• Ingen resultater fundet

The Main Theorem for r = 1

In document Afhandling (Sider 84-93)

and assume

n | d (i) 1 (a j )

for all

i

. This holdsfor

j = 1

. Then

a j ∈ Z[[t]] ⊗ L j

,

and we onsider the image of under

d (i) 1

:

Z[[t]] ⊗ R L j d (i) 1

−→ Z[[t]] (i)R M j ,

f j x j + . . . + f r x r 7→ f j (x j − y j ) + . . . + f r (x r − y r ).

Byassumption,

f j (x j − y j ) + . . . + f r (x r − y r ) = nb

forsome

b

. Nowwe use

the projetion

π j : M j −→ M j /M j+1

, whihindues amap

Z[[t]] (i) ⊗ R M j π j

−→ Z[[t]] (i) ⊗ R M j /M j+1 , f j (x j − y j ) + . . . + f r (x r − y r ) = nb 7→ f j (x j − y j ) = n · π j (b).

We wish to map

M j /M j+1 −→ Z

. For now, assume

r > 1

. If

j > 1

, we use

the map

q

from (69), (70). Sine

q(x j − y j ) = 1

for

j > 1

, we get

Z[[t]] (i) ⊗ R M j /M j+1

−→ q Z[[t]] (i) ⊗ R Z,

f j (x j − y j ) = n · π j (b) 7→ f j = n · qπ j (b).

(72)

If

j = 1

,we use the well-dened map

q 1 (x) = 1

,

q 1 (y) = 0

,and get the same

result. The onlusion is that

f j (t) ∈ n · Z[[t]] (i)R Z

for all

i

. By Lemma

6.7this implies

f j (t) ∈ nZ[[t]]

. Sine

a j = f j (t)x j + a j+1

, indutively we get

n | d (i) 1 (a j+1 )

for all

i

. This nishestheindution step. Thisindution shows

that

n | f j (t)

for all

j = 1, . . . , r

, so

a ∈ nK hS 1 1 (Σ F 0 , ∗ )

.

Now take

r = 1

. Then

j = 1

. We use the map

q : M 1 /M 2 −→ Z

from

(69). Then in (72), we get instead

2f 1 (t) ∈ n · Z[[t]] ⊗ R Z

. By Lemma 6.7,

2f 1 (t) ∈ nZ[[t]]

, and

2a ∈ nK hS 1 1 (Σ F 0 , ∗ )

.

Lemma 6.14. There is an

S 1

transfer map

τ

on

K

-theory, whih ts into

the followingexat sequene,

−→ K 0 (X) −→ τ K hS 1 1 (X) −→ ϕ K hS 1 1 (X) −→ q K 1 (X) −→ τ K hS 0 1 (X) −→

where

K (BS 1 ) = Z[[t]]

, and the map

ϕ

is multipliation by

− t

.

Proof. Let

T −→ BS 1

denote the standard omplex line bundle, as usual.

Let

p : ES 1 × S 1 X −→ BS 1

,beprojetionontherstfator, andlet

ξ = p T

denotethe pullbak. As in(48), we use the ober sequene,

S(ξ) −→ D(ξ) −→ T h(ξ).

As shown in (49),

S(ξ) ∼ = ES 1 × X ≃ X

. The long exat sequene on

K

-theory beomes, using the Thom isomorphism,f. [Atiyah℄ Cor. 2.7.3,

K ∗− 1 (X) −→ δ K (ES 1 × S 1 X) −→ ϕ K (ES 1 × S 1 X) −→ K (X) −→ δ

The map

ϕ

is given by multipliation with

Λ 1 (T ) = 1 − T = − t

, sine

T

is

alinebundle. Wedenethe

S 1

transfer map

τ

tobethe boundary map

δ

in

the long exat sequene.

By exatness, Im

(τ ) =

Ker

(ϕ)

, and sowe willneed the kernel of

t

:

Lemma 6.15. The kernel of the map givenby multipliation by

t

,

t : Z[[t]] (k) ⊗ R Z[x]/x 2 −→ Z[[t]] (k) ⊗ R Z[x]/x 2

is

Zp k − 1 (t)x

, where

(t + 1) k − 1 = tp k − 1 (t)

.

Proof. Firstwerelate thekernelof

t

tothe kernelof

u : M −→ M

(this part

holds for all

r

). Reall

R = R(S 1 ) = Z[U, U 1 ]

, and let

u = U − 1

. Then

M

is an

R

-module by

u 7→ (x − y)/(1 + y)

, and

Z[[t]] (k)

is an

R

-module by

u 7→ (t + 1) k − 1

. Consider the exat sequene

0 // Z[[t]] t // Z[[t]] // Z // 0.

Tensoring with

M

over

R

yieldsthe exat sequene

0 //

Tor

R 1 (Z, M ) // Z[[t]] (k) ⊗ R M t // Z[[t]] (k) ⊗ R M

To ompute Ker

(t) ∼ =

Tor

R 1 (Z, M )

, we use the following free resolution of

Z

over

R

:

0 // R u // R // Z // 0.

Again, we tensorover

R

with

M

and nd

0 //

Tor

R 1 (Z, M) // R ⊗ R M u // R ⊗ R M // Z[[t]] (k) ⊗ R Z // 0.

so Tor

R

1 (Z, M ) ∼ =

Ker

(u)

. All we need to know is how to translate from

Ker

(u)

toKer

(t)

. The followingdiagram,

0 //

Ker

(u) //

R ⊗ R M u //

p k− 1 (t) ⊗

id

R ⊗ R M //

Z[[t]] (k) ⊗ R Z

id

0 //

Ker

(t) // Z[[t]] (k)R M t // Z[[t]] (k)R M // Z[[t]] (k)R Z

isommutative,sine

tp k − 1 (t) = (t + 1) k − 1 = u

. Fromthis diagram,wesee

that Ker

(t) = p k − 1 (t)

Ker

(u)

.

So all that remains is to determine Ker

(u)

. This an be done for any

r

,

but it is espeially easy when

r = 1

, and

M = Z[x]/x 2

, where

u1 = 2x

and

ux = 0

. Clearly Ker

(u) = Zx

, and so Ker

(t) = Zp k − 1 (t)x

.

Wean now provethe MainTheorem inase

r = 1

:

Proof of Theorem6.13. Bythe exat sequene

K hS 1 1 (Σ F 0 , ∗ ) −→ δ K ˜ hS 1 1 ( F ∞ / F 0 ) −→ K hS 1 1 ( F ∞ ) −→ 0,

we see that

K hS 1 1 (LCP 1 ) = K hS 1 1 ( F ∞ )

is isomorphi to the okernel Cok

(δ)

of

δ

. Sine

r = 1

,

K hS 1 1 (Σ F 0 , ∗ ) = Z[[t]] · h

, solet

f(t) ∈ Z[[t]]

begiven, and

assumethat

δ(f (t)h)

is divisibleby

2

. We willshow thatthis implies

f(t)

is

divisible by

2

,meaning that there isno

2

-torsion inCok

(δ)

.

For ontradition, assume that

f (t)

is not divisible by

2

. Then, without

loss of generality,

f(t)

has the form

t l g(t)

, where

g(t) = 1 + tp(t)

for some

p(t) ∈ Z[[t]]

. Here

l

is the rst exponentin

f(t)

with anodd oeient,and

so

2 | δ(f(t)h)

ifandonlyif

2 | δ(t l g(t)h)

. Then

g (t)

isaunitin

Z[[t]]

,sosine

δ

is a

Z[[t]]

-module homomorphism,

2 | δ(t l g (t)h)

if and only if

2 | δ(t l h)

.

We haveshown that if

δ(f (t)h)

is divisibleby

2

,but

f(t)

is not divisible by

2

,then

δ(t N 1 h)

is alsodivisible by

2

forall

N > l

.

We will now show that this leads to a ontradition if

N = 2 n > l

.

Consider the omposite map, whih we all

d (N) 2

,

K hS 1 1 (Σ F 0 , ∗ ) δ // K ˜ hS 1 1 ( F ∞ / F 0 ) // K ˜ hS 1 1 ( F 2N / F 0 ) P

N // K ˜ hS 1 1 ( F 2 (N ) / F 0 )

Then

d (N) 2 (t N 1 h)

is divisible by

2

, sine

δ(t N 1 h)

is. We will investigate

d (N) 2 (t N 1 h)

via the following diagram:

Σ F 0 oo ( F 1 / F 0 ) (2N)

'' O O O O O O O O O O O

( F 1 / F 0 ) (N) //

OO

( F 2 / F 0 ) (N ) //

gg OOO

OOO OOO OOO O

( F 2 / F 1 ) (N)

The maps into

Σ F 0

are the ones induing the various dierentials in the Morse spetral sequenes. The map

( F 1 / F 0 ) (2N) −→ ( F 2 / F 1 ) (N )

is simply

the omposite of the two other maps inthe triangle

( F 1 / F 0 ) (2N) P

(N)

2 // ( F 2 / F 0 ) (N) // ( F 2 / F 1 ) (N ) .

On

S 1

-equivariant

K

-theory this beomes

K hS 1 1 (Σ F 0 ) d

(2N) 1 //

d (N) 1

d (N) 2

))

S S S S S S S S S S S S S S S

K ˜ hS 1 1 (( F 1 / F 0 ) (2N) )

K ˜ hS 1 1 (( F 1 / F 0 ) (N) ) oo i K ˜ hS 1 1 (( F 2 / F 0 ) (N) )

k

OO

K ˜ hS 1 1 (( F 2 / F 1 ) (N ) )

oo j E N

ii SSS

SSSS SSSS SSSS

(73)

with the lowerrow short exat (

i

surjetive and

j

injetive). When

N = 2 n

,

we have

p N − 1 (t) = t 1 ((t + 1) 2 n − 1) = t N 1 + 2q(t),

for some polynomial

q(t)

, sine allbinomial oeients

2 j n

are divisible by

2

for

j 6 = 0, 2 n

. Sinewe havededued that

d (N 2 ) (t N 1 h)

isdivisible by

2

, we

therefore get

d (N) 2 (p N 1 (t)h)

is also divisible by

2

, say

d (N) 2 (p N 1 (t)h) = 2a

forsome

a

in

K ˜ hS 1 1 (( F 2 / F 0 ) (N) )

. ByLemma5.3weseethat

d (N) 1 (p N 1 (t)h) = 2p N − 1 (t)x

. Sine the diagram(73) is ommutative, we get

i(a) = p N − 1 (t)x

,

sine the group

K ˜ hS 1 1 (( F 1 / F 0 ) (N) )

istorsion-free, see Lemma6.4.

We now use the

S 1

transfer, see Lemma 6.14. We an hoose a transfer

lass

e ∈ K 1 ( F 1 / F 0 )

, suh that

τ(e) = p N − 1 (t)x

by Lemma 6.15. We an

liftthis transfer lassto

e ¯ ∈ K 1 ( F 2 / F 0 )

, so

i(τ(¯ e)) = τ (e) = p N − 1 (t)x

. Thus

we have an element

w = a − τ(¯ e) ∈ K ˜ hS 1 1 (( F 2 / F 0 ) (N ) )

with

i(w) = 0

. By

exatness of thelowerrowin(73), thereisanelement

z ∈ K ˜ hS 1 1 (( F 2 / F 1 ) (N) )

with

j(z) = w

. By ommutativity of (73), we get

E N (z) = k(w) = k(a − τ(¯ e)) = k(a) − k(τ(¯ e)),

so let us ompute this. Sine

2a = d (N) 2 (p N − 1 h)

, we see that

k(2a) =

d (2N 1 ) (p N − 1 h) = 2p N − 1 x

, and sine

K ˜ hS 1 1 (( F 1 / F 0 ) (2N) )

is torsion-free,

k(a) =

p N 1 x

. But

k(τ (¯ e))

isin the image of the transfer map, so by Lemma 6.15,

k(τ (¯ e)) = mp 2N − 1 (t)x

for some

m ∈ Z

. In onlusion,

E N (z) = k(w) = (p N − 1 (t) − mp 2N − 1 (t))x.

(74)

Toinvestigatethis equality,wewillneed touse

F 2

-oeients,and to

deter-mine the map

E N

. This isdone in the followinglemmas:

Lemma 6.16. As

K (BS 1 ) = Z[[t]]

-modules,

K ˜ hS 1 1 (( F 1 / F 0 ) (2 k ) ; F 2 ) ∼ = (F 2 [t]/t 2 k )1 ⊕ (F 2 [t]/t 2 k )x.

Proof. As explainedin the beginning,

K ˜ hS 1 1 (( F 1 / F 0 ) (2 k ) ; F 2 ) ∼ = Z[[t]] (2 k )R M ⊗ Z F 2 ,

where

M = Z[x]/x 2

,and

u1 = 2x

,

ux = 0

. Soweseethat

M ⊗ Z F 2 = F 2 ⊕ F 2

istrivial asan

R = Z[U, U 1 ]

-module. So

Z[[t]] (2 k ) ⊗ R M ⊗ Z F 2 = (Z[[t]] (2 k ) ⊗ R F 2 )1 ⊕ (Z[[t]] (2 k ) ⊗ R F 2 )x.

On

Z[[t]] (2 k )

,

u

atsas

(t + 1) 2 k − 1 ≡ t 2 k (

mod

2)

. Therefore,

Z[[t]] (2 k ) ⊗ R F 2 = F 2 [t]/t 2 k

. This shows the Lemma.

Lemma 6.17. The map

E N

is multipliation by

1 − (t + 1) N

.

Proof. Wemust determinethemap induedby

( F 1 / F 0 ) (2N ) −→ ( F 2 / F 1 ) (N )

,

whih is the

(N )

-twistingof the omposite map

( F 1 / F 0 ) (2) −→ F P 2 2 / F 0 −→ F 2 / F 1 .

We willrst study this untwistedase. The indued map, allit

E

, isgiven

asfollows:

K ˜ hS 1 1 ( F 2 / F 1 ) = //

K ˜ hS 1 1 (T h(µ 2 )) Φ 2 //

K hS 0 1 (G 2 (r))

E

˜

K hS 1 1 (( F 2 / F 1 ) (2) ) = // K ˜ hS 1 1 (T h((µ 1 ) (2) )) Φ 1 // K hS 0 1 (G(r) (2) )

wherethe rstisomorphismsare Morse theory,and the

Φ j

denotethe Thom

isomorphisms (the index indiates whih negative bundle). Also,

G 2 (r)

is

the geodesis of length2, whih asan

S 1

-spae isisomorphi to

G(r) (2)

,the

(2)

-twistedspae of simple losedgeodesis of length 1.

This isaspeialase of thefollowinggeneralsituation: Forabundle and

a subbundle,

ξ ⊆ η

, overa spae

X

, the following diagramommutes

K ˜ (T h(η)) // K ˜ (T h(ξ))

K (X)

Φ η ∼ =

OO

Λ // K (X)

Φ ξ ∼ =

OO

The vertial maps are the Thom isomorphisms. Then the indued map on

K

-theory of the base spae is given by multipliation by the Euler lass

Λ = Λ 1 (η − ξ)

of the bundle

η − ξ

, i.e. the (orthogonal) omplement of

ξ

inside

η

.

Sowe need the negative bundle

µ 2 = ε 2 ⊕ ν 2

over

G 2 (r)

,see Proposition 4.2. I have given

ε

and

ν

anindex, so one an distinguish between them for

µ 2

and

µ 1

. Now

1 ) (2)

isnotapriori asubbundleof

µ 2

, butsine

µ 1 = ε 1

wherethe

S 1

ationistrivialonthebers,weseethat

1 ) (2) = ε 2

asbundles

over

G(r) (2) ∼ = G 2 (r)

, sothat

µ 2 − (µ 1 ) (2) = ν 2 = ν

, where

ν

is the omplex

bundlefound inthe proof ofProposition4.2. From here, weknowthatfor a

geodesi

f

of length 2, parametrizedas

f(t)

for

t ∈ [0, 1]

, the berof

ν

over

f

is given by

g(t)if (t)

for

t ∈ [0, 1]

, where

g ∈

span

R { cos(2πt), sin(2πt) }

.

The rotationation of

S 1

isgiven by, for

θ ∈ [0, 1]

:

θ ∗ (f (t), cos(2πt)if (t)) = (f (t − θ), cos(2πt − 2πθ)if (t − θ))

and similarly for

sin(2πt)

. The omplex struture

J

found in the proof of

Proposition4.2 is

J (cos(2πt)) = sin(2πt)

.

Nowletusomparethis tothebundle

T

,i.e. thebundleomingfromthe

standard representation of

S 1

. Ignoring the

S 1

ation,

T

is just a produt

bundle

G 2 (r) × C

. The

S 1

ation of

θ ∈ [0, 1]

is given by

θ ∗ (f (t), c) = (f(t − θ), e 2πiθ c),

for

t ∈ [0, 1].

Wewillnow onstrut a map

ϕ : T −→ ν

, given by

ϕ(f, c)(t) = (f(t), c cos(2πt)if (t)).

We hek that this is

S 1

-equivariant, i.e. that the following diagram

om-mutes (it sues to hek

c = 1

):

T ϕ //

θ ∗

ν

θ ∗

(f (t), 1) ϕ //

_

θ ∗

(f (t), cos(2πt)if (t))

_

θ ∗ ?

T ϕ // ν (f (t − θ), e 2πiθ ) ϕ // (f(t − θ), e 2πiθ cos(2πt)if (t − θ))

This ommutes, sine

e 2πiθ = cos(2πθ) + i sin(2πθ)

is multipliedon

cos(2πt)

as

e 2πiθ cos(2πt) = cos(2πθ) cos(2πt) + sin(2πθ)J(cos(2πt))

= cos(2πθ) cos(2πt) + sin(2πθ) sin(2πt)

= cos(2π(t − θ))

by the trigonometri formula. So

ϕ

is

S 1

-equivariant. Then

ϕ

denes an

isomorphism of

S 1

bundles, sine it is learly an isomorphism on the bers.

We haveshown

µ 2 − (µ 1 ) (2) = ν ∼ = T

.

Now let us look at the

(N)

-twisted ase. We get again

1 ) (2N ) = ε 2N

,

and so

2 ) (N) − (µ 1 ) (2N) = ν (N ) ∼ = T (N)

, by the above isomorphism. Now,

T (N )

is the bundle with

S 1

ationof

θ ∈ [0, 1]

given by

θ ∗ (f (t), c) = (f (t − θ), (e 2πit ) N c),

for

t ∈ [0, 1].

This shows that this is the same bundle as

T N

, so the map

E N

is

multi-pliation by the Euler lass of

T N

, and sine this is a line bundle, we get

Λ 1 (T N ) = 1 − T N = 1 − (t + 1) N

.

Using the previous two lemmas, we an now investigate equation (74)

in

K ˜ hS 1 1 (( F 1 / F 0 ) (2N ) ; F 2 )

, where

N = 2 n

. As already noted,

p N − 1 (t) ≡ t N 1 (

mod

2)

, and so the left-hand side of (74) is

(t N 1 − mt 2N 1 )x

mod-ulo 2. The right-hand side is

E N (z) = (1 − (t + 1) 2 n )z ≡ − t 2 n z(

mod

2)

.

So

(t N 1 − mt 2N 1 )x = − t N z ∈ (Z[t]/t 2N )1 ⊕ (Z[t]/t 2N )x

Clearly,thisisimpossible,sinetheterm

t N 1 x

annotbeanelledby

− t N z

in

(Z[t]/t 2N )1 ⊕ (Z[t]/t 2N )x

. This gives a ontradition, so the given

f

we

startedwith must be divisible by

2

. This proves the Theorem.

Notation

Inthistableanbefoundsomeofthefrequentlyusednotationinthis paper:

(between topologialspaes): homotopy equivalent.

F C

or

H

.

G(r)

The spae of simple parametrizedlosed geodesis on

FP r

.

Sometimes written

G(HP r )

or

G(CP r )

tobe spei.

G n (r)

The spae of parametrizedlosed geodesis of length

n

,

an be obtained by iterating

n

times the elements of

G(r)

.

∆(r)

The quotient

S 1 \ G(r)

under the rotationation of

S 1

.

EG

A ontratiblespae with a freeation of the group

G

;

unique up to homotopy.

BG EG/G

,the lassifying spaeof

G

.

X hS 1 ES 1 × S 1 X

, where

X

is an

S 1

-spae.

K hS 1 (X) K (X hS 1 )

.

K hS 1 (X, ∗ )

The relativegroup

K (X hS 1 , BS 1 )

.

T

The standard omplex linebundle over

BS 1 = CP

, or its

pullbak to

X hS 1

underthe map pr

1 : ES 1 × S 1 X −→ BS 1

.

Also used forthe lass of this bundle inK-theory.

t

the lass

T − 1

, see

T

.

F n E 1 (] − ∞ , n 2 ])

,the

n

th term inthe Morse ltration.

µ n

the negativebundle for the ritial manifold

G n (r)

.

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In document Afhandling (Sider 84-93)