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A374135
Nonmultiples of 3 whose sum of divisors is also a nonmultiple of 3.
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
1,2
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.
MATHEMATICA
Select[Range[380], Mod[#, 3]!=0&&Mod[DivisorSigma[1, #], 3]!=0 &] (* Stefano Spezia, Jul 03 2024 *)
PROG
(PARI)
A070563(n) = ((n%3) && (sigma(n)%3));
isA374135 = A070563;
CROSSREFS
Intersection of A001651 and A329963.
Cf. A000203, A000594, A070563 (characteristic function).
Sequence in context: A191201 A084089 A202822 * A137827 A045090 A213382
KEYWORD
nonn
AUTHOR
Antti Karttunen, Jul 03 2024
STATUS
approved