http://ift.tt/2fbqA81 Theorems have the tendency to pop into existence when you least expect them: taking a bath, during your sleep, dreaming away during a dull lecture, waiting for an airplane, bicycling, whatever. One of the most famous (and useful) lemmas was dreamed up in the Parisian Gare du Nord station, during a conversation between Saunders Mac Lane (then in his mid 40ties) and a young Japanese mathematician, half his age, Nobuo Yoneda . Here’s the story: Yoneda’s story In the announcement of the death of his friend Yoneda on the catlist , the computer scientist Yoshiki Kinoshita writes: “Prof. Yoneda was born on 28 March, 1930. He studied mathematics in the University of Tokyo; in the last year of his studies he followed the seminar of Prof. Shokiti Iyanaga , where he became interested in algebraic topology. Soon after that (or maybe when he was still an undergraduate), Prof. Samuel Eilenberg visited Japan, and Yoneda traveled around Japan with him, as a translator a...