for the equivariantζ-function.

ZˇγLL(z) =−X

λ6=0

λ−zX

t

t(−1)tind(γW(λ;t),W(λ;t))

+

χγ(T)ZγW(0;0)(z) if χ≡1

0 if χ6≡1

The second term is0because χγ(T) is0 for γL=L˜g0 which we show now.

Recall thatH0,q(T, T ×C) = Λq t(0,1)

and that the~γ action on the cohomology of T×C is given byγ =Lg0 : Λq t(0,1)

→Λq t(0,1)

.

But since the Lie algebra consists of left invariant vector fields the~γ action on H0,q(T, T×C) is trivial.

We conclude:

χγ(T) =χ(T) =

n

X

k=0

(−1)k n

k

= (1−1)n= 0.

Naturally, we get a corollary out of Theorem 5.2 that holds for isolated, non-degenerated fixed points ofγG/T.

Corollary 5.48:

In the setting from Theorem 5.2.

If g01,G˜(g0) generates a maximal torus inG.

Then the expression for the equivariantζ-function simplifies to:

ZˇγLL(z) =

detT1,0

[x0]G/T(1−γ[x−1

0]) −1

· X

[n]∈W(T)

Z˜L

ˇ γ[x˜

0·n]

(z).

Here, [x0]∈(G/T)γ denotes an arbitrary chosen fixed point of LG/Tg0 that always exists (compare Lemma 5.40). Further on, W(T) denotes the Weyl group of T in G.

The equivariant ζ-function on the right hand side is that of the holomorphic line bundle π˜Lχ: L˜χ:= ˆT×χC −→ T

with a group action γ[x˜L

0·n]:=LL˜ˆ

Ω([x0·n]): L˜χ −→ L˜χ

t, zˆ

χ 7−→ h

Ω([xˆ 0·n])·ˆt, z i

χ

where Ωˆ is constructed as above (compare Corollary 5.47).

Proof.

Starting at the formula for the equivariantζ-function of Theorem 5.2 and compare this with the formula from Corollary 5.36, we obtain:

ZˇγLL(z) = X

[x]∈(G/T)γ

detT1,0

[x]G/T

1−γ[x]−1−1

·ZL[x]

ˇ γL[x](z)

Recall that every fixed point[x]differs from [x0] by an element of W(T), i.e. there is an [n]∈W(T) such that

[x] =δ(nT,[x0]) = [x0]·[n]

(compare Equation (60)).

In the proof of Lemma 5.43 we saw, thatT[x](G/T)is isomorphic to mas complex vector space and furthermore, that the mapγ[x]−1 corresponds to the map Ad(Ω([x])) via that isomorphism.

5.6 An example of legitimate group actions

It follows, using theΩ-δ-equivariance, i.e. Equation (61), that we obtain:

detT1,0

Thus, it does not depend on the choice of the fixed point[x0].

At last, we simplify the expressionZ

L|π−1

Now, look at the map

[˜x0·n]˜ρ˜: L˜ −→ L|π−1

G ([x0·n])

[ˆs, z]χ 7−→ [˜x0·n˜·s, z]ˆ χ. (62) With the notations for the local trivialisations from Remark 5.23, take ani∈I such that [x0·n]∈Ui.

In particular, Equation (62) defines a biholomorphic isometry becauseρ(ˆ˜t) is one and so is(φi,x)◦Φi,x (compare Lemma 5.24).

The actionγL restricted to L|

In the situation of Corollary 5.48, ifG has rank (which is defined as the real dimension of the maximal torusT) greater than2 than the equivariant holomorphic ζ-function simplifies to

ZˇγLL(z) =

χγ(T)ZγW(0;0)(z) if χ≡1

0 if χ6≡1

because here the equivariant holomorphic torsion of a holomorphic line bundle vanishes.

In particular, the equivariant holomorphic torsion vanishes, i.e.

τL(ˇγL) = 0.

Starting at the definition of the equivariantζ-function (compare Def. 2.43), we obtain for largeRe(z):

Note that the Eigenspace decomposes as follows Eigλ((0,q)Lx

and note furthermore thatˇγ[xL˜χ·n] covers a left transition onT that acts trivially on the

5.6 An example of legitimate group actions

The determinant can be expressed using weights of the adjoint representation.

Corollary 5.50:

In the situation of Corollary 5.48 where the rank ofG equals 2, we compute the equivariant torsion via:

where the product goes over all the positive roots of the Lie groupG and wheree−2πiα denotes the global root corresponding to −α∈R:

e−2πiα : T −→ U(1)

t= exp(X) 7−→ e−2πiα(X).

Proof. Corollary 5.48 directly implies the following formula for the equivariant holomorphic torsion ofL:

τL(ˇγL) = detT1,0

Now, observe thatγ[x

0]=T[x0]LG/Tg0 and furthermore, that

Usingg0·x0 =x0·Ω([x0]), we now compute:

T[x0]LG/Tg0 ◦(Tx0πG)◦(TeLx0) (X) =

Tx0·Ω([x0])πG

◦Tx0Lg0 ◦(TeLx0) (X)

=

Tx0·Ω([x0])πG

◦Tx0Lg0 ◦(TeLx0) (X)

= (Tx0πG)◦(TeLx0)◦Ad (Ω([x0])) (X) Consequently, we obtain:

detT1,0

[x0]G/T

1−γ[x−1

0]

= detL

α∈R+gα

1−Ad (Ω([x0]))−1 We apply the root space composition and finally obtain:

detT1,0

[x0]G/T

1−γ[x−1

0]

= Y

α∈R+

1−e2πiα(Ω([x0])−1) which finishes the proof.

This finally reduces the computation of the equivariant holomorphic torsion for the holomorphic, Hermitian line bundleL= ˜G×χC→Gwith natural Hermitian metric and a holomorphic structure∂¯L= ¯∂Lχ+ε(πG(ω))for the action Lg˜0 :L→L to a

computation of the equivariant torsion of the Hermitian, holomorphic line bundle L˜χ →T with holomorphic structure ∂¯L˜χ : ˜L→L˜ for different actionsγ[x]˜L .

In particular, the result does not depend on the(0,1)-form ω.

A Appendix: Linear algebra

In this part of the appendix, we introduce the linear algebra setting that we need.

In particular, give a definition of an almost complex vector space and recall some useful definitions and properties of Hermitian vector spaces and operators thereon.

Furthermore, we state two identities for the Hodge-Star-operator that we need throughout this thesis.

This part of the appendix is more or less common knowledge. Nonetheless, some conventions have to be made. Furthermore, in order to simplify the process of

understanding for the reader, we choose to repeat some of the necessary definitions and results.

Throughout this thesis we will constantly use the fact, that the fibre of the (co-)tangent space over a point in a complex manifold is a complex vector space and that some operators like the Hodge-Star operator can be described by linear algebra purely.

Therefore, it sometimes suffices to understand these objects on such a low level.

The first object of interest will be an almost complex vector space.

Definition A.1:

• The tuple(V, JV) whereV is a real even dimensional vector space andJV is an automorphism ofV such thatJV2 =−idV is calledalmost complex vector space.

The map JV is called almost complex structure onV.

• The triple(V, JV, gV) is calledHermitian, almost complex vector space if (V, JV) is an almost complex vector space andgV is a Euclidean metric which is compatible with the almost complex structureJV, i.e. JV is an isometry of (V, gV).

Remark A.2:

• Every almost complex vector space (V, JV)has a natural orientation given by a basee1, JV(e1), . . . , en, JV(en).

• We may define an almost complex structureJV[ on the dual space V, via

JV[(α)

(v) :=α(JV(v)) for any α∈V and v∈V.

• In particular, for a Hermitian, almost complex vector space, ife1, . . . , en is

orthonormal with respect togV and if furthermoreJV(ek)does not lie in the linear

hull of e1, . . . , enfor any k= 1. . . n, thene1, JV(e1). . . , en, JV(en)forms an orthonormal base of V.

In particular, this base defines an orientation onV that does not depend on the choice of the e1, . . . , en.

Further on, its dual base given bye1, JV[(e1), . . . , en, JV[(en) is orthonormal as well.

• For a Hermitian, almost complex vector space, we obtain thecanonical volume form:

dvolV :=e1∧JV(e1)∧. . .∧en∧JV(en)∈Λ2nV.

Let from now on for the time being(V, JV, gV)be a Hermitian, almost complex vector space.

Remark A.3:

• The complexified vector space VC:=V ⊗RC splits into itsholomorphic part, V(1,0) := Ker(JV −i),

and its antiholomorphicpart, V(0,1) := Ker(JV +i), i.e. we obtain:

VC=V(1,0)⊕V(0,1). Likewise, its dual spaceV

C :=VRCsplits, too:

VC =V∗(1,0)⊕V∗(0,1).

This splitting may be extended to the entire exterior algebra ofVC: ΛqVC= M

r+s=q

ΛrV∗(1,0)∧ΛsV∗(1,0) =: M

r+s=q

Λ(r,s)V

• OnVC, we have a Hermitian product hV induced by gV given through hV(v⊗Rz,vˆ⊗Ry) :=gV(v,ˆv)·zy.¯

We denote its analog onV

C by h[V and in abuse of notation,h[V will also be used for the extended Hermitian form on the exterior algebra Λ·V

C.

The operator we define right now helps us to compute the dual of a certain operator, namely the∂¯-operator, on complexified differential forms with coefficients in a Hermitian vector bundle.

Definition A.4:

Let(W, hW) be a finite dimensional Hermitian vector space.

There is a natural operator

¯∗V⊗W : Λ(p,q)V

⊗W −→ Λ(n−p,n−q)V

⊗W, theHodge-Star operator.

It is given implicitly through:

h[V ⊗hW

(α⊗w, β⊗w0)dvolgVV =α∧ ιw¯∗V⊗W(β⊗w0) extended linearly onto the whole tensor product.

For w∈W, the operator ιw denotes the mapping W

C to C putting winto the first component, i.e. (ιwκ)(w1, . . . , wq−1) :=κ(w, w1, . . . , wq−1).

Remark A.5:

• Recall that

h[V ⊗hW

(α⊗w, β⊗w0) =h[V(α, β)·hW(w, w0) and therefore

¯∗V⊗W(β⊗w0) = (¯∗Vβ)⊗hW(·, w0) (63) for an operator

¯

V : Λ(p,q)V →Λ(n−p,n−q)V.

• ¯∗V⊗W and ¯∗V areC-antilinear maps.

Now, that we have defined, what the Hodge-Star operator is, we will proof a small Lemma, that we use for the splitting of the∂¯ operator into a horizontal and a vertical part. Actually it will be used directly, when we introduce the induced holomorphic structure of a line bundle, if we restrict it to a complex submanifold, in our case this submanifold is the fibre of a holomorphic fibre bundle.

Lemma A.6:

Let (V, JV, gV), (U, JU, gU) be two Hermitian, almost complex vector spaces and let ϕ:U ,→V be an isometric embedding that is compatible with the almost complex structures, i.e. ϕ(JU(u)) =JV(ϕ(u)).

Then for any Hermitian vector space (W, hW) the following two statements hold.

¯

U⊗W ◦ϕ = ϕ◦¯∗V⊗W ◦ε(dvolϕ(U))

¯

V⊗W |Λ·ϕ(U) = ε

dvolϕ(U)

◦¯∗V⊗W ◦ε

dvolϕ(U)

|Λ·ϕ(U)

Here, ε(α) denotes the wedge-product with α from the left hand side and ϕ(U) is the orthogonal complement of ϕ(U) in V regarding gV.

Proof.

Because of Equation (63), the first statement is true if and only if it is true for¯∗U and¯∗V without theW part.

Observe now thatV =ϕ(U)⊕ϕ(U) which leads to a splitting Λ·V = Λ·(ϕ(U))∧Λ·

ϕ(U) ,

using the musical isomorphismv7→gV(·, v) fromV to V. In particular, we obtaindvolV = dvolϕ(U)∧dvolϕ(U). Now letω be inΛp(ϕ(U)RC)∧Λq

ϕ(U)

RC

with non-zeroq.

We observe that, on the left hand side, the pullbackϕω= 0 and, on the right hand side, ε(dvolϕ(U))ω= 0.

Therefore, the statement holds for these forms.

So let from now onq equal zero, i.e. ω∈Λp(ϕ(U)RC).

Now, look at the defining equation for the Hodge-Star-operator¯∗V on V. The expression:

h[V(β, ω∧dvolϕ(U))dvolV =β∧¯∗V

ω∧dvolϕ(U)

is zero ifβ is not of the typeβ =β0∧dvolϕ(U) for a β0 inΛ·(ϕ(U)RC). On the other hand, we get forβ =β0∧dvolϕ(U):

h[V(β, ω∧dvolϕ(U))dvolV0∧¯∗V

ω∧dvolϕ(U)

∧dvolϕ(U). (64) Further on, computing the left hand side, we obtain:

h[V(β, ω∧dvolϕ(U))dvolV =h[V0, ω)dvolV

=h[U0), ϕ(ω))dvolϕ(U)∧dvolϕ(U). (65)

Now, we compare Equation (64) and Equation (65) and derive:

ϕ

β0∧¯∗V

ω∧dvolϕ(U)

=h[U0), ϕ(ω))ϕdvolϕ(U)

0))∧¯∗Uω)

which, holding for anyβ0, completes the proof of the first statement.

The second statement can be easily seen, using an orthonormal frame.

B Fréchet space of sections in a vector bundle

The content of this section is to define and understand the Fréchet structure of the vector space of smooth sections from a manifold into a vector bundle.

First of all, we define what a Fréchet space is (compare [30]).

Definition B.1:

A Fréchet space is a topological vector spaceF, with the following properties:

• F is metrizable.

• F is complete.

• F is locally convex, i.e. there is a basis{Bk}k∈K of the topology ofF, such that each base setBk is a convex subset of F.

For maps into a Fréchet spaceF, there is a concept of differentiability.

Definition B.2:

LetF be a Fréchet space,Ω⊂Rn be an open subset andh: Ω−→F be a map.

• h ispartially differentiable in x∈Ωif for eachl= 1, . . . , nthere is an element

∂h

∂xl(x)∈Fsuch that

06=t→0lim t−1(h(x+t·el)−h(x)) = ∂h

∂xl(x).

• h isdifferentiable in x∈Ωif it is partially differentiable in x and if the following equation holds:

y→xlim h(y)−h(x)−

n

X

l=1

(yl−xl)· ∂h

∂xl(x)

!

= 0.

• h is(partially) differentiable onΩif it is (partially) differentiable in x for every x inΩ.

LetM be a compact Riemannian manifold andQ −→M be a smooth vector bundle with Euclidean metric and a metric connection∇.

Lemma B.3:

Let V ⊂M be an open subset.

Then the space of smooth sections from V into Q denoted byΓ(V,Q |V) becomes a Fréchet space.

In particular, the space Γ(M,Q) is a Fréchet space.

Proof. First, we have to define a topology on Γ(V,Q |V).

Take therefore the following set of semi-norms onΓ(V,Q |V). For each compact subset K⊂V and eachl∈N, define for s∈Γ(V,Q |V):

kskl,K := sup

t≤l

sup

x∈K

k∇tskx

, where∇t denotes the map

t: Γ(V,Q |V) −→ Γ V, Nt

i=1TM

⊗ Q |V ,

induced be the metric connection∇as well as by the Levi-Civita connection on T M. The norm onΓ(V,Nt

i=1TM⊗ Q |V)|x is given by the metric on M as well as the Euclidean metric onQ.

ThatΓ(V,Q |V) is a real topological vector space is obvious.

The local convexity can be seen by takingεballs inΓ(V,Q |V).

ThatΓ(V,Q |V) is metrizable follows from the fact that every open set U ⊂V can be approximated by a sequence of compact sets{Ki}i∈N such thatKi ⊂int(Ki+1)⊂U for everyi∈Nand the fact that

kskl,Ki ≤ kskk,Ki+1

for everyi∈N(compare [30, Ch. 10.3] for the analogous result for C(V,Rm)).

The completeness follows now from the theorem of uniform convergence (compare again [30, Ch. 10.3] for theC(V,Rm) case).

Now,Q →M is a vector bundle. Hence, we may chose a finite covering{Vj}j∈J of M such thatQ −→M becomes locally trivial, i.e. such that there are smooth maps

ψj : Q |V

j −→ Vj ×Rm. Lemma B.4:

Let now Ω⊂Rn be an open subset.

A map

ϕ: Ω −→ Γ(M,Q)

is differentiable if and only if the induced maps ϕ|V

j: Ω −→ Γ(Vj,Q |V

j) are differentiable for all j∈J.

Proof. "⇒": Letϕ: Ω−→Γ(M,Q)be differentiable.

Consequently, there are ∂x∂ϕk(x)∈Γ(M,Q) for every x∈Ω such that

Almost the same observation for the continuity of ∂ϕ|∂xVjk (x).

y→xlim

We now use the sheave property of the vector space of sections, i.e. we use that if two sectionssi ∈Γ(Vi,Q |V

Observe that we have not made use of the fact thatM is compact in the "⇒"-direction of this proof.

We conclude that by "⇒" the following equation holds.

∂ϕ|V

Now, we show the two limit formulae for the differentiability ofϕ:

This property extends to higher differentiabilities by induction.

Corollary B.5:

Let Ω⊂Rn be an open subset.

A map

ϕ: Ω −→ Γ(M,Q)

isC-smooth if and only if the induced maps ϕ|V

Without restrictions to generalityVj is a chart, i.e. there is a diffeomorphism φj :Vj −→ Uj

withUi⊂Rp open.

We identify nowVj withUj to make it less difficult.

The space m becomes a Fréchet space if we apply the standard topology, i.e.

LetK⊂Vj be a compact subset andl∈N, define the semi-norm k · kl,K for f ∈C(Vj,Rm) to be:

kfkl,K := sup

|~t|≤l

sup

x∈K

~tf

∂x(x)

! ,

where for a tuple~t= (t0, . . . , tp)⊂Nn the degree of~tis given by |~t|:=t1+. . .+tn and the operator ∂x~t is given by

~tf

∂x :=

p

Y

i=1

ti

∂xtii

! f.

Now, take as∈Γ(Vj,Q |V

j) and take aK ⊂Vj, then there is are positive constants C1, C2 such that:

C1kskl,K ≤ kproj2◦ψj(s)kl,K ≤C2kskl,K.

This is due to the fact that the Euclidean metrics onN

TM⊗Vj differ from the standard metric onN

(Rp)⊗Rm by smooth functions. These functions have onK a maximum as well as a minimum.

Corollary B.6:

Let Ω⊂Rn be an open subset.

A map

ϕ: Ω −→ Γ(M,Q)

isC-smooth if and only if the induced maps proj2◦ψj◦ϕ|V

j: Ω −→ C(Vj,Rm) areC-smooth for all j∈J.

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Erklärung

Die hier vorgelegte Dissertation habe ich eigenständig und ohne unerlaubte Hilfe angefertigt.

Die Dissertation wurde in der vorgelegten oder in ähnlicher Form noch bei keiner anderen Institution eingereicht.

Ich habe bisher keine erfolglosen Promotionsversuche unternommen.

Thomas Ueckerdt Düsseldorf, 21. Januar 2014