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.

• Homotopy associative algebra

• 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

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