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Erdős problem 346

Let A={1a1<a2<}A=\{1\leq a_1< a_2<\cdots\} be a set of integers such that A\BA\backslash B is complete for any finite subset BB and not complete for any infinite subset BB. If an+1/an1+ϵa_{n+1}/a_n \geq 1+\epsilon for all nn, must limnan+1/an=(1+5)/2\lim_n a_{n+1}/a_n=(1+\sqrt{5})/2? Under the reading where the ratio limit is assumed to exist, a Lean-verified argument forces the limit to be the golden ratio; a separate construction disproves the literal statement where convergence is not assumed.

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sha256:5194281796a5abbc79792b92fdc32c109a1fa119286627991a022392563d0e5b
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sha256:251a4854c72031baf8fdfa681bf793f14acdf443c409928b5773f63789513038
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sha256:8c823d621b7e1256c8e47c60a5f1c54c016a5507e6f27b2bab537f6f5f232067
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sha256:309b4735342d7c95ac7e41e9bf63d77289e46d394b2f409c47efd19c76eca90f
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sha256:a956b84c437202e5a02cc9e036a621bd14a302b34a75758115730bdbb77c52a4
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sha256:c9d14c459c518937e758918b5897dc3b22f1a55f07739afe99502f5b046c907a
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2415f78e850aeee50afdca525c6f2e0ea606f207

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