About the problem
In dimension four, it is known that it is possible to fully fill a symplectic ball by \(k\) symplectic balls of the same radius whenever \(k \ge 10\). Here “fully fill” means that one can find a symplectomorphism under which the images of the balls take up all but \(\epsilon\) of the volume of the target ball, for any arbitrarily small \(\epsilon > 0\). However, the proof is not at all explicit. It remains an important open problem to find explicit constructions of these embeddings.
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Prompt
Fix $n=2$. Let $k \ge 10$ be an integer and take $\epsilon \in (0,1)$. For $i = 1,\dots,k$, let $B_i \subset \R^{2n}$ denote the closed unit ball centered at $(3i-3,0,\dots,0)$.
Your goal is to find an algorithm that takes $k$ and $\epislon$ as inputs and produces as output an explicit smooth Hamiltonian function $H_\epsilon: \R^{2n} \times [0,1] \rightarrow \R$ whose time-$1$ Hamiltonian flow $\phi: \R^{2n} \ra \R^{2n}$ satisfies $\phi(\sqcup_{i=1}^{k}B_i) \subset B^{2n}(R)$, where $B^{2n}(R) \subset \R^{2n}$ is the closed ball centered at the origin of radius $R$, and where we require the volume ratio of $\bigsqcup\limits_{i=1}^{k} B_i$ to $B^{2n}(R)$ to satisfy $\dfrac{k}{R^{2n}} > 1 - \epsilon$.
Present the algorithm as a Python program that takes $k$ and $\epislon$ as inputs. The algorithm's output must itself be a Python program that implements the Hamiltonian for the given $k$ and $\epislon$. This program should be in a file called Hamiltonian.py which implements a function Hamiltonian(Q, P, t) that has the following properties.
- Takes inputs Q (positions), P (momenta), and t (time).
- Must be implemented using PyTorch operations to support automatic differentiation.
- Inputs Q and P will be `torch.Tensor`s of shape `(N, n)` (where N is the size of a batch size of points sampled from the boundaries of the input balls).
- Input t will be a scalar float.
- Returns a `torch.Tensor` of shape `(N,)` representing the Hamiltonian value for each point in the batch.
- Is smooth, to ensure existence and uniqueness of flow.
A solution to this problem is meant to rest on finding a family of embeddings that has a natural algorithmic dependence on $\epsilon$. As such, the algorithm that produces the Hamiltonians for a given $\epsilon$ must finish running in under an hour on a typical laptop. The Hamiltonian function itself will be called multiple times during solution verification and must return within ten seconds on each call.