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 A115228 #21 Oct 19 2017 03:15:00 %S A115228 4,12,24,40,49,60,76,84,112,121,144,148,162,171,175,180,184,212,220, %T A115228 256,264,292,312,328,364,387,400,412,416,420,423,436,472,480,490,508, %U A115228 512,544,580,612,616,625,637,652,684,688,712,722,724,760,796,808,812 %N A115228 Nonsquarefree numbers n such that 2n+1 is also nonsquarefree (A013929). %C A115228 For any distinct primes p, q with q odd, contains all n such that n == 0 (mod p^2) and n == -1/2 (mod q^2). - _Robert Israel_, Oct 21 2016 %H A115228 Robert Israel, <a href="/A115228/b115228.txt">Table of n, a(n) for n = 1..10000</a> %F A115228 a(n) ~ n/(1 - 14/Pi^2 + 3*k/2 ) as n -> infinity, where k is the Feller-Tornier constant (A065474). - _Robert Israel_, Oct 21 2016 %e A115228 24 is in the sequence because 2^2 divides 24 and 7^2 divides 24*2 + 1. %p A115228 select(n -> not numtheory:-issqrfree(n) and not numtheory:-issqrfree(2*n+1), [$1..2000]); # _Robert Israel_, Oct 21 2016 %t A115228 fQ[n_] := ! SquareFreeQ[n] && ! SquareFreeQ[2 n + 1]; Select[Range[1000], fQ] (* _Robert G. Wilson v_, Oct 21 2016 *) %o A115228 (PARI) isok(n) = !issquarefree(n) && ! issquarefree(2*n+1); \\ _Michel Marcus_, Oct 22 2016 %Y A115228 Cf. A013939, A065474, A115170. %K A115228 easy,nonn %O A115228 1,1 %A A115228 _Don Reble_, Mar 05 2006 %E A115228 Corrected by _Zak Seidov_, Oct 21 2016