ScalingStacks

Remark 3.7 . [03NU]

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Remark 3.7.

A partial verification of Conjecture 3.6 in the case M=T2M=T^{2} is provided by Haug [28]. He defines a version of the derived Fukaya category Dbℱ(T2)D^{b}{\mathbin{\mathscr{F}}}(T^{2}) in which the objects are twisted complexes built out of pairs (L,E)(L,E) for LL a compact, spin, graded, embedded Lagrangian in T2T^{2}, and E→LE\rightarrow L a local system, and proves that Dbℱ(T2)D^{b}{\mathbin{\mathscr{F}}}(T^{2}) is idempotent complete.

Haug remarks [28, §1] that for T2T^{2}, including local systems E→LE\rightarrow L has the effect of making Dbℱ(T2)D^{b}{\mathbin{\mathscr{F}}}(T^{2}) idempotent complete, and that Dbℱ(T2)D^{b}{\mathbin{\mathscr{F}}}(T^{2}) would not be idempotent complete if we took objects to be twisted complexes of Lagrangians LL rather than pairs (L,E)(L,E). This shows that including local systems E→LE\rightarrow L in objects (L,E,b)(L,E,b) is necessary for our programme, since otherwise Conjecture 3.6 and hence Conjecture 3.2 would be false even for M=T2M=T^{2}. We will see in §3.4 how nontrivial local systems are needed for some kinds of surgeries.

Haug’s definition of Dbℱ(T2)D^{b}{\mathbin{\mathscr{F}}}(T^{2}) is not quite the same as ours. He does not include bounding cochains bb in his objects (L,E)(L,E) (the simplicity of dimension 1 permits this). He fixes 𝔽=ℂ{\mathbin{\mathbb{F}}}={\mathbin{\mathbb{C}}}. His local systems E→LE\rightarrow L [28, §3.1.1] are not 𝔽{\mathbin{\mathbb{F}}}-local systems, as in §2.5, but Λnov\Lambda_{\rm nov}-local systems of arbitrary finite rank, such that (roughly) the eigenvalues of Hol(∇E)\mathop{\rm Hol}\nolimits(\nabla_{E}) lie in 𝔽∗⊂Λnov∗{\mathbin{\mathbb{F}}}^{*}\subset\Lambda_{\rm nov}^{*} to leading order.

I expect this should be related to our definition of Dbℱ(T2)D^{b}{\mathbin{\mathscr{F}}}(T^{2}) as follows. In dimension 1, the combination of a rank one 𝔽{\mathbin{\mathbb{F}}}-local system E→LE\rightarrow L and a bounding cochain bb is essentially equivalent to a rank one Λnov\Lambda_{\rm nov}-local system Enov→LE_{\rm nov}\rightarrow L satisfying Haug’s condition, where the holonomies satisfy Hol(∇Enov)​[γ]=Hol(∇E)​[γ]⋅e∫γb\mathop{\rm Hol}\nolimits(\nabla_{E_{\rm nov}})[\gamma]=\mathop{\rm Hol}\nolimits(\nabla_{E})[\gamma]\cdot e^{\int_{\gamma}b} for [γ]∈π1​(L)[\gamma]\in\pi_{1}(L). Also, I expect that for T2T^{2}, considering rank one local systems E→LE\rightarrow L on immersed Lagrangians has a similar effect to considering higher rank local systems E→LE\rightarrow L on embedded Lagrangians.

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