A280927 Odd numbers k such that phi(k) and cototient(k) have the same prime signature.

1735, 2469, 4341, 4569, 4989, 5469, 5637, 5961, 6879, 7149, 7407, 8675, 9969, 11569, 12949, 13057, 13089, 13707, 15829, 15969, 16407, 18597, 18969, 19959, 20109, 20487, 20721, 21081, 21309, 21729, 22107, 22221, 22513, 23469, 24355, 25269, 25617, 26305, 27021

Offset: 1

Author

Altug Alkan, Jan 10 2017

Keywords

• nonn

Comments

46347 = 3*7*2207 is the least term that has 3 distinct prime factors.

Examples

			1735 is a term because phi(1735) = 1384 = 2^3 * 173 and cototient(1735) = 1735 - phi(1735) = 351 = 3^3 * 13.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000010, A051953.

Programs

Mathematica

Select[Range[5, 30000, 2], Sort @ FactorInteger[(phi = EulerPhi[#])][[;;, 2]] == Sort @ FactorInteger[# - phi][[;;, 2]] &] (* Amiram Eldar, Jan 02 2021 *)

PARI

is(n) = vecsort(factor(eulerphi(n))[, 2]) == vecsort(factor(n-eulerphi(n))[, 2]) && n%2==1;