Jáchim, František. “The Principle, Which Provided a Comprehensive View on Ray Optics (The 350th Anniversary of the Death of Pierre De Fermat)”. MATHEMATICS–PHYSICS–INFORMATICS 24, no. 5 (November 1, 2015): 397–400. Accessed July 3, 2024. https://mfi.upol.cz/index.php/mfi/article/view/240.