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 A115599 #27 Jan 17 2017 11:36:59
%S A115599 2,9,50,289,1682,9801,57122,332929,1940450,11309769,65918162,
%T A115599 384199201,2239277042,13051463049,76069501250,443365544449,
%U A115599 2584123765442,15061377048201,87784138523762,511643454094369,2982076586042450,17380816062160329,101302819786919522
%N A115599 Consider all Pythagorean triples (X,X+1,Z) ordered by increasing Z; sequence gives Z-X values.
%C A115599 Old name was A001653(n) - A001652(n).
%H A115599 L. J. Gerstein, <a href="http://www.jstor.org/stable/30044157">Pythagorean triples and inner products</a>, Math. Mag., 78 (2005), 205-213.
%H A115599 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (7,-7,1).
%F A115599 a(n) = (2+(3-2*sqrt(2))^n+(3+2*sqrt(2))^n)/4.
%F A115599 a(n) = 7*a(n-1)-7*a(n-2)+a(n-3).
%F A115599 G.f.: -x*(x^2-5*x+2) / ((x-1)*(x^2-6*x+1)).
%t A115599 CoefficientList[Series[(x^2 - 5 x + 2)/((1 - x) (x^2 - 6 x + 1)), {x, 0, 33}], x] (* _Vincenzo Librandi_, Nov 26 2015 *)
%t A115599 LinearRecurrence[{7,-7,1},{2,9,50},30] (* _Harvey P. Dale_, Jan 17 2017 *)
%o A115599 (PARI) Vec(-x*(x^2-5*x+2)/((x-1)*(x^2-6*x+1)) + O(x^100)) \\ _Altug Alkan_, Nov 26 2015
%Y A115599 Identical to A055997 without leading term.
%K A115599 nonn,easy
%O A115599 1,1
%A A115599 _N. J. A. Sloane_, Mar 14 2006
%E A115599 Corrected and edited by _Colin Barker_, Jul 31 2013