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Casper Oelen

Publications and source records attributed to Casper Oelen.

4 recordsLinked to original sources
arXiv2025

Non-Birkhoff periodic orbits in symmetric billiards

We study non-Birkhoff periodic orbits in symmetric convex planar billiards. Our main result provides a quantitative criterion for the existence of such orbits with prescribed minimal period, rotation number, and spatiotemporal symmetry. We exploit this criterion to find sufficient conditions for a symmetric billiard to possess infinitely many non-Birkhoff periodic orbits. It follows that arbitrarily small analytical perturbations of the circular billiard have non-Birkhoff periodic orbits of any rational rotation number and with arbitrarily long periods. We also generalize a known result for elliptical billiards to other $\mathbb{D}_2$-symmetric billiards. Lastly, we provide Matlab codes which can be used to numerically compute and visualize the non-Birkhoff periodic orbits whose existence we prove analytically.

math.DS↗
arXiv2024

Normal forms of elliptic automorphic Lie algebras and Landau-Lifshitz type of equations

We present normal forms of elliptic automorphic Lie algebras with dihedral symmetry of order 4, which arise naturally in the context of Landau-Lifshitz type of equations. These normal forms provide a transparent description and allow a classification of such Lie algebras over $\mathbb{C}$. Using this perspective, we show that a Lie algebra introduced by Uglov, as well as the hidden symmetry algebra of the Landau-Lifshitz equation by Holod, are both isomorphic to an elliptic $\mathfrak{sl}(2,\mathbb{C})$-current algebra. Furthermore, we realise the Wahlquist-Estabrook algebra of the Landau-Lifshitz equation in terms of elliptic automorphic Lie algebras. This construction reveals that, as complex Lie algebras, it is isomorphic to the direct sum of an $\mathfrak{sl}(2,\mathbb{C})$-current algebra and the two-dimensional abelian Lie algebra $\mathbb{C}^2$. Finally, we explicitly implement the automorphic Lie algebra framework in the context of an $n$-component generalisation of the Landau-Lifshitz equation by Golubchik and Sokolov in the case of $n=3$.

math.RA↗
arXiv2024

A classification of automorphic Lie algebras on complex tori

We classify the automorphic Lie algebras of equivariant maps from a complex torus to $\mathfrak{sl}_2(\mathbb{C})$. For each case we compute a basis in a normal form. The automorphic Lie algebras correspond precisely to two disjoint families of Lie algebras parametrised by the modular curve of $\mathrm{PSL}_2(\mathbb{Z})$, apart from four cases, which are all isomorphic to Onsager's algebra.

math.RA↗