%I #20 Sep 08 2022 08:46:18

%S 6,21,30,42,66,78,102,105,110,114,138,155,174,186,210,222,231,246,253,

%T 258,273,282,310,318,330,354,357,366,390,399,402,426,438,462,465,474,

%U 483,498,506,510,534,546,570,582,602,606,609,618,642,651,654,678,690,714,759,762,770,777,786,798,822

%N Squarefree numbers k such that 2^k - 1 is divisible by a square > 1.

%C Intersection of A049094 and A005117. - _Michel Marcus_, Dec 31 2016

%H Amiram Eldar, <a href="/A280296/b280296.txt">Table of n, a(n) for n = 1..89</a>

%e 6 is in this sequence because 2^6 - 1 = 63 is divisible by 9 = 3^2.

%o (Magma) [n: n in [1..200] | IsSquarefree(n) and not IsSquarefree(2^n-1)];

%Y Cf. A005117, A049094, A237043, A280203, A280208, A280209.

%K nonn

%O 1,1

%A _Juri-Stepan Gerasimov_, Dec 31 2016

%E a(38)=498 inserted by _Amiram Eldar_, Oct 23 2019