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 A058075 Numbers k such that gcd(sigma(k), sigma(k+1)) = 1, where sigma(k) is sum of positive divisors of k. 4
 1, 2, 3, 4, 7, 8, 9, 15, 16, 18, 24, 25, 31, 35, 36, 48, 63, 64, 72, 80, 81, 97, 99, 100, 120, 121, 127, 128, 143, 144, 162, 200, 224, 225, 241, 255, 256, 288, 289, 323, 337, 399, 400, 483, 511, 512, 528, 529, 575, 576, 577, 578, 624, 625, 675, 721, 722, 728, 729 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Ivan Neretin, Table of n, a(n) for n = 1..3000 EXAMPLE 8 is included because sigma(8) = 15 is relatively prime to sigma(9) = 13. MATHEMATICA Select[Range@800, GCD @@ DivisorSigma[1, {#, # + 1}] == 1 &] (* Ivan Neretin, Jan 27 2017 *) Drop[Position[Partition[DivisorSigma[1, Range[800]], 2, 1], _?(GCD@@#== 1&)]// Flatten, 2] (* Harvey P. Dale, Jul 31 2019 *) PROG (PARI) lista(nn) = {for(n=1, nn, if (gcd(sigma(n), sigma(n+1)) == 1, print1(n, ", "))); } \\ Michel Marcus, May 19 2014 CROSSREFS Cf. A058074. Sequence in context: A307561 A152037 A329295 * A243495 A029783 A112736 Adjacent sequences:  A058072 A058073 A058074 * A058076 A058077 A058078 KEYWORD nonn AUTHOR Leroy Quet, Nov 11 2000 STATUS approved

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Last modified June 7 05:20 EDT 2020. Contains 334837 sequences. (Running on oeis4.)