Floer homology and status of the Arnold conjecture. - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-21T12:48:41Z http://mathoverflow.net/feeds/question/26203 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/26203/floer-homology-and-status-of-the-arnold-conjecture Floer homology and status of the Arnold conjecture. Thomas Kragh 2010-05-27T22:07:06Z 2010-06-03T19:56:32Z <p>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.</p> <p>On wikipedia it says that the Arnold conjecture is solved in many cases using Floer homology. </p> <p>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.</p> <p><strong>Question</strong>: What is the current status of this weak Arnold conjecture precisely, and what are the refferences for these results?</p> <p>Added: I know most of the details of the monotone case, so I am mostly interested in the more exotic cases.</p> http://mathoverflow.net/questions/26203/floer-homology-and-status-of-the-arnold-conjecture/26208#26208 Answer by Tim Perutz for Floer homology and status of the Arnold conjecture. Tim Perutz 2010-05-27T23:05:12Z 2010-06-03T19:56:32Z <hr> <p><i>V. I. Arnol'd, June 12, 1937 - June 3, 2010.</i></p> <p>The very sad news of his death is reported today <a href="http://www.france-info.com/ressources-afp-2010-06-03-l-eminent-mathematicien-russe-vladimir-arnold-mort-subitement-en-450299-69-69.html" rel="nofollow">here</a>.</p> <hr> <p>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.</p> <p>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 </p> <blockquote> Hofer, H.; Salamon, D. A. "Floer homology and Novikov rings." The Floer memorial volume, 483--524, Progr. Math., 133, Birkhäuser, Basel, 1995; MR1362838. </blockquote> <p>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:</p> <blockquote> Fukaya, Kenji; Ono, Kaoru. "Arnold conjecture and Gromov-Witten invariant". Topology 38 (1999), no. 5, 933-1048. MR1688434 </blockquote> <blockquote> Liu, Gang; Tian, Gang, "Floer homology and Arnold conjecture", J. Differential Geom. 49 (1998), no. 1, 1-74. MR1642105 </blockquote> <p>[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</p> <blockquote> Salamon, Dietmar, "Lectures on Floer homology". MR1702944 </blockquote> <p>The technical complications of virtual transversality theory are notorious, and one could wish for a fully detailed textbook account.</p> <p>What's left?</p> <p>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.</p>