Past exam of the mathematics course of the University of Cambridge / 2021 / iii / Paper 146 / 2

An almost complex structure $J$ on $(M,{\omega }_M)$ is a compatible almost complex structure when $J^2=-I$, ${\omega }_M(Ju,Jv)={\omega }_M(u,v)$, andis a positive-definite inner product. To prove existence, choose any Riemannian metric $h$ and define $A$ by ${\omega }_M(u,v)=h(Au,v)$. The metric construction of a compatible almost complex structureis smooth and compatible, so the space is nonempty.

Identify the compact symplectic manifold $N$ with its image under the symplectic embedding. Along $N$ there is a symplectic splittingChoose compatible almost complex structures on both summands and take their direct sum. Its associated metric makes the two summands orthogonal. Extend this metric from the closed submanifold $N$ to all of $M$ using a partition of unity, and apply the metric construction again. Along $N$ it recovers the prescribed direct sum, so the resulting global compatible $J$ satisfies $J(TN)=TN$. This is the relative extension of a compatible almost complex structure.

For the two compact complex curves, use the supplied holomorphic coordinates at their transverse intersection. There $C_1\cup C_2$ is $\{xy=0\}$. Replace it in a small ball by the complex annulus $\{xy=ϵ\}$ and use a cutoff in a surrounding annulus to rejoin the unchanged curves. For sufficiently small nonzero $ϵ$, the result is an embedded symplectic surface $\Sigma$. This local replacement is a bordism between the old and new cycles, soThis is the symplectic smoothing of a positive transverse node.

Regard $X={\mathbb{CP}}^2$ as a Kähler manifold with its standard structure, letand choose $ϵ\ne 0$. The smooth conicrepresents $2H=[C_1]+[C_2]$. The standard complex structure is compatible with the Fubini-Study form, and $C_1$, $C_2$, and $\Sigma$ are all its complex submanifolds. It therefore supplies the required example.

Take the Hirzebruch surface $X={\mathbb{F}}_2$ with its standard toric Kähler structure. Let $C_1=S$ be its negative section of a Hirzebruch surface, and let $C_2=F$ represent the fiber class of a Hirzebruch surface and meet $S$ once. ThusThe preceding smoothing produces an embedded symplectic $\Sigma$ in class $S+F$.

Suppose a compatible almost complex structure made all three surfaces almost complex. Since $S$ and $\Sigma$ are distinct, Positive intersection of J-holomorphic curves would imply $S\cdot \Sigma \ge 0$. Homologically, however,a contradiction. Hence this example cannot admit such a simultaneous compatible structure.