ScalingStacks

Multiplicity issues [04G0]

Original official author HTML, exact retained edition. Historical TeX conversion verdicts remain unchanged. Cited-edition alignment and mathematical self-containment are not assessed.

Complete original source context · Original author HTML

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 LiL_{i} converging to a multiple of a Lagrangian current LL. The underlying Lagrangian current contains only the support information and the multiplicity, which can be imagined as the number of sheets in LiL_{i}. Much geometric information, however, is not captured this way:

  • •

    Take two Lagrangians L1,L2L_{1},L_{2} which are both C∞C^{\infty} close to a given immersed Lagrangian L′L^{\prime}, but whose Lagrangian potentials differ by approximately a constant. In the limit L1∪L2→2​L′L_{1}\cup L_{2}\to 2L^{\prime} as currents, but the potential information is lost. On the other hand, the potential clustering property can restore this information.

  • •

    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.

  • •

    Let QQ be a closed smooth manifold. Abouzaid [3] showed that the wrapped Fukaya category of the cotangent bundle T∗​QT^{*}Q is generated by any cotangent fibre Tq∗​QT_{q}^{*}Q, and the wrapped Floer cochain complex of Tq∗​QT_{q}^{*}Q is A∞A_{\infty}-equivalent to C−⁣∗​(Ωq​Q)C_{-*}(\Omega_{q}Q) for the based loop space Ωq​Q\Omega_{q}Q. In particular, for any (compact, embedded, exact) Lagrangian L⊂T∗​QL\subset T^{*}Q, the Floer cohomologies H​W∗​(Tq∗​Q,L)HW^{*}(T_{q}^{*}Q,L) and H​F∗​(L,L)HF^{*}(L,L) are representations of H−⁣∗​(Ωq​Q)H_{-*}(\Omega_{q}Q). 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 Db​F​u​k​(X)D^{b}Fuk(X) 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 Db​F​u​k​(X)D^{b}Fuk(X) 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 E′E^{\prime} fitting into a short exact sequence 0→E→E′→E→00\to E\to E^{\prime}\to E\to 0. 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.

Original mathematics by the credited authors. Source-backed reader collection; mathematical self-containment is not assessed.