@matematico314
Ah! One of my all-time favorite classes was a course on the Lebesgue integral taught by the inimitable David Bressoud. We studied it in historical order: encountering problems as mathematicians encountered them in the 19th century, proving a theorem one week that turned out to be false the next week, sharpening definitions as we went in true Lakatos style. It was wonderful.
Thanks for the meme-y memories!
@matematico314 Suggested alt text for the image (which you can add by editing the post):
A cartoon of two Christmas trees side by side, both with Lego-like block shapes. On the left is “Riemann’s Christmas Tree,” composed of vertical bars. On the right is “Lebesque’s Christmas tree,” composed of horizontal bars.