cp's OEIS Frontend

A348741 Odd numbers k for which A161942(k) < k, where A161942 is the odd part of sigma.

%I A348741 #19 Nov 30 2024 23:46:54
%S A348741 3,5,7,11,13,15,17,19,21,23,27,29,31,33,35,37,39,41,43,45,47,51,53,55,
%T A348741 57,59,61,63,65,67,69,71,73,75,77,79,83,85,87,89,91,93,95,97,99,101,
%U A348741 103,105,107,109,111,113,115,117,119,123,125,127,129,131,133,135,137,139,141,143,145,147,149,151,153,155,157
%N A348741 Odd numbers k for which A161942(k) < k, where A161942 is the odd part of sigma.
%C A348741 Differs from A088828 for the first time at n=1080, where a(1080) = 2207, while A088828(1080) = 2205 = A348743(1), the value which is missing from this sequence.
%H A348741 Antti Karttunen, <a href="/A348741/b348741.txt">Table of n, a(n) for n = 1..16291</a>
%H A348741 <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>
%t A348741 odd[n_] := n/2^IntegerExponent[n, 2]; Select[Range[1, 150, 2], odd[DivisorSigma[1, #]] < # &] (* _Amiram Eldar_, Nov 02 2021 *)
%o A348741 (PARI)
%o A348741 A000265(n) = (n >> valuation(n, 2));
%o A348741 isA348741(n) = ((n%2)&&A000265(sigma(n))<n);
%Y A348741 Setwise difference A088828 \ A348743.
%Y A348741 Cf. A161942, A348742 (complement among the odd numbers).
%Y A348741 Cf. also A348738, A348753, A348931.
%K A348741 nonn
%O A348741 1,1
%A A348741 _Antti Karttunen_, Nov 02 2021