Comlan Koudjinan
Publications:
Koudjinan C. E., Kaloshin V.
On Some Invariants of Birkhoff Billiards Under Conjugacy
2022, vol. 27, no. 5, pp. 525537
Abstract
In the class of strictly convex smooth boundaries each of which has no strip around its boundary foliated by invariant curves, we prove that the Taylor coefficients of the ``normalized'' Mather's $\beta$function
are invariant under $C^\infty$conjugacies.
In contrast, we prove that any two elliptic billiard maps are $C^0$conjugate near their respective boundaries, and $C^\infty$conjugate, near the boundary and away from a line passing through the center of the underlying ellipse. We also prove that, if the billiard maps corresponding to two ellipses are topologically conjugate, then the two ellipses are similar.

Chierchia L., Koudjinan C. E.
V. I. Arnold’s ''Global'' KAM Theorem and Geometric Measure Estimates
2021, vol. 26, no. 1, pp. 6188
Abstract
This paper continues the discussion started in [10] concerning Arnold's legacy on classical KAM theory and (some of) its modern developments. We prove a
detailed and explicit ''global'' Arnold's KAM theorem, which yields, in particular, the Whitney conjugacy of a nondegenerate,
realanalytic, nearlyintegrable Hamiltonian system to an integrable system on a closed, nowhere dense, positive measure subset of the phase space. Detailed measure estimates on the Kolmogorov set are provided in case the phase space is: (A)
a uniform neighbourhood of an arbitrary (bounded) set times the $d$torus and
(B) a domain with $C^2$ boundary times the $d$torus. All constants are explicitly given.
