arXiv subjects

René Langøen

Publications and source records attributed to René Langøen.

3 recordsLinked to original sources

arXiv2026

Curvature Decomposition for Riemannian Lie Algebroids

Classical curvature formulas for Lie groups and Riemannian submersions are recast in terms of Lie groupoids with right-invariant source-fibre metrics and Riemannian Lie algebroids. We derive a sectional-curvature formula for such groupoids extending the Arnold-Milnor ``1-2-3-4'' formula. For transitive Riemannian Lie algebroids, we factor the base and vertical Levi-Civita connections through Lie algebroids of metric derivations and obtain Lie-algebroid analogues of O'Neill's curvature formulas. Applied to the action of the diffeomorphism group of the torus on densities, these formulas recover Arnold's curvature of the measure-preserving diffeomorphism group and give explicit Wasserstein sectional curvatures at the uniform density. Finite-dimensional examples include the rotational action algebroid of the round sphere and the WGS84 ellipsoid, and a smooth family of negative curvatures on non-abelian isotropy groups over the real projective line.

math.DG↗

arXiv2025

Curvature of Measure-Preserving Diffeomorphism Groups of Non-Orientable Surfaces

We study curvatures of the groups of measure-preserving diffeomorphisms of non-orientable compact surfaces. For the cases of the Klein bottle and the real projective plane we compute curvatures, their asymptotics and the normalized Ricci curvatures in many directions. Extending the approach of V. Arnold, and A. Lukatskii we provide estimates of weather unpredictability for natural models of trade wind currents on the Klein bottle and the projective plane.

math.DG↗

arXiv2024

Stokes graphs of the Rabi problem with real parameters

The goal of this paper is to study the geometry of the Stokes graphs associated with the problem, which was introduced by Isidor Rabi in 1937 to model reactions of atoms to the harmonic electric field with frequency close to the natural frequency of the atoms. In the standard Garnier form, the Rabi model is a matrix linear differential equation with three physical parameters, which are: the level of separation of the fermion mode $Δ$, the boson-fermion coupling $g$, and the eigenvalue $E$ of the Hamiltonian relevant to this model. The qualitative behavior of solutions of this type of problems is often described in terms of the Stokes graphs of associated quadratic differential, which in the case of Rabi problem can be represented in the form $Q_0(z)\, dz^2 = -\frac{z^4+c_3z^3+c_2z^2+c_1z+c_0}{(z-1)^2(z+1)^2}\, dz^2$ with the coefficients $c_k$, $k=0,1,2,3$, depending on the parameters $Δ$, $g$, and $E$. In this paper, we first give a complete classification of possible generic topological types of domain configurations and Stokes graphs of this quadratic differential assuming that its coefficients $c_k$ are real and the zeros of its numerator are distinct from its poles. Then we identify the set of coefficients $(c_3,c_2,c_1,c_0)\in \mathbb{R}^4$, which correspond to particular choices of the physical parameters $Δ$, $g$, and $E$. The structure of Stokes graphs and domain configurations of quadratic differentials, which appear as asymptotic cases when the parameters of the Rabi problem tend to infinity, also will be discussed.

math-ph↗