(September 21, 2018 at 10:09 pm)DodosAreDead Wrote: Ask people the biggest thing they've learned as they've got older and they'll start spouting all sorts of philosophical mumbo jumbo. Nothing academic.
In other words, we don't need no school.
Well, if you want to go that direction, I'd say the Atiyah-Singer Index Theorem is the biggest thing I've learned. But the most useful is probably the Urysohn Lemma.