A374135 Nonmultiples of 3 whose sum of divisors is also a nonmultiple of 3.

1, 4, 7, 13, 16, 19, 25, 28, 31, 37, 43, 52, 61, 64, 67, 73, 76, 79, 91, 97, 100, 103, 109, 112, 121, 124, 127, 133, 139, 148, 151, 157, 163, 172, 175, 181, 193, 199, 208, 211, 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

Offset: 1

Author

Antti Karttunen, Jul 03 2024

Keywords

• nonn

Comments

Numbers k such that Ramanujan tau(k), A000594(k), is not a multiple of 3.

Not a multiplicative semigroup. For example, although 7 is included, 49 = 7*7 is not.

Links

• Antti Karttunen, Table of n, a(n) for n = 1..20000
• Index entries for sequences related to sigma(n)

Crossrefs

Intersection of A001651 and A329963.

Cf. A000203, A000594, A070563 (characteristic function).

Programs

Mathematica

Select[Range[380],Mod[#,3]!=0&&Mod[DivisorSigma[1,#],3]!=0 &] (* Stefano Spezia, Jul 03 2024 *)

PARI

A070563(n) = ((n%3) && (sigma(n)%3));
isA374135 = A070563;