Floer Talk 4: Section 8.6 - 8.8: Setup for Computing the Index

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See Floer Reading Group Fall 2020

Summary/Outline

Outline

What we’re trying to prove:

  • 8.1.5: (dF)u is a Fredholm operator of index μ(x)μ(y).

What we have so far:

  • Define L:W1,p(R×S1;R2n)Lp(R×S1;R2n)YYs+J0Yt+S(s,t)Y where S:R×S1Mat(2n;R)S(s,t)s±S±(t).

Outline

  • Took R±:ISp(2n;R): symplectic paths associated to S±

  • These paths defined μ(x),μ(y)

  • Section 8.7: R±S:={R(t) | R(0)=id, det(R(1)id)0}L is Fredholm.

  • WTS 8.8.1: Ind(L)Thm?=μ(R(t))μ(R+(t))=μ(x)μ(y).

From Yesterday

  • Han proved 8.8.2 and 8.8.4.
    • So we know Ind(L)=Ind(L1)
  • Today: 8.8.5 and 8.8.3:
    • Computing Ind(L1) by computing kernels.

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8.8.5: dimkerF,F

Recall

L:W1,p(R×S1;R2n)Lp(R×S1;R2n)YYs+J0Yt+S(s,t)YL1:W1,p(R×S1;R2n)Lp(R×S1;R2n)YYs+J0Yt+S(s)YL1:W1,q(R×S1;R2n)Lq(R×S1;R2n)ZZs+J0Zt+S(s)tZ

Here 1p+1q=1 are conjugate exponents.

Reductions

L1=s+J0t+S(s)t.

  • Since cokerL1kerL1, it suffices to compute kerL1

  • We have

J10:=[0110]J0=[J10J10J10]ni=1Mat(2;R).

  • This allows us to reduce to the n=1 case.

Setup

L1 used a path of diagonal matrices constant near : S(s):=(a1(s)00a2(s)), with ai(s):={aiif ss0a+iif ss0.

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Statement of Later Lemma (8.8.5)

Let p>2 and define F:W1,p(R×S1;R2)Lp(R×S1;R2)YYs+J0Yt+S(s)Y.

Note: F is L1 for n=1: L1:W1,p(R×S1;R2n)Lp(R×S1;R2n)YYs+J0Yt+S(s)Y.

Statement of Lemma

F:W1,p(R×S1;R2)Lp(R×S1;R2)YYs+J0Yt+S(s)Y.

Suppose a±i2πZ.

  • Suppose a1(s)=a2(s) and set a±:=a±1=a±2. Then

dimKerF=2{Z | 2π(a,a+)R}dimKerF=2{Z | 2π(a+,a)R}.

  • Suppose supsRS(s)<1, then

dimKerF={i{1,2} |  ai<0 and a+i>0}dimKerF={i{1,2} |  a+i<0 and ai>0}.

Statement of Lemma

In words:

  • If S(s) is a scalar matrix, set a±=a±1=a±2 to the limiting scalars and count the integer multiples of 2π between a and a+.

  • Otherwise, if S is uniformly bounded by 1, count the number of entries the flip from positive to negative as s goes from .

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Proof of Assertion 1

  • Suppose a1(s)=a2(s) and set a±:=a±1=a±2. Then

dimKerF=2{Z | 2π(a,a+)R}dimKerF=2{Z | 2π(a+,a)R}.

Step 1: Transform to Cauchy-Riemann Equations

  • Write a(s):=a1(s)=a2(s).
  • Start with equation on R2, Y(s,t)=[Y1(s,t),Y2(s,t)].
  • Replace with equation on C: Y(s,t)=Y1(s,t)+iY2(s,t).

Assertion 1, Step 1: Reduce to CR

  • Expand definition of the PDE F(Y)=0¯Y+SY=0sY+(0110)tY+(a(s)00a(s))Y=0.

  • Change of variables: want to reduce to ¯˜Y=0

  • Choose BGL(1,C) such that ¯B+SB=0

  • Set Y=B˜Y, which (?) reduces the previous equation to ¯˜Y=0.

Assertion 1, Step 1: Reduce to CR

Can choose (and then solve) B=[b(s)00b(s)]wherebs=a(s)b(s)b(s)=exps0a(σ) dσ:=expA(s).

Remarks:

  • For some constants Ci, we have

A(s)={C1+as,sσ0C2+a+s,sσ0.

  • The new ˜Y satisfies CR, is continuous and L1loc, so elliptic regularity C.

  • The real/imaginary parts of ˜Y are C and harmonic.

Assertion 1, Step 2: Solve CR

  • Identify s+itR×S1 with u=e2πz

  • Apply Laurent’s theorem to ˜Y(u) on C{0} to obtain an expansion of ˜Y in z.

  • Deduce that the solutions of the system are given by ˜Y(u)=Zcu˜Y(s+it)=Zce(s+it)2π. where {c}ZC converges for all s,t.

Assertion 1, Step 2: Solve CR

Use es+it=es(cos(t)+isin(t)) to write in real coordinates: ˜Y(s,t)=Ze2πs[cos(2πt)sin(2πt)sin(2πt)cos(2πt)][αβ].

Use Y=B˜Y=[eA(s)00eA(s)]˜Y

to write Y(s,t)=Ze2πs[eA(s)00eA(s)][cos(2πt)sin(2πt)sin(2πt)cos(2πt)][αβ].

For ss0 this yields for some constants K,K: Y(s,t)=Ze2πa[eK(αcos(2πt)βsin(2πt))eK(αsin(2πt)+βcos(2πt))].

Condition on Lp Solutions

For ss0 we had Y(s,t)=Ze(2πa)s[eK(αcos(2πt)βsin(2πt))eK(αsin(2πt)+βcos(2πt))]

and similarly for ss0, for some constants C,C we have: Y(s,t)=Ze(2πa+)s[eC(αcos(2πt)βsin(2πt))eC(αsin(2πt)+βcos(2πt))].

Then YLpexponential terms0.

Condition on Lp Solutions: Small Tails

Y(s,t)=Ze(2πa)s[eK(αcos(2πt)βsin(2πt))eK(αsin(2πt)+βcos(2πt))]

  • 0: Need α=β=0 or 2π<a+
  • =0: Need both
    • α0=0 or a+>0 and
    • β0=0 or a+>0.

Counting Solutions

{α=β=0 or 2π(a,a+)0(α0=0or0(a,a+))and(β0=0or0(a,a+))=0.

  • Finitely many such that satisfy these conditions
  • Sufficient conditions for Y(s,t)W1,p.

Compute dimension of space of solutions: ` \begin{align*} \operatorname{dim} \operatorname{Ker} F &=2 \cdot {\sharp}\left{{\ell \in \mathbb{Z}^{*} {~\mathrel{\Big\vert}~} 2\pi\ell \in (a^-, a^+) }\right}

  • 2\cdot \indic{0 \in (a^-, a^+)} \ &=2 \cdot {\sharp}\left{\ell \in \mathbb{Z} {~\mathrel{\Big\vert}~}2\pi\ell \in (a^-, a^+) \right} .\end{align*} `{=html}

Note: not sure what Z is: most likely Z{0}.

Counting Solutions

Use this to deduce dimkerF:

  • YkerFZ(s,t):=Y(s,t) is in the kernel of the operator \begin{align*} \tilde F: W^{1, q}\left(\mathbb{R} \times S^{1} ; \mathbb{R}^{2}\right) &\longrightarrow L^{p}\left(\mathbb{R} \times S^{1} ; \mathbb{R}^{2}\right) \\ Z &\mapsto \frac{\partial Z}{\partial s}+J_{0} \frac{\partial Z}{\partial t}+S({\color{red}-s}) Y .\end{align*}

  • Obtain \ker F^* \cong \ker \tilde F.

  • Formula for \dim \ker \tilde F almost identical to previous formula, just swapping a^- and a^+.

Assertion 2

Assertion 2: Suppose \sup_{s\in {\mathbb{R}}} {\left\lVert {S(s)} \right\rVert} < 1, then \begin{align*} \operatorname{dim} \operatorname{Ker} F &= {\sharp}\left\{i \in\{1,2\} {~\mathrel{\Big\vert}~}~a_{i}^{-}<0 < a_{i}^{+}\right\}\\ \operatorname{dim} \operatorname{Ker} F^{*} &={\sharp}\left\{i \in\{1,2\} {~\mathrel{\Big\vert}~}~ a_{i}^{+}<0 < a_{i}^{-} \right\} .\end{align*}

We use the following:

  • Lemma 8.8.7: \begin{align*} \sup_{s\in {\mathbb{R}}} {\left\lVert { S(s) } \right\rVert} < 1 \implies \text{the elements in }\ker F,~ \ker F^* \text{ are independent of }t .\end{align*}

  • Proof: in subsection 10.4.a.

Proof of Assertion 2

\begin{align*} F: W^{1, p}\left(\mathbb{R} \times S^{1} ; \mathbb{R}^{2}\right) &\longrightarrow L^{p}\left(\mathbb{R} \times S^{1} ; \mathbb{R}^{2}\right) \\ Y &\mapsto \frac{\partial Y}{\partial s}+J_{0} \frac{\partial Y}{\partial t}+S(s) Y .\end{align*}

  • Given as a fact: ` \begin{align*} \mathbf{Y} \in \ker F \implies {\frac{\partial }{\partial s},}\mathbf{Y} = \mathbf{a}(s)\mathbf{Y} ~~~
    -a_1(s) & 0 \\
    0 & -a_2(s)
    \end{bmatrix}
    \mathbf{Y}
    .\end{align*}
    <span>`{=html}
    
  • Therefore we can solve to obtain \begin{align*} \mathbf{Y}(s) = \mathbf{c}_0 \exp{-\mathbf{A}(s)}{\quad \operatorname{where} \quad} \mathbf{A}(s) = \int_0^s -\mathbf{a}(\sigma) ~d\sigma .\end{align*}

Proof of Assertion 2

  • Explicitly in components: \begin{align*} \begin{cases} {\frac{\partial Y_1}{\partial s}\,} &= -a_1(s) Y_1 \\ {\frac{\partial Y_s}{\partial s}\,} &= -a_2(s) Y_2 \\ \end{cases} \quad \implies \quad Y_i(s) = c_i e^{-A_i(s)}, \quad A_i(s) &= \int_0^s -a_i(\sigma) ~d\sigma .\end{align*}

  • As before, for some constants C_{j, i}, \begin{align*} A_i(s) = \begin{cases} C_{1, i} + a_i^-\cdot s & s \leq -\sigma_0 \\ C_{2, i} + a_i^+\cdot s & s \geq \sigma_0 \\ \end{cases} .\end{align*}

  • Thus \begin{align*} Y_i \in W^{1, p} \iff 0 \in (a_i^-, a_i^+) ,\end{align*}

    establishing

\begin{align*} \dim \ker F = {\sharp}\left\{i \in\{1,2\} {~\mathrel{\Big\vert}~}0 \in (a_i^-, a_i^+) \right\} .\end{align*}

8.8.3: \operatorname{Ind}(L_1) = k^- - k^+

Statement and Outline

Statement: let k^\pm \coloneqq\operatorname{Ind}(R^\pm); then \operatorname{Ind}(L_1) = k^- - k^+.

  • Consider four cases, depending on parity of k^\pm - n
  • Show all 4 lead to \operatorname{Ind}(L_1) = k^- - k^+
  • k^- \equiv k^+ \equiv n \operatorname{mod}2.
  • k^- \equiv n, k^+ \equiv n-1 \operatorname{mod}2
  • k^- \equiv n-1, k^+ \equiv n \operatorname{mod}2.
  • k^- \equiv k^+ \equiv n-1 \operatorname{mod}2
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Case 1: k^+ \equiv k^- \equiv n \operatorname{mod}2

\begin{align*} S_{k^-} & = \begin{bmatrix} -\pi & & & & & & & \\ & -\pi & & & & & & \\ & & \ddots & & & & & \\ & & & & -\pi & & & \\ & & & & & -\pi & & \\ & & & & & & (n-1-k^-)\pi & \\ & & & & & & & (n-1-k^-)\pi \\ \end{bmatrix} \\ S_{k^+} & = \begin{bmatrix} -\pi & & & & & & & \\ & -\pi & & & & & & \\ & & \ddots & & & & & \\ & & & & -\pi & & & \\ & & & & & -\pi & & \\ & & & & & & (n-1-{\color{blue}k^+})\pi & \\ & & & & & & & (n-1-{\color{blue}k^+})\pi \\ \end{bmatrix} .\end{align*}

Case 1: k^- \equiv k^+ \equiv n \operatorname{mod}2

  • Take a_1(s) = a_2(s) so a_1^\pm = a^\pm
  • Apply the proved lemma to obtain

\begin{align*} \dim \ker L_1 &= 2\cdot {\sharp}\left\{{\ell \in {\mathbb{Z}}{~\mathrel{\Big\vert}~}2\ell \in (n-1-k^-, n-1-k^+)}\right\} \\ &= \begin{cases} k^- - k^+ & k^- > k^+ \\ 0 & \text{else} \end{cases} \\ \\ \dim \ker L_1^* &= 2\cdot {\sharp}\left\{{ \ell \in {\mathbb{Z}}{~\mathrel{\Big\vert}~}2\ell \in (k^- - n + 1, k^+ - n + 1)}\right\} \\ &= \begin{cases} k^+ - k^- & k^+ > k^- \\ 0 & \text{otherwise} \end{cases} \\ \\ \implies \operatorname{Ind}(L_1) &= \qty{k^- - k^+ \over 2} - \qty{k^+ - k^- \over 2} = k^- - k^+ .\end{align*}

Case 2: k^+ \not\equiv k^- \equiv n \operatorname{mod}2

\begin{align*} S_{k-} & = \begin{bmatrix} -\pi & & & & & & & \\ & -\pi & & & & & & \\ & & \ddots & & & & & \\ & & & & -{\color{red}{\varepsilon}}\pi & & & \\ & & & & & -{\color{red}{\varepsilon}}\pi & & \\ & & & & & & (n-1-{\color{red}k^-})\pi & \\ & & & & & & & (n-1-{\color{red}k^-})\pi \\ \end{bmatrix} \\ S_{k^+} & = \begin{bmatrix} -\pi & & & & & & & \\ & -\pi & & & & & & \\ & & \ddots & & & & & \\ & & & & {\color{red}{\varepsilon}} & & & \\ & & & & & -{\color{red}{\varepsilon}} & & \\ & & & & & & (n-{\color{red}2}-k^+)\pi & \\ & & & & & & & (n-{\color{red}2}-k^+)\pi \\ \end{bmatrix} .\end{align*}

Case 2: k^+ \not\equiv k^- \equiv n \operatorname{mod}2

  • Take a_1(s) = a_2(s) everywhere except the n-1st block, where we can assume \sup_{s\in {\mathbb{R}}} {\left\lVert {S(s)} \right\rVert} < 1.
  • Assertion 2 applies and we get

\begin{align*} \dim \ker L_1 &= 2\cdot {\sharp}\left\{{\ell \in {\mathbb{Z}}{~\mathrel{\Big\vert}~}2\ell \in (n-1-k^-, n-2-k^+)}\right\} + 1 \\ &= \begin{cases} \qty{k^- - k^+ - 1} + 1 & k^- > k^+ \\ 1 & \text{otherwise} \end{cases} \\ \\ \dim \ker L_1^* &= 2\cdot {\sharp}\left\{{\ell \in {\mathbb{Z}}{~\mathrel{\Big\vert}~}2\ell \in (k^- - n + 1, k^+ - n + 2)}\right\} \\ &= \begin{cases} k^+ - k^- + 1, & k^+ > k^- \\ 0 & \text{otherwise} \end{cases} \\ \implies \operatorname{Ind}(L_1) &= \qty{ {k^- - k^+ -1 \over 2} + 1} - \qty{k^+ - k^- + 1 \over 2} = k^- - k^+ .\end{align*}

The other 2 cases involve different matrices S_{k^\pm}, but proceed similarly.

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