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 A137210 Numbers k such that abundance(k) = abundance(sigma(k)). 0
 1, 84, 252, 12717, 177744, 189264, 143747328 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(8) > 3.4*10^10, if it exists. - Amiram Eldar, Apr 06 2024 LINKS Table of n, a(n) for n=1..7. EXAMPLE abundance(84) = sigma(84) - 2(84) = 56 = abundance(224) = abundance(sigma(84)), so 84 is a term in the sequence. MATHEMATICA abund[n_] := DivisorSigma[1, n]-2n; l = {}; For[i = 1, i <= 10^6, i++, If[abund[i] == abund[DivisorSigma[1, i]], l = Append[l, i]]]; l PROG (PARI) is(k) = {my(s = sigma(k)); s - 2*k == sigma(s) - 2*s; } \\ Amiram Eldar, Apr 06 2024 CROSSREFS Cf. A000203 (sigma), A033880 (abundance). Sequence in context: A112066 A096383 A220043 * A220009 A160321 A249969 Adjacent sequences: A137207 A137208 A137209 * A137211 A137212 A137213 KEYWORD nonn,more AUTHOR Joseph L. Pe, Mar 05 2008 EXTENSIONS a(7) from Donovan Johnson, Apr 27 2008 STATUS approved

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Last modified August 8 16:18 EDT 2024. Contains 375023 sequences. (Running on oeis4.)