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I'm trying to find out the motivations that led V. Arnold to formulate his famous conjecture (I guess theorem by now) in the following form:

Let $(M,\omega)$ be a closed symplectic manifold (add whatever monotonicity condition here to avoid the need of virtual methods) and let $\phi$ be an Hamiltonian diffeomorphism, then the number of fixed points of $\phi$ is bounded below by the sum of the Betti numbers of $M$

Now Hamiltonian diffeomorphisms correspond to canonical transformations in physics and give rise to a family of Hamiltonians $\{H_t\}_{t \in [0,1]}$.

Q1 Why do we require in the standard statement of the weak Arnold conjecture, that the family of Hamiltonians is (say) $1$-periodic? Is it due to some interesting physic application?

In order to set up the action functional on the loop space of $M$ (the approach of Conley, Zehnder and Floer for example), we have to restrict to contractible periodic orbits $x(t)$ solutions of $$ \dot{x(t)}=X_{H_t}(x(t))$$ where $X_{H_t}$ is the Hamiltonian vector field associated with $H_t$.

Q2 Since there is a 1-1 bijection between fixed points of $\phi=\varphi^1_{X_{H_t}}$ and the periodic orbits defined above, does it mean that I'm potentially not counting certain fixed points which arises as NON-contractible periodic orbits?

Thanks a lot for any clarifications

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    $\begingroup$ If the Hamiltonian is not $1$-periodic, having a fixed point is not so meaningful, as the point will stay fixed after $1$ unit of time but could move over the next $1$ unit. $\endgroup$
    – Will Sawin
    Commented Feb 13, 2022 at 23:22
  • $\begingroup$ @WillSawin that's an interesting observation, thanks for sharing! so we ask for periodicity in order to deal with "meaningful" fixed points? is this idea inspired by physics somehow? $\endgroup$
    – Riccardo
    Commented Feb 13, 2022 at 23:30
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    $\begingroup$ I don't know if the idea was inspired by physics, but physicists are certainly interested in states of systems that are stable for a long time - for example because they make it easy to predict what will happen for a long time, or because that's where you want to put your satellite, or for a number of reasons. If the Hamiltonian is periodic with period $t$, a fixed point of the evolution for time $t$ map will be stable for a long time, but if the Hamiltonian is not periodic, it won't be. $\endgroup$
    – Will Sawin
    Commented Feb 14, 2022 at 0:10
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    $\begingroup$ In the title you mention the motivation for the conjecture. If you want to read a very beautiful paper of Arnold which explains the thinking/philosophy behind some of his conjectures, try his "Topological problems in wave propagation and topological economy principle in algebraic geometry". $\endgroup$ Commented Feb 14, 2022 at 8:24
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    $\begingroup$ @JonnyEvans That paper is quite nice to read! It reads like a script, thanks a lot for sharing it! $\endgroup$
    – Riccardo
    Commented Feb 14, 2022 at 16:43

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