This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A387720 #6 Sep 06 2025 21:40:04
%S A387720 1,3,5,7,9,11,13,17,19,23,25,27,29,31,33,35,37,39,41,43,47,49,51,53,
%T A387720 55,57,59,61,65,67,69,71,73,77,79,81,83,85,87,89,91,93,95,97,99,101,
%U A387720 103,107,109,111,113,115,117,119,121,123,125,127,129,131,133,137,139,141,143,145,147,149,151,153,155,157,159
%N A387720 Numbers k for which A107758(k) < 2*k, where A107758 is sigma+, multiplicative function with a(p^e) = 1+sigma(p^e).
%H A387720 Antti Karttunen, <a href="/A387720/b387720.txt">Table of n, a(n) for n = 1..10000</a>
%o A387720 (PARI)
%o A387720 A107758(n) = { my(f = factor(n)); prod(k=1, #f~, 1+sigma(f[k, 1]^f[k, 2])); };
%o A387720 is_A387720(n) = (A107758(n)<(2*n));
%Y A387720 Cf. A107758.
%Y A387720 Complement of (A052396 U A387721).
%Y A387720 Cf. also A005100, A246281, A387710.
%K A387720 nonn,new
%O A387720 1,2
%A A387720 _Antti Karttunen_, Sep 06 2025