Ricci curvature for fluid models on the torus via Zeitlin's quantization

Abstract

Ricci curvature measures the average stability of geodesics under transverse perturbations, but how it should be defined in infinite dimensions is often unclear. This paper proposes a definition of Ricci curvature on the space of Hamiltonian diffeomorphisms on the two-dimensional flat torus HDiff(𝕋²), the state space for ideal fluids. Our definition is based on Zeitlin's model, which approximates HDiff(𝕋²) by finite-dimensional Lie groups SU(N). We derive a formula for the Ricci curvature tensor on SU(N) and provide numerical evidence for its convergence in the large-N limit to our conjectured finite value. Additionally, we explore potential applications for hydrodynamics through the Lyapunov stability of gravest wave modes and Arnold's tradewind estimates for long-term weather predictability. Our framework extends to a wide range of settings. We demonstrate this by introducing Ricci curvature on the state spaces of fluids on rectangular domains, the Lagrangian averaged Euler equation induced by the H¹-Sobolev metric, and the quasi-geostrophic equation incorporating the Coriolis effect.

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BibTeX RIS

BibTeX

@misc{ishida2026ricci,
  title = {{Ricci curvature for fluid models on the torus via Zeitlin's quantization}},
  author = {Sadashige Ishida and Alex Suri},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.01259},
  eprint = {2609.01259},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.01259},
  note = {CC BY 4.0},
}

RIS

TY  - UNPB
TI  - Ricci curvature for fluid models on the torus via Zeitlin's quantization
AU  - Sadashige Ishida
AU  - Alex Suri
PY  - 2026
DA  - 2026/09/01
PB  - arXiv
DO  - 10.48550/arxiv.2609.01259
UR  - https://arxiv.org/abs/2609.01259
AB  - Ricci curvature measures the average stability of geodesics under transverse perturbations, but how it should be defined in infinite dimensions is often unclear. This paper proposes a definition of Ricci curvature on the space of Hamiltonian diffeomorphisms on the two-dimensional flat torus HDiff(𝕋^2), the state space for ideal fluids. Our definition is based on Zeitlin's model, which approximates HDiff(𝕋^2) by finite-dimensional Lie groups SU(N). We derive a formula for the Ricci curvature tensor on SU(N) and provide numerical evidence for its convergence in the large-N limit to our conjectured finite value. Additionally, we explore potential applications for hydrodynamics through the Lyapunov stability of gravest wave modes and Arnold's tradewind estimates for long-term weather predictability. Our framework extends to a wide range of settings. We demonstrate this by introducing Ricci curvature on the state spaces of fluids on rectangular domains, the Lagrangian averaged Euler equation induced by the H^1-Sobolev metric, and the quasi-geostrophic equation incorporating the Coriolis effect.
ER  -

Abstract

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