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This is Numberphile. We mainly post videos about mathematics and just numbers in general.

The Dehn Invariant

The Dehn Invariant

Additional information

It was #3 on Hilbert's list of the most important problems in mathematics - until his student solved it.

Featuring Daniel Litt, currently at the Institute for Advanced Study - https://twitter.com/littmath

David Hilbert's problems: https://en.wikipedia.org/wiki/Hilbert%27s_problems

Max Dehn's famed proof: https://archive.org/details/mathematischean33behngoog/page/n488

James Card Trick

James Card Trick