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 is the union of transverse immersed Lagrangians with bounding cochains respectively, and we have morphisms for . The key assumption here is that the morphisms only go in one direction from to , not vice versa. We assume that is a bounding cochain for the immersed Lagrangian , and in particular all intersection points in satisfy the Novikov positivity condition . We can write out the Mauer-Cartan equation
in component form: for any ,
The key observation is that this is precisely how one would define twisted complexes built on , in the presence of the bounding cochains and the data , 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 is the finite union of transversely intersecting immersed Lagrangians, with a bounding cochain . Then can be decomposed as a twisted complex built from some , such that whenever , and the Lagrangian potential has connected range for each .
Proof.
The decomposition can continue as long as there exists a real number , such that the Lagrangian components can be partitioned into two types, with Lagrangian potential strictly smaller than (resp. greater than ). As long as whenever , the Novikov positivity condition on the Lagrangian intersection points would imply that the entries of can only go in the direction and not vice versa, so the immersed Lagrangian is necessarily of the twisted complex form. This algorithm stops in finitely many steps since there are only finitely many components involved. ∎