Genius mathematician and an excellent expositor as well! One year, he was invited to give the inaugural lecture for undergraduate math in my university. The t

Genius mathematician and an excellent expositor as well! One year, he was invited to give the inaugural lecture for undergraduate math in my university. The title of the talk was something like "four derivations of the heat equation". And it was really that, he explained some independent physical models to reach the same mathematical formulation. One of them was particles moving in random walks, another an out-of focus lens, etc. At the end he said that it was important for mathematicians to understand the connections between these interpretations because they lead to many new insights into the equation. For linear equations the story was already pretty well-known, but developing the corresponding methods for nonlinear equations was an ongoing story on which he was actively participating. It's not an exaggeration to say that most undergrads could follow most of the talk, which may be a unique occurrence as inaugural lessons go. Typically lower-level mathematicians that give these talks tend to explain complicated proofs; but he did just the opposite: only talk about modelling, with subtle side hints at methods of proof. The talk was so engaging that many attendees became extremely excited to work on nonlinear PDE. (And some of them did, to great success!)