A015715 - OEIS

%I #25 Dec 27 2024 04:12:02

%S 1,3,15,35,105,357,1045,1485,3135,3339,5049,10659,12441,16065,24871,

%T 24969,29029,33915,35343,39105,39585,50065,58435,64285,71145,74613,

%U 87087,87685,99693,124355,124605,132957,137885,140335,145145,150195

%N Odd integers m such that phi(m) | sigma(m).

%C Subsequence of A236693. Proof: if n is in this sequence, then 2^phi(n) - 1 is divisible by n and 2^sigma(n) - 1 is divisible by 2^phi(n) - 1. Therefore, 2^sigma(n) == 1 (mod n) and n is in A236693. - _Jinyuan Wang_, Mar 13 2020

%H Amiram Eldar, <a href="/A015715/b015715.txt">Table of n, a(n) for n = 1..10000</a> (calculated using data from Jud McCranie; terms 1..1000 from Donovan Johnson)

%t Select[Range[1,151001,2],Divisible[DivisorSigma[1,#],EulerPhi[#]]&] (* _Harvey P. Dale_, Sep 16 2016 *)

%o (PARI) isok(m) = (m % 2) && !(sigma(m) % eulerphi(m)); \\ _Michel Marcus_, Mar 14 2020

%Y Subsequence of A020492 and A236693.

%Y Cf. A000010, A000203.

%K nonn

%O 1,2

%A _Robert G. Wilson v_

%E Offset corrected by _Donovan Johnson_, Jan 18 2012