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In symplectic topology, a Fukaya category of a symplectic manifold ${\displaystyle (X,\omega )}$ is a category ${\displaystyle {\mathcal {F}}(X)}$ whose objects are Lagrangian submanifolds of ${\displaystyle X}$, and morphisms are Lagrangian Floer chain groups: ${\displaystyle \mathrm {Hom} (L_{0},L_{1})=CF(L_{0},L_{1})}$. Its finer structure can be described as an A_∞-category.

They are named after Kenji Fukaya who introduced the ${\displaystyle A_{\infty }}$ language first in the context of Morse homology, and exist in a number of variants. As Fukaya categories are A_∞-categories, they have associated derived categories, which are the subject of the celebrated homological mirror symmetry conjecture of Maxim Kontsevich. This conjecture has now been computationally verified for a number of examples.

Formal definition

Let ${\displaystyle (X,\omega )}$ be a symplectic manifold. For each pair of Lagrangian submanifolds ${\displaystyle L_{0},L_{1}\subset X}$ that intersect transversely, one defines the Floer cochain complex ${\displaystyle CF^{*}(L_{0},L_{1})}$ which is a module generated by intersection points ${\displaystyle L_{0}\cap L_{1}}$. The Floer cochain complex is viewed as the set of morphisms from ${\displaystyle L_{0}}$ to ${\displaystyle L_{1}}$. The Fukaya category is an ${\displaystyle A_{\infty }}$ category, meaning that besides ordinary compositions, there are higher composition maps

${\displaystyle \mu _{d}:CF^{*}(L_{d-1},L_{d})\otimes CF^{*}(L_{d-2},L_{d-1})\otimes \cdots \otimes CF^{*}(L_{1},L_{2})\otimes CF^{*}(L_{0},L_{1})\to CF^{*}(L_{0},L_{d}).}$

It is defined as follows. Choose a compatible almost complex structure ${\displaystyle J}$ on the symplectic manifold ${\displaystyle (X,\omega )}$. For generators ${\displaystyle p_{d-1,d}\in CF^{*}(L_{d-1},L_{d}),\ldots ,p_{0,1}\in CF^{*}(L_{0},L_{1})}$ and ${\displaystyle q_{0,d}\in CF^{*}(L_{0},L_{d})}$ of the cochain complexes, the moduli space of ${\displaystyle J}$-holomorphic polygons with ${\displaystyle d+1}$ faces with each face mapped into ${\displaystyle L_{0},L_{1},\ldots ,L_{d}}$ has a count

${\displaystyle n(p_{d-1,d},\ldots ,p_{0,1};q_{0,d})}$

in the coefficient ring. Then define

${\displaystyle \mu _{d}(p_{d-1,d},\ldots ,p_{0,1})=\sum _{q_{0,d}\in L_{0}\cap L_{d}}n(p_{d-1,d},\ldots ,p_{0,1})\cdot q_{0,d}\in CF^{*}(L_{0},L_{d})}$

and extend ${\displaystyle \mu _{d}}$ in a multilinear way.

The sequence of higher compositions ${\displaystyle \mu _{1},\mu _{2},\ldots ,}$ satisfy the ${\displaystyle A_{\infty }}$ relations because the boundaries of various moduli spaces of holomorphic polygons correspond to configurations of degenerate polygons.

This definition of Fukaya category for a general (compact) symplectic manifold has never been rigorously given. The main challenge is the transversality issue, which is essential in defining the counting of holomorphic disks.

See also

• Homotopy associative algebra

References

1. Kenji Fukaya, Morse homotopy, ${\displaystyle A_{\infty }}$ category and Floer homologies, MSRI preprint No. 020-94 (1993)
2. Kontsevich, Maxim, Homological algebra of mirror symmetry, Proceedings of the International Congress of Mathematicians, Vol. 1, 2 (Zürich, 1994), 120–139, Birkhäuser, Basel, 1995.

Bibliography

• Denis Auroux, A beginner's introduction to Fukaya categories.

• Paul Seidel, Fukaya categories and Picard-Lefschetz theory. Zurich lectures in Advanced Mathematics
• Fukaya, Kenji; Oh, Yong-Geun; Ohta, Hiroshi; Ono, Kaoru (2009), Lagrangian intersection Floer theory: anomaly and obstruction. Part I, AMS/IP Studies in Advanced Mathematics, vol. 46, American Mathematical Society, Providence, RI; International Press, Somerville, MA, ISBN 978-0-8218-4836-4, MR 2553465
• Fukaya, Kenji; Oh, Yong-Geun; Ohta, Hiroshi; Ono, Kaoru (2009), Lagrangian intersection Floer theory: anomaly and obstruction. Part II, AMS/IP Studies in Advanced Mathematics, vol. 46, American Mathematical Society, Providence, RI; International Press, Somerville, MA, ISBN 978-0-8218-4837-1, MR 2548482

External links

• The thread on MathOverflow 'Is the Fukaya category "defined"?'

• Symplectic geometry
• Categories in category theory