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 A233089 #22 Dec 19 2014 12:04:44
%S A233089 8,128,228,648,3240,5976,13160,23760,23940,24840,32768,37224,78540,
%T A233089 82800,139248,166716,238368,278520,280368,288360,516528,633420,664668,
%U A233089 731808,734448,1145520,1211100,1377240,1425816,1484568,1627640,2055060,2131080,2292780
%N A233089 Numbers n such that 2n-1 and 2n+1 divide 2^n-1.
%C A233089 Intersection of A081856 and A081858.
%C A233089 Numbers n such that 4n^2-1 divides 2^n-1. - _Charles R Greathouse IV_, Dec 04 2013
%H A233089 Charles R Greathouse IV, <a href="/A233089/b233089.txt">Table of n, a(n) for n = 1..5000</a>
%e A233089 2^8-1=255 is divisible by 2*8-1=15 and by 2*8+1=17.
%t A233089 Select[Range[23*10^5],PowerMod[2,#,2#+{1,-1}]=={1,1}&] (* _Harvey P. Dale_, Dec 19 2014 *)
%o A233089 (PARI) isok(n) = !((2^n-1) % (2*n-1)) && !((2^n-1) % (2*n+1));
%o A233089 (PARI) is(n)=Mod(2,4*n^2-1)^n==1 \\ _Charles R Greathouse IV_, Dec 04 2013
%K A233089 nonn
%O A233089 1,1
%A A233089 _Michel Marcus_, Dec 04 2013