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 A158560 #30 Mar 09 2023 04:19:24
%S A158560 29,119,269,479,749,1079,1469,1919,2429,2999,3629,4319,5069,5879,6749,
%T A158560 7679,8669,9719,10829,11999,13229,14519,15869,17279,18749,20279,21869,
%U A158560 23519,25229,26999,28829,30719,32669,34679,36749,38879,41069,43319,45629,47999,50429
%N A158560 a(n) = 30*n^2 - 1.
%C A158560 The identity (30*n^2-1)^2 - (225*n^2-15) * (2*n)^2 = 1 can be written as a(n)^2 - A158559(n) * A005843(n)^2 = 1.
%H A158560 Vincenzo Librandi, <a href="/A158560/b158560.txt">Table of n, a(n) for n = 1..10000</a>
%H A158560 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%F A158560 G.f.: x*(-29 - 32*x + x^2)/(x - 1)^3.
%F A158560 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3).
%F A158560 From _Amiram Eldar_, Mar 09 2023: (Start)
%F A158560 Sum_{n>=1} 1/a(n) = (1 - cot(Pi/sqrt(30))*Pi/sqrt(30))/2.
%F A158560 Sum_{n>=1} (-1)^(n+1)/a(n) = (cosec(Pi/sqrt(30))*Pi/sqrt(30) - 1)/2. (End)
%t A158560 LinearRecurrence[{3, -3, 1}, {29, 119, 269}, 50] (* _Vincenzo Librandi_, Feb 14 2012 *)
%t A158560 30*Range[50]^2-1 (* _Harvey P. Dale_, May 30 2018 *)
%o A158560 (Magma) I:=[29, 119, 269]; [n le 3 select I[n] else 3*Self(n-1)-3*Self(n-2)+1*Self(n-3): n in [1..50]]; // _Vincenzo Librandi_, Feb 14 2012
%o A158560 (PARI) for(n=1, 40, print1(30*n^2-1", ")); \\ _Vincenzo Librandi_, Feb 14 2012
%Y A158560 Cf. A005843, A158559.
%K A158560 nonn,easy
%O A158560 1,1
%A A158560 _Vincenzo Librandi_, Mar 21 2009
%E A158560 Comment rewritten by _R. J. Mathar_, Oct 16 2009