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 A387721 #14 Sep 07 2025 21:54:35
%S A387721 6,10,12,14,15,18,20,21,22,24,26,28,30,34,36,38,40,42,44,45,46,48,50,
%T A387721 52,54,56,58,60,62,66,68,70,72,74,75,76,78,80,82,84,86,88,90,92,94,96,
%U A387721 98,100,102,104,105,106,108,110,112,114,116,118,120,122,124,126,130,132,134,135,136,138,140,142,144,146,148
%N A387721 Numbers k for which A107758(k) > 2*k, where A107758 is sigma+, multiplicative function with a(p^e) = 1+sigma(p^e).
%C A387721 Note that in contrast to analogous sequences like A005101, A246282, A387711, here it is not guaranteed that multiples of any term are also terms. For example, 21 is a term, but 3*21 = 63 is in A052396. Also 15, 45, 75 are in this sequence, but their multiple 225 is in A387720.
%H A387721 Antti Karttunen, <a href="/A387721/b387721.txt">Table of n, a(n) for n = 1..10000</a>
%o A387721 (PARI)
%o A387721 A107758(n) = { my(f = factor(n)); prod(k=1, #f~, 1+sigma(f[k, 1]^f[k, 2])); };
%o A387721 is_A387721(n) = (A107758(n)>(2*n));
%Y A387721 Cf. A107758.
%Y A387721 Complement of (A052396 U A387720).
%Y A387721 Disjoint union of A387722 and A387723.
%Y A387721 Positions of positive terms in A387725.
%Y A387721 Cf. also A005101 (subsequence), A246282, A387711.
%K A387721 nonn,new
%O A387721 1,1
%A A387721 _Antti Karttunen_, Sep 06 2025