Poincaré's `[Sur les solutions périodiques et le principe de moindre action](https://gallica.bnf.fr/ark:/12148/bpt6k30799/f915.item)'.

in *Comptes Rendus des Séances de l'Académie des Sciences*, t. 123, p 915-918, 1896

This paper humbled me. It made me realize how much the founders of the subject(s) I work in  really knew, and how far ahead they could see. I am in awe of how Poincaré could give such a detailed trip report of his investigations without the formal language we use today being in place.  (In many ways, formal language often gets in the way.) In these 2 and a half pages, Poincaré does most everything I did in a 13 page paper 102 years later. He does it more clearly and elegantly.