A058075 Numbers k such that gcd(sigma(k), sigma(k+1)) = 1, where sigma(k) is sum of positive divisors of k.

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

Offset

1,2

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..3000 from Ivan Neretin)

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 A340324 A029783

Adjacent sequences: A058072 A058073 A058074 * A058076 A058077 A058078

Keyword

nonn

Author

Leroy Quet, Nov 11 2000

Status

approved