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 A057637 a(n) is the largest number such that sigma(a(n)) = n, where sigma is the sum of divisors function (A000203). 9
 1, 0, 2, 3, 0, 5, 4, 7, 0, 0, 0, 11, 9, 13, 8, 0, 0, 17, 0, 19, 0, 0, 0, 23, 0, 0, 0, 12, 0, 29, 25, 31, 0, 0, 0, 22, 0, 37, 18, 27, 0, 41, 0, 43, 0, 0, 0, 47, 0, 0, 0, 0, 0, 53, 0, 39, 49, 0, 0, 59, 0, 61, 32, 0, 0, 0, 0, 67, 0, 0, 0, 71, 0, 73, 0, 0, 0, 45, 0, 79, 0, 0, 0, 83, 0, 0, 0, 0, 0, 89 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Right border of A299762. - Omar E. Pol, Mar 14 2018 LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 EXAMPLE x = 11 is the largest x such that sigma(x) = 12, so a(12) = 11. MATHEMATICA a[n_] := Module[{k = n}, While[k > 0 && DivisorSigma[1, k] != n, k--]; k]; Array[a, 90] (* Amiram Eldar, Jan 05 2020 *) CROSSREFS Cf. A000203, A051444, A054973, A299762. Sequence in context: A240667 A051444 A299762 * A258913 A167485 A294141 Adjacent sequences:  A057634 A057635 A057636 * A057638 A057639 A057640 KEYWORD nonn AUTHOR Jud McCranie, Oct 10 2000 STATUS approved

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Last modified January 26 20:11 EST 2020. Contains 331288 sequences. (Running on oeis4.)