A146747 Numbers k such that sigma_1(k)*sigma_0(k)/(sigma_1(k)-sigma_0(k)) is an integer.

2, 3, 5, 6, 7, 20, 30

Offset: 1

Author

Ctibor O. Zizka, Nov 01 2008

Keywords

• nonn
• more

Comments

Numbers k such that A000203(k)*A000005(k)/(A000203(k)-A000005(k)) is an integer.

Crossrefs

Cf. A000005, A000203, A063647, A083795.

Programs

Mathematica

q[n_] := Module[{s = DivisorSigma[1, n], d = DivisorSigma[0, n]}, Divisible[s*d, s - d]]; Select[Range[2, 100], q] (* Amiram Eldar, Apr 07 2024 *)

PARI

is(n) = {my(f = factor(n), s = sigma(f), d = numdiv(f)); n > 1 && !((s*d) % (s-d));} \\ Amiram Eldar, Apr 07 2024