cp's OEIS Frontend

A349749 Odd numbers k for which the 3-adic valuation of sigma(k) is equal to the 3-adic valuation of k, where sigma is the sum of divisors function.

%I A349749 #17 Dec 04 2021 12:30:55
%S A349749 1,7,13,15,19,25,31,33,37,43,61,67,69,73,79,87,91,97,103,105,109,121,
%T A349749 123,127,133,139,141,147,151,153,157,163,175,177,181,193,195,199,211,
%U A349749 217,223,229,231,241,247,249,259,271,277,283,285,289,301,303,307,313,325,331,337,339,343,349,367,373,375,379,393,397
%N A349749 Odd numbers k for which the 3-adic valuation of sigma(k) is equal to the 3-adic valuation of k, where sigma is the sum of divisors function.
%C A349749 Odd numbers for which sigma (A000203) preserves the 3-adic valuation (A007949).
%H A349749 <a href="/index/O#opnseqs">Index entries for sequences where odd perfect numbers must occur, if they exist at all</a>
%H A349749 <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>
%t A349749 Select[Range[1, 400, 2], IntegerExponent[DivisorSigma[1, #], 3] == IntegerExponent[#, 3] &] (* _Amiram Eldar_, Dec 01 2021 *)
%o A349749 (PARI) isA349749(n) = ((n%2)&&valuation(sigma(n),3)==valuation(n,3));
%Y A349749 Cf. A000203, A007949, A329963, A349750, A349751.
%Y A349749 Cf. A349169, A349752, A349755 (subsequences).
%K A349749 nonn
%O A349749 1,2
%A A349749 _Antti Karttunen_, Nov 30 2021