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3.7.1 Moduli integral formula for the Solomon functional [04C2]

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3.7.1 Moduli integral formula for the Solomon functional

Assuming automatic transversality, we can rewrite the Solomon functional (20) as a moduli space integral in terms of the notations introduced in section 3.5. Let u:Σ→Xu:\Sigma\to X be a holomorphic polygon, with first order deformation vector fields v1,…​vn−1v_{1},\ldots v_{n-1}, so we can define a holomorphic function FF via (33). In clockwise order on ∂Σ\partial\Sigma, we encounter the degree one intersections on LL, an intersection p∈C​F0​(L,L0)p\in CF^{0}(L,L_{0}), the degree one self intersections on L0L_{0}, and an intersection q∈C​F0​(L0,L)q\in CF^{0}(L_{0},L). As before, we fix the additive constant by F⁡(q)=0F(q)=0.

In the following calculation, we will use the complex orientation on Σ\Sigma, and the counterclockwise orientation on ∂Σ\partial\Sigma. The Solomon functional contains a term −∫𝒞λ∧Im(e−i​θ^Ω).-\int_{\mathcal{C}}\lambda\wedge\text{Im}(e^{-i\hat{\theta}}\Omega). Now −∫𝒞λ∧Ω-\int_{\mathcal{C}}\lambda\wedge\Omega can be expressed as an integral of the following (n−1)(n-1)-form over the (n−1)(n-1)-dimensional moduli spaces ℳ\mathcal{M} of holomorphic curves:

∫Σλ∧Ω⁡(⋅,v1,…​vn−1)=∫Σλ∧𝑑F.\int_{\Sigma}\lambda\wedge\Omega(\cdot,v_{1},\ldots v_{n-1})=\int_{\Sigma}\lambda\wedge dF.

Notice that since u:Σ→Xu:\Sigma\to X is a holomorphic curve and Ω\Omega is an (n,0)(n,0)-form, adjusting viv_{i} by a vector field tangent to Σ\Sigma does not change this integrand, and all viv_{i} must hit Ω\Omega instead of the 1-form λ\lambda. After integration by part,

∫Σλ∧𝑑F=∫ΣF​𝑑λ−∫∂ΣF​λ=∫ΣF​ω−∫∂ΣF​λ.\int_{\Sigma}\lambda\wedge dF=\int_{\Sigma}Fd\lambda-\int_{\partial\Sigma}F\lambda=\int_{\Sigma}F\omega-\int_{\partial\Sigma}F\lambda.

The Solomon functional contains another two terms ∫LfL​Im​(e−i​θ^​Ω)\int_{L}f_{L}\text{Im}(e^{-i\hat{\theta}}\Omega) and −∫L0fL0Im(e−i​θ^Ω)-\int_{L_{0}}f_{L_{0}}\text{Im}(e^{-i\hat{\theta}}\Omega). Now ∫LfL​Ω\int_{L}f_{L}\Omega can be expressed as an integral of the following (n−1)(n-1)-form over the moduli spaces ℳ\mathcal{M}:

−∫∂Σ∩LfLΩ(⋅,v1,…vn−1)=−∫∂Σ∩LfLdF.-\int_{\partial\Sigma\cap L}f_{L}\Omega(\cdot,v_{1},\ldots v_{n-1})=-\int_{\partial\Sigma\cap L}f_{L}dF.

The abused notation ∂Σ∩L\partial\Sigma\cap L means the part of ∂Σ\partial\Sigma mapping to LL instead of L0L_{0}. Similarly −∫L0fL0Ω-\int_{L_{0}}f_{L_{0}}\Omega is the moduli space integral of the (n−1)(n-1)-form

−∫∂Σ∩L0fL0Ω(⋅,v1,…vn−1)=−∫∂Σ∩L0fL0dF.-\int_{\partial\Sigma\cap L_{0}}f_{L_{0}}\Omega(\cdot,v_{1},\ldots v_{n-1})=-\int_{\partial\Sigma\cap L_{0}}f_{L_{0}}dF.

The extra minus sign comes from the fact that ∂𝒞\partial\mathcal{C} sweeps out the cycle −L0-L_{0} instead of L0L_{0}.

Combining all the three contributions, the Solomon functional is the moduli space integral with integrand

ℐ=Im​∫Σe−i​θ^​F​ω−Im​∫∂Σ∩Le−i​θ^​d​(fL​F)−Im​∫∂Σ∩L0e−i​θ^​d​(fL0​F).\mathcal{I}=\text{Im}\int_{\Sigma}e^{-i\hat{\theta}}F\omega-\text{Im}\int_{\partial\Sigma\cap L}e^{-i\hat{\theta}}d(f_{L}F)-\text{Im}\int_{\partial\Sigma\cap L_{0}}e^{-i\hat{\theta}}d(f_{L_{0}}F).

The last two terms involve total derivatives, so can be integrated along boundary segments between the corner points, to yield

−Im∫∂Σ∩Le−i​θ^d(fLF)−Im∫∂Σ∩L0e−i​θ^d(fL0F)=Im​∑all cornerse−i​θ^​F​f|−+,\begin{split}&-\text{Im}\int_{\partial\Sigma\cap L}e^{-i\hat{\theta}}d(f_{L}F)-\text{Im}\int_{\partial\Sigma\cap L_{0}}e^{-i\hat{\theta}}d(f_{L_{0}}F)\\ &=\text{Im}\sum_{\text{all corners}}e^{-i\hat{\theta}}Ff|^{+}_{-},\end{split} (38)

where f|−+f|^{+}_{-} stands for the difference of the potentials fL+−fL−f_{L_{+}}-f_{L-} at a Lagrangian intersection point, such that ∂Σ\partial\Sigma moves from L+L_{+} to L−L_{-} in the clockwise direction. In the more general framework of Floer theory with Novikov coefficients, f|−+f|^{+}_{-} have the interpretation as the Novikov exponents of these intersection points. The bounding cochain elements have f|−+≥0f|^{+}_{-}\geq 0, while p∈C​F0​(L,L0),q∈C​F0​(L0,L)p\in CF^{0}(L,L_{0}),q\in CF^{0}(L_{0},L) may have negative Novikov exponents.

Proposition 3.35.

(Moduli space integral formula) The Solomon functional 𝒮⁡(L)\mathcal{S}(L) is the integral of the following complex valued volume form over the (n−1)(n-1)-dimensional moduli spaces of holomorphic curves:

𝒮⁡(L)=∫ℳℐ,ℐ=Im​∫Σe−i​θ^​F​ω+Im​∑all cornerse−i​θ^​F​f|−+.\mathcal{S}(L)=\int_{\mathcal{M}}\mathcal{I},\quad\mathcal{I}=\text{Im}\int_{\Sigma}e^{-i\hat{\theta}}F\omega+\text{Im}\sum_{\text{all corners}}e^{-i\hat{\theta}}Ff|^{+}_{-}. (39)
Remark 3.16.

The normalization F⁡(q)=0F(q)=0 is convenient, but changing FF by a constant along Σ\Sigma does not affect ℐ\mathcal{I}, due to the energy identity

∫Σω+∑all cornersf|−+=0.\int_{\Sigma}\omega+\sum_{\text{all corners}}f|^{+}_{-}=0.
Remark 3.17.

We have focused the discussion on the holomorphic curves with boundary on both LL and L0L_{0}, which are the only curves relevant for the bordism current 𝒞\mathcal{C} in the almost calibrated case. In general we need also curves involving corners at C​F−1​(L,L)CF^{-1}(L,L) or C​F−1​(L0,L0)CF^{-1}(L_{0},L_{0}), and the formula (39) takes into account all these contributions.

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