# Floer homology and status of the Arnold conjecture.

The Arnold conjecture on a closed symplectic manifold $(M,\omega)$ says in the weakest version that for a non-degenerate Hamiltonian there are at least $k$ 1-periodic orbits where $k$ is the sum of the beti numbers of $M$. It is easy to show that one can assume that $\omega$ is integral, so I do so in the following.

On wikipedia it says that the Arnold conjecture is solved in many cases using Floer homology.

However, I was given the impression that this version of the Arnold conjecture has been solved in all cases, but are scattered around in several papers - due to several different complications.

Question: What is the current status of this weak Arnold conjecture precisely, and what are the refferences for these results?

Added: I know most of the details of the monotone case, so I am mostly interested in the more exotic cases.

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You might be interested by what is written about Arnold conjecutre here www2.maths.ox.ac.uk/hms – Dmitri May 27 '10 at 22:32
@Dmitri: Link is now dead. – Qmechanic Aug 21 '15 at 20:56

## 1 Answer

V. I. Arnol'd, June 12, 1937 - June 3, 2010.

The very sad news of his death is reported today here.

After Floer, the main difficulty in solving the weak Arnol'd conjecture on a compact symplectic manifold $M$ lies in defining a Floer chain complex generated by 1-periodic orbits of an arbitrary non-degenerate Hamiltonian $H: S^1\times M \to \mathbb{R}$, in such a way that the homology is independent of $H$. Once one has that, the remaining step (proving an isomorphism with Morse homology) can be done either by a computation with small autonomous Hamiltonians, or by a "PSS" isomorphism.

When $M$ is monotone, the crucial compactness theorems for solutions to Floer's equation (used to define the candidate-differential on the Floer complex, to prove that it squares to zero, and, in a variant, to prove the invariance of the theory) can be proved using index considerations. When $M$ is Calabi-Yau, compactness needs an additional idea, that holomorphic spheres generically don't hit cylinders solving Floer's equation. This is beautifully worked out in

Hofer, H.; Salamon, D. A. "Floer homology and Novikov rings." The Floer memorial volume, 483--524, Progr. Math., 133, Birkhäuser, Basel, 1995; MR1362838.

In general, where there may be holomorphic spheres with small negative Chern number, one has little choice but to allow "stable trajectories" consisting of broken Floer trajectories with holomorphic bubble-trees attached. Transversality is proved by introducing multi-valued perturbations to the equations, and this forces one to use rational coefficients. References:

Fukaya, Kenji; Ono, Kaoru. "Arnold conjecture and Gromov-Witten invariant". Topology 38 (1999), no. 5, 933-1048. MR1688434
Liu, Gang; Tian, Gang, "Floer homology and Arnold conjecture", J. Differential Geom. 49 (1998), no. 1, 1-74. MR1642105

[Edit: both these references offer proofs of the weak Arnol'd conjecture with rational coefficients.] For a detailed introduction to these "virtual transversality" methods, see

Salamon, Dietmar, "Lectures on Floer homology". MR1702944

The technical complications of virtual transversality theory are notorious, and one could wish for a fully detailed textbook account.

What's left?

So far as I know, there is no proof for general manifolds that the number $h$ of 1-periodic orbits of a non-degenerate Hamiltonian is at least the sum of the mod $p$ Betti numbers. The strong Arnol'd conjecture for non-degenerate Hamiltonians, that $h$ is at least the minimum number of critical points of a Morse function, is wide open.

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Very nice - thank you! In the mathSciNet review of the last reference it states:"The lectures serve as a good introduction to symplectic Floer homology and the proof of the Arnolʹd conjecture for general symplectic manifolds.", which seems to indicate that the Arnold conjecture has been proven? – Thomas Kragh May 28 '10 at 11:12
In fact On page 4 of the last reference it is stated that it has been proved and provides references - so I have my general answer. Thanks again! – Thomas Kragh May 28 '10 at 12:15
Yes, I should have said clearly that the Fukaya-Ono and Liu-Tian papers offer proofs of the rational Arnol'd weak conjecture on general symplectic manifolds. Good luck understanding the details... – Tim Perutz May 28 '10 at 13:13