%0 Journal Article
%A H. Stachel 
%T Isometric Billiards in Ellipses and Focal Billiards in Ellipsoids
%J Journal for geometry and graphics
%D 2021
%P 97-118
%V 25
%N 1
%I mathdoc
%U http://geodesic.mathdoc.fr/item/JGG_2021_25_1_JGG_2021_25_1_a8/
%F JGG_2021_25_1_JGG_2021_25_1_a8