[Math] Manifolds distinguished by Gromov-Witten invariants

ag.algebraic-geometrygromov-witten-theorysg.symplectic-geometry

What is a simplest example of a manifold $M^{2n}$ that admits two symplectic structures with isotopic almost complex structures, and such that Gromov-Witten invariants of these symplectic structures are different?
(unfortunately I don't know any example…) If we don't impose the condition that almost complex structures are isotopic, such examples exist in dimension 6.

Added

Refined question. Is there a manifold $M^{2n}$ with two symplectic forms $\omega_1$, $\omega_2$,
such that the cohomology classes of $\omega_1$ and $\omega_2$ are the same and
the corresponding almost complex structures are homotopic,
but at the same time the Gromov-Witten invariants are different?

Best Answer

The examples in Ruan's paper "Symplectic topology on algebraic 3-folds" (JDG 1994) seem to qualify: take any two algebraic surfaces V and W which are homeomorphic but such that V is minimal and W isn't. These are nondiffeomorphic, but VxS2 and WxS2 are diffeomorphic, and Ruan gives lots of examples (starting with V equal to the Barlow surface and W equal to the 8-point blowup of CP2) where the diffeomorphism can be arranged to intertwine the first Chern classes, whence by a theorem of Wall the almost complex structures are isotopic. However, the distinction between the GW invariants between V and W (which holds because V is minimal and W isn't) survives to VxS2 and WxS2, so VxS2 and WxS2 aren't symplectic deformation equivalent.

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