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%I #26 Sep 26 2023 02:15:21
%S 1,2,5,7,10,11,13,14,17,19,22,23,26,29,31,34,35,37,38,41,43,46,47,53,
%T 55,58,59,61,62,65,67,70,71,73,74,77,79,82,83,85,86,89,91,94,95,97,
%U 101,103,106,107,109,110,113,115,118,119,122,127,130,131
%N Numbers m such that 3*m is squarefree.
%C These are the numbers from A005117 that are not divisible by 3. See the Maple program by _Robert Israel_. - _Wolfdieter Lang_, Aug 21 2015
%C Squarefree numbers divisible by 3: 3, 6, 15, 21, 30, 33, 39, 42, 51, 57, 66, 69, 78, 87, 93, 102, ...
%F a(n) ~ 2*Pi^2*n/9. - _Charles R Greathouse IV_, Aug 07 2015
%F Sum_{n>=1} 1/a(n)^s = (3^s)*zeta(s)/((1+3^s)*zeta(2*s)), s>1. - _Amiram Eldar_, Sep 26 2023
%e 10 is in this sequence because 3*10 = 30 is squarefree.
%p select(numtheory:-issqrfree, [seq(seq(3*i+j,j=1..2),i=0..1000)]); # _Robert Israel_, Aug 07 2015
%t Select[Range[0, 200], SquareFreeQ[3 #] &] (* _Vincenzo Librandi_, Aug 08 2015 *)
%o (Magma) [n: n in [1..200] | IsSquarefree(3*n)];
%o (PARI) is(n)=n%3 && issquarefree(n) \\ _Charles R Greathouse IV_, Aug 07 2015
%Y Cf. A045344, A088245.
%Y Numbers m such that k*m is squarefree: A005117 (k = 1), A056911 (k = 2), this sequence (k = 3), A274546 (k = 5), A276378 (k = 6).
%K nonn,easy
%O 1,2
%A _Juri-Stepan Gerasimov_, Aug 07 2015
%E Corrected and extended by _Vincenzo Librandi_, Aug 08 2015
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