Poincare's `Sur les Solutions Periodiques et le principe de moindre action'.
in Comptes rendus l'Academie de 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 Poincare 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 this 2 and a half pages, Poincare does most everything I did in a 13 page 102 years later. He does it more clearly and elegantly.
