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 A280296 #20 Sep 08 2022 08:46:18 %S A280296 6,21,30,42,66,78,102,105,110,114,138,155,174,186,210,222,231,246,253, %T A280296 258,273,282,310,318,330,354,357,366,390,399,402,426,438,462,465,474, %U A280296 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 A280296 Squarefree numbers k such that 2^k - 1 is divisible by a square > 1. %C A280296 Intersection of A049094 and A005117. - _Michel Marcus_, Dec 31 2016 %H A280296 Amiram Eldar, <a href="/A280296/b280296.txt">Table of n, a(n) for n = 1..89</a> %e A280296 6 is in this sequence because 2^6 - 1 = 63 is divisible by 9 = 3^2. %o A280296 (Magma) [n: n in [1..200] | IsSquarefree(n) and not IsSquarefree(2^n-1)]; %Y A280296 Cf. A005117, A049094, A237043, A280203, A280208, A280209. %K A280296 nonn %O A280296 1,1 %A A280296 _Juri-Stepan Gerasimov_, Dec 31 2016 %E A280296 a(38)=498 inserted by _Amiram Eldar_, Oct 23 2019