Positioned as the hardest question on a Putnam exam (#6, 1992)
Open with LBRYA link to the full video is at the bottom of the screen.
Or, for reference: https://youtu.be/OkmNXy7er84
Editing from the original video into this short by Dawid Kołodziej
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https://www.youtube.com/watch?v=IstyL9vnybg