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 A287405 #34 Mar 15 2018 12:58:36
%S A287405 3,35,931,27587,830403,25054435,756187747,22824258947,688917131651,
%T A287405 20793986742179,627637106311971,18944339609269571,571808137046942019,
%U A287405 17259221092289630307,520945214725090792931,15723995613526902256387,474606601742375424297731
%N A287405 a(n) = (7*(cot(1*Pi/7))^2)^n + (7*(cot(2*Pi/7))^2)^n + (7*(cot(4*Pi/7))^2)^n.
%C A287405 a(n) = x1^n + x2^n + x3^n, where x1, x2, x3 are the roots of x^3 - 35*x^2 + 147*x - 49, x1 = 7*(cot(1*Pi/7))^2, x2 = 7*(cot(2*Pi/7))^2, x3 = 7*(cot(4*Pi/7))^2.
%H A287405 Colin Barker, <a href="/A287405/b287405.txt">Table of n, a(n) for n = 0..650</a>
%H A287405 Roman Witula, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL12/Witula/witula17.html">Ramanujan Type Trigonometric Formulas: The General Form for the Argument 2Pi/7</a>, J. Integer Seq., 12 (2009), Article 09.8.5.
%H A287405 Roman Witula and Damian Slota, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL10/Slota/witula13.html">New Ramanujan-Type Formulas and Quasi-Fibonacci Numbers of Order 7</a>, Journal of Integer Sequences, Vol. 10 (2007), Article 07.5.6.
%H A287405 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (35,-147,49).
%F A287405 a(n) = 35*a(n-1) - 147*a(n-2) + 49*a(n-3), a(0) = 3, a(1) = 35, a(2) = 931.
%F A287405 Bisection of A215575: a(n) = A215575(2*n).
%F A287405 G.f.: (3 - 7*x)*(1 - 21*x) / (1 - 35*x + 147*x^2 - 49*x^3). - _Colin Barker_, May 26 2017
%t A287405 LinearRecurrence[{35,-147,49},{3,35,931},30] (* _Harvey P. Dale_, Mar 15 2018 *)
%o A287405 (PARI) Vec((3 - 7*x)*(1 - 21*x) / (1 - 35*x + 147*x^2 - 49*x^3) + O(x^30)) \\ _Colin Barker_, May 26 2017
%o A287405 (PARI) polsym(x^3 - 35*x^2 + 147*x - 49, 20) \\ _Joerg Arndt_, May 26 2017
%Y A287405 Cf. A108716, A033304, A094648, A274220, A215076, A274075 A274032, A275195, A215575, A274975.
%K A287405 nonn,easy
%O A287405 0,1
%A A287405 _Kai Wang_, May 24 2017