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 A270889 #20 Jan 23 2019 12:31:51
%S A270889 3,6,27,150,867,5046,29403,171366,998787,5821350,33929307,197754486,
%T A270889 1152597603,6717831126,39154389147,228208503750,1330096633347,
%U A270889 7752371296326,45184131144603,263352415571286,1534930362283107,8946229758127350,52142448186480987,303908459360758566,1771308307978070403
%N A270889 Integers n such that the circular graph C_n has a square size deficiency.
%C A270889 Define the size deficiency of a graph G as the number of edges needed to complete G. If G is a cycle graph C_n, this sequence gives the values of n for which C_n has a size deficiency which is a perfect square.
%H A270889 Colin Barker, <a href="/A270889/b270889.txt">Table of n, a(n) for n = 0..1000</a>
%H A270889 J. R. M. Antalan and I. F. Callano, <a href="https://www.researchgate.net/publication/297871520">On the Size Deficiency of Cycle Graphs and Some Integer Sequences</a>, Asian Journal of Mathematics and Computer Research, 11 (3) (2016),192-200.
%H A270889 Mathematics Stack Exchange, <a href="http://math.stackexchange.com/questions/1439519/solving-the-nonlinear-diophantine-equation-x^2-3x-2y2">Solving the nonlinear Diophantine equation x2-3x=2y^2</a>
%H A270889 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (7,-7,1).
%F A270889 a(n+2) = 6*a(n+1) - a(n) - 6; a(0) = 3 , a(1) = 6.
%F A270889 G.f.: 3*(1-5*x+2*x^2)/((1-x)*(1-6*x+x^2)). - _Joerg Arndt_, Mar 25 2016
%F A270889 a(n) = 3 * A055997(n+1). - _Joerg Arndt_, Mar 25 2016
%F A270889 a(n) = 7*a(n-1)-7*a(n-2)+a(n-3) for n>2. - _Colin Barker_, Apr 03 2016
%F A270889 a(n) = 3*(2+(3-2*sqrt(2))^n+(3+2*sqrt(2))^n)/4. - _Colin Barker_, Apr 03 2016
%t A270889 a[0] = 3; a[1] = 6; a[n_] := a[n] = 6 a[n - 1] - a[n - 2] - 6; Table[a@ n, {n, 0, 24}] (* _Michael De Vlieger_, Mar 25 2016 *)
%t A270889 LinearRecurrence[{7,-7,1},{3,6,27},30] (* _Harvey P. Dale_, Jan 23 2019 *)
%o A270889 (PARI) is(n)=issquare(n*(n-3)/2) \\ _Charles R Greathouse IV_, Mar 25 2016
%o A270889 (PARI) a(n)=([0,1,0;0,0,1;1,-7,7]^n*[3;6;27])[1,1] \\ _Charles R Greathouse IV_, Mar 25 2016
%o A270889 (PARI) Vec(3*(1-5*x+2*x^2)/((1-x)*(1-6*x+x^2)) + O(x^50)) \\ _Colin Barker_, Apr 03 2016
%K A270889 nonn,easy
%O A270889 0,1
%A A270889 _John Rafael M. Antalan_, Mar 25 2016