Here is a 3x3 square poem -- inspired by a recently-found margin-note I made in Differential and Integral Calculus (by Ross R Middlemiss) when it was my text for an introductory calculus course at Westminster College all those years ago:
The sum of
While the pages of text near the note go on with discussions and diagrams of slices and sums and limits -- they introduce the centroid, the moment of inertia, and the radius of gyration, and are importantly informative -- it is the margin-note that has today delighted me. I wonder if the girl who wrote it saw it as I do today. I like the mystery.