New constraints on Lagrangian embeddings and the shape invariant

For a large class of toric domains in $\mathbb{R}^4$ we determine which product Lagrangian tori can be mapped into the domain by a Hamiltonian diffeomorphism. In other words, we compute the Hamiltonian shape invariant of these toric domains, as defined by Hind and Zhang. The argument relies on new intersection results for product Lagrangian tori in symplectic polydisks. For Hamiltonian diffeomorphisms which map certain Lagrangian product tori back into the polydisk, we establish intersections between the images and a one-parameter family of product Lagrangian tori that includes (is based at) the original torus. For symplectic polydisks with area ratios less than two, we strengthen this to establish intersections between the Hamiltonian images and the original Lagrangian torus. As a soft complement to these intersection results we also present an embedding construction which demonstrates that this intersection rigidity vanishes when the one-parameter family of product Lagrangian tori is replaced by a natural packing by Lagrangian tori.

💡 Research Summary

The paper investigates which product Lagrangian tori can be placed inside a large class of toric domains in ℝ⁴ by Hamiltonian diffeomorphisms, thereby computing the Hamiltonian shape invariant introduced by Hind and Zhang. The authors begin with a two‑dimensional illustration: a Lagrangian circle L(s) inside a disk D(b) must intersect a one‑parameter family of circles L(t) whenever a Hamiltonian image of L(s) stays inside D(b). This motivates the four‑dimensional analogue, where the standard product torus L(r,s)= {π|z₁|²=r, π|z₂|²=s} lives in the symplectic polydisk P(a,b)= {π|z₁|²<a, π|z₂|²<b}.

The central result (Theorem 1.3) states that for ½ ≤ r < 1 and 1 ≤ s < b/2, any Hamiltonian diffeomorphism φ with φ(L(r,s))⊂P(1,b) must intersect the codimension‑one family {L(r,t) | t∈