Multiplicity issues [04G0]
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Multiplicity issues
The same underlying geometric Lagrangian can conceivably support many different objects in the Fukaya category. A possible source of this problem is a sequence of immersed Lagrangians converging to a multiple of a Lagrangian current . The underlying Lagrangian current contains only the support information and the multiplicity, which can be imagined as the number of sheets in . Much geometric information, however, is not captured this way:
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Take two Lagrangians which are both close to a given immersed Lagrangian , but whose Lagrangian potentials differ by approximately a constant. In the limit as currents, but the potential information is lost. On the other hand, the potential clustering property can restore this information.
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Immersed Lagrangians may be nontrivial (branched) covers over other immersed Lagrangians. When this happens, the monodromy information is not remembered by the underlying current. On the other hand, it is conceivable that some (generalized) local system data can restore this information.
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Let be a closed smooth manifold. Abouzaid [3] showed that the wrapped Fukaya category of the cotangent bundle is generated by any cotangent fibre , and the wrapped Floer cochain complex of is -equivalent to for the based loop space . In particular, for any (compact, embedded, exact) Lagrangian , the Floer cohomologies and are representations of . This cotangent bundle case can be viewed as the local model of Lagrangians contained in a small neighbourhood of a given embedded Lagrangian.
It is interesting to ask how much of such information can still make sense for Lagrangian currents.
Remark 5.23.
Multiple covers of Lagrangians may be related to the following problem of the Fukaya category. Given a class in the Grothendieck group of represented by a Lagrangian, one may ask if the primitive of this class is also represented by a Lagrangian. Such questions are related to the idempotent closure problem of in Joyce’s program, which seems very delicate.
Remark 5.24.
Construction of special Lagrangian branched multiple covers over given special Lagrangians is currently studied by S. Donaldson [29] and S. He among others.
Remark 5.25.
A holomorphic vector bundle analogue for multiply covered Lagrangians is the (multiple) extension of the bundle by itself, such as the fitting into a short exact sequence . In the HYM setting these are prototypical sources of semistable but not stable bundles, and it would not be surprising if similar phenomenon happens in the Thomas-Yau program.