A332217 Numbers k for which the 2-adic valuation of sigma(k) is zero and its 3-adic valuation is 1 (so that sigma(k) is an odd multiple of 3, but not of 9).

2, 8, 18, 49, 50, 72, 128, 162, 169, 196, 200, 242, 361, 441, 450, 512, 578, 648, 676, 784, 961, 968, 1058, 1152, 1225, 1250, 1369, 1444, 1458, 1521, 1682, 1764, 1800, 1849, 2178, 2312, 2704, 3136, 3200, 3249, 3362, 3721, 3844, 3969, 4050, 4225, 4232, 4418, 4489, 4608, 4802, 4900, 5000

Offset: 1

Author

Antti Karttunen, Feb 11 2020

Keywords

• nonn

Links

• Antti Karttunen, Table of n, a(n) for n = 1..10000
• Index entries for sequences related to sigma(n)

Crossrefs

Subsequence of A067051, which is a subsequence of A028982.

Cf. A332218 (a subsequence).

Cf. A000203, A007949, A332216.

Programs

Mathematica

Select[Range[5*10^3], IntegerExponent[DivisorSigma[1, #], {2, 3}] === {0, 1} &] (* Michael De Vlieger, Feb 12 2020 *)

PARI

isA332217(n) = ((sigma(n)%2)&&(valuation(sigma(n),3)==1));

Formula

{n: A000035(A000203(n))*A007949(A000203(n))==1}.