A small puzzle about sorting envelopes and stamps: Post's Correspondence Problem with a postal skin on top. Twelve stages. The first six can be solved by hand; the last six are instances taken from the literature, and cannot.
If you have not met the problem before, this is a way to get a feel for it: the rule is all there is to it, and it is undecidable in general.
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[ 3.1 ms ] story [ 10.8 ms ] threadIf you have not met the problem before, this is a way to get a feel for it: the rule is all there is to it, and it is undecidable in general.
Made to celebrate every instance of PCP[3,4] finally being resolved: https://github.com/Mojashi/pcp-3-4-proofs