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 A158554 #28 Mar 09 2023 04:18:28
%S A158554 27,111,251,447,699,1007,1371,1791,2267,2799,3387,4031,4731,5487,6299,
%T A158554 7167,8091,9071,10107,11199,12347,13551,14811,16127,17499,18927,20411,
%U A158554 21951,23547,25199,26907,28671,30491,32367,34299,36287,38331,40431,42587,44799,47067
%N A158554 a(n) = 28*n^2 - 1.
%C A158554 The identity (28*n^2 - 1)^2 - (196*n^2 - 14)*(2*n)^2 = 1 can be written as a(n)^2 - A158553(n)*A005843(n)^2 = 1.
%H A158554 Vincenzo Librandi, <a href="/A158554/b158554.txt">Table of n, a(n) for n = 1..10000</a>
%H A158554 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%F A158554 G.f.: x*(-27 - 30*x + x^2)/(x-1)^3.
%F A158554 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3).
%F A158554 From _Amiram Eldar_, Mar 09 2023: (Start)
%F A158554 Sum_{n>=1} 1/a(n) = (1 - cot(Pi/sqrt(28))*Pi/sqrt(28))/2.
%F A158554 Sum_{n>=1} (-1)^(n+1)/a(n) = (cosec(Pi/sqrt(28))*Pi/sqrt(28) - 1)/2. (End)
%t A158554 28Range[50]^2-1 (* _Harvey P. Dale_, Mar 17 2011 *)
%t A158554 LinearRecurrence[{3, -3, 1}, {27, 111, 251}, 40] (* _Vincenzo Librandi_, Feb 14 2012 *)
%o A158554 (Magma) I:=[27, 111, 251]; [n le 3 select I[n] else 3*Self(n-1)-3*Self(n-2)+Self(n-3): n in [1..50]]; // _Vincenzo Librandi_, Feb 14 2012
%o A158554 (PARI) for(n=1, 40, print1(28*n^2-1", ")); \\ _Vincenzo Librandi_, Feb 14 2012
%Y A158554 Cf. A005843, A158553.
%K A158554 nonn,easy
%O A158554 1,1
%A A158554 _Vincenzo Librandi_, Mar 21 2009
%E A158554 Comment rewritten by _R. J. Mathar_, Oct 16 2009