ScalingStacks

The union of several components [04HE]

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The union of several components

In our convention an immersed Lagrangian can have several components. Of particular interest is the case where LL is the union of transverse immersed Lagrangians L1,…​LNL_{1},\ldots L_{N} with bounding cochains b1,…,bNb_{1},\ldots,b_{N} respectively, and we have morphisms bi​j∈C​F1​((Li,bi),(Lj,bj))b_{ij}\in CF^{1}((L_{i},b_{i}),(L_{j},b_{j})) for i>ji>j. The key assumption here is that the morphisms only go in one direction from LiL_{i} to LjL_{j}, not vice versa. We assume that b=∑bi+∑i>jbi​jb=\sum b_{i}+\sum_{i>j}b_{ij} is a bounding cochain for the immersed Lagrangian LL, and in particular all intersection points in bi​jb_{ij} satisfy the Novikov positivity condition fLi≥fLjf_{L_{i}}\geq f_{L_{j}}. We can write out the Mauer-Cartan equation

m0+m1​(b)+m2​(b,b)+…=0m_{0}+m_{1}(b)+m_{2}(b,b)+\ldots=0

in component form: for any i>ji>j,

∑l∑k≤l∑i=i0>…>ik=jml​(bik,…​bik,bik−1​ik,…,bi1,…​bi1,bi0​i1,bi0,…,bi0)=0.\sum_{l}\sum_{k\leq l}\sum_{i=i_{0}>\ldots>i_{k}=j}m_{l}(b_{i_{k}},\ldots b_{i_{k}},b_{i_{k-1}i_{k}},\ldots,b_{i_{1}},\ldots b_{i_{1}},b_{i_{0}i_{1}},b_{i_{0}},\ldots,b_{i_{0}})=0.

The key observation is that this is precisely how one would define twisted complexes built on L1,…​LNL_{1},\ldots L_{N}, in the presence of the bounding cochains b1,…​bNb_{1},\ldots b_{N} and the data bi​jb_{ij}, when no further degree shifts are involved (cf. the exact setting in section 6.1). In this sense, we say that ‘immersed Lagrangians geometrises twisted complexes’. In other words, if the unobstructed immersed Lagrangians are admitted into the Fukaya category, then there is no need to formally add twisted complexes.

Lemma 6.3.

(Blocking together connected components based on potential clustering) Assume LL is the finite union of transversely intersecting immersed Lagrangians, with a bounding cochain bb. Then LL can be decomposed as a twisted complex built from some L1,…​LNL_{1},\ldots L_{N}, such that infLifLi>supLjfLj\inf_{L_{i}}f_{L_{i}}>\sup_{L_{j}}f_{L_{j}} whenever i>ji>j, and the Lagrangian potential fLif_{L_{i}} has connected range for each LiL_{i} .

Proof.

The decomposition can continue as long as there exists a real number cc, such that the Lagrangian components can be partitioned into two types, with Lagrangian potential strictly smaller than cc (resp. greater than cc). As long as infLifLi>supLjfLj\inf_{L_{i}}f_{L_{i}}>\sup_{L_{j}}f_{L_{j}} whenever i>ji>j, the Novikov positivity condition on the Lagrangian intersection points would imply that the entries bi​j∈C​F1​(Li,Lj)b_{ij}\in CF^{1}(L_{i},L_{j}) of bb can only go in the direction i>ji>j and not vice versa, so the immersed Lagrangian LL is necessarily of the twisted complex form. This algorithm stops in finitely many steps since there are only finitely many components involved. ∎

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