Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #15 Jan 02 2021 04:50:01
%S 1735,2469,4341,4569,4989,5469,5637,5961,6879,7149,7407,8675,9969,
%T 11569,12949,13057,13089,13707,15829,15969,16407,18597,18969,19959,
%U 20109,20487,20721,21081,21309,21729,22107,22221,22513,23469,24355,25269,25617,26305,27021
%N Odd numbers k such that phi(k) and cototient(k) have the same prime signature.
%C 46347 = 3*7*2207 is the least term that has 3 distinct prime factors.
%H Amiram Eldar, <a href="/A280927/b280927.txt">Table of n, a(n) for n = 1..10000</a>
%e 1735 is a term because phi(1735) = 1384 = 2^3 * 173 and cototient(1735) = 1735 - phi(1735) = 351 = 3^3 * 13.
%t Select[Range[5, 30000, 2], Sort @ FactorInteger[(phi = EulerPhi[#])][[;;, 2]] == Sort @ FactorInteger[# - phi][[;;, 2]] &] (* _Amiram Eldar_, Jan 02 2021 *)
%o (PARI) is(n) = vecsort(factor(eulerphi(n))[, 2]) == vecsort(factor(n-eulerphi(n))[, 2]) && n%2==1;
%Y Cf. A000010, A051953.
%K nonn
%O 1,1
%A _Altug Alkan_, Jan 10 2017