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 A105289 #12 Sep 05 2019 11:13:02
%S A105289 6,12,18,20,21,33,36,48,51,54,55,60,75,78,87,99,100,108,110,112,114,
%T A105289 120,129,132,133,144,147,153,154,155,159,162,165,174,177,180,182,183,
%U A105289 185,195,210,219,225,228,230,234,237,245,261,267,270,275,285,290,297,310
%N A105289 Indices of Lucas 5-step numbers A074048 which have a nontrivial divisor in common with index.
%C A105289 Extension by _T. D. Noe_. Wanted: closed-form formula for this as exists for Fibonacci and Lucas numbers. See also A105764 (indices of prime Lucas 5-step numbers).
%H A105289 Amiram Eldar, <a href="/A105289/b105289.txt">Table of n, a(n) for n = 1..10000</a>
%F A105289 gcd(a(n), A074048(a(n))) > 1.
%e A105289 gcd(6, A074048(6)) = gcd(6,57) = 3,
%e A105289 gcd(20, A074048(20)) = gcd(20,743775) = 5.
%e A105289 gcd(21, A074048(21)) = gcd(21,1462223) = 7.
%t A105289 m=300; s = LinearRecurrence[{1, 1, 1, 1, 1}, {5, 1, 3, 7, 15}, m+1]; Select[Range[m], !CoprimeQ[#, s[[#+1]]] &] (* _Amiram Eldar_, Sep 05 2019 *)
%Y A105289 Cf. A074048, A104714, A105764.
%K A105289 easy,nonn
%O A105289 1,1
%A A105289 _Jonathan Vos Post_, Apr 25 2005
%E A105289 More terms from _Amiram Eldar_, Sep 05 2019