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 A374135 #9 Jul 03 2024 20:06:52 %S A374135 1,4,7,13,16,19,25,28,31,37,43,52,61,64,67,73,76,79,91,97,100,103,109, %T A374135 112,121,124,127,133,139,148,151,157,163,172,175,181,193,199,208,211, %U A374135 217,223,229,241,244,247,256,259,268,271,277,283,289,292,301,304,307,313,316,325,331,337,343,349,364,367,373,379 %N A374135 Nonmultiples of 3 whose sum of divisors is also a nonmultiple of 3. %C A374135 Numbers k such that Ramanujan tau(k), A000594(k), is not a multiple of 3. %C A374135 Not a multiplicative semigroup. For example, although 7 is included, 49 = 7*7 is not. %H A374135 Antti Karttunen, <a href="/A374135/b374135.txt">Table of n, a(n) for n = 1..20000</a> %H A374135 <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a> %t A374135 Select[Range[380],Mod[#,3]!=0&&Mod[DivisorSigma[1,#],3]!=0 &] (* _Stefano Spezia_, Jul 03 2024 *) %o A374135 (PARI) %o A374135 A070563(n) = ((n%3) && (sigma(n)%3)); %o A374135 isA374135 = A070563; %Y A374135 Intersection of A001651 and A329963. %Y A374135 Cf. A000203, A000594, A070563 (characteristic function). %K A374135 nonn %O A374135 1,2 %A A374135 _Antti Karttunen_, Jul 03 2024