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 A137880 #19 Jun 07 2021 04:42:30
%S A137880 1,49,225,23409,108241,11282881,52171729,5438325025,25146664929,
%T A137880 2621261378961,12120640323841,1263442546333969,5842123489426225,
%U A137880 608976686071593889,2815891401263116401,293525499243961920321,1357253813285332678849,141478681658903574000625
%N A137880 Indices k of perfect squares among 17-gonal numbers A051869(k) = k*(15*k - 13)/2.
%C A137880 Corresponding perfect squares are listed in A137878.
%C A137880 Note that all a(n) are perfect squares themselves, their square roots are listed in A137881.
%H A137880 Matthew House, <a href="/A137880/b137880.txt">Table of n, a(n) for n = 1..746</a>
%H A137880 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,482,-482,-1,1).
%F A137880 A051869( a(n) ) = A137878(n); a(n) = A137881(n)^2.
%F A137880 From _Max Alekseyev_, Oct 19 2008: (Start)
%F A137880 a(n) = 482*a(n-2) - a(n-4) - 208.
%F A137880 a(2n) = ( (15 - sqrt(30))/30 * (11 + 2*sqrt(30))^n + (15 + sqrt(30))/30 * (11 - 2*sqrt(30))^n )^2.
%F A137880 a(2n+1) = ( (15 + sqrt(30))/30 * (11 + 2*sqrt(30))^n + (15 - sqrt(30))/30 * (11 - 2*sqrt(30))^n )^2. (End)
%F A137880 a(n) = a(n-1) + 482*a(n-2) - 482*a(n-3) - a(n-4) + a(n-5). - _Matthew House_, Jun 18 2016
%F A137880 G.f.: x*(1 + 48*x - 306*x^2 + 48*x^3 + x^4) / ((1-x)*(1 - 22*x + x^2)*(1 + 22*x + x^2)). - _Colin Barker_, Jun 18 2016
%t A137880 Rest@ CoefficientList[Series[x (1 + 48 x - 306 x^2 + 48 x^3 + x^4)/((1 - x) (1 - 22 x + x^2) (1 + 22 x + x^2)), {x, 0, 18}], x] (* _Michael De Vlieger_, Jun 18 2016 *)
%o A137880 (PARI) Vec(x*(1+48*x-306*x^2+48*x^3+x^4)/((1-x)*(1-22*x+x^2)*(1+22*x+x^2)) + O(x^20)) \\ _Colin Barker_, Jun 18 2016
%Y A137880 Cf. A051869 (17-gonal numbers), A137878 (17-gonal numbers that are perfect squares), A137879, A137881.
%K A137880 nonn,easy
%O A137880 1,2
%A A137880 _Alexander Adamchuk_, Feb 19 2008
%E A137880 Edited and extended by _Max Alekseyev_, Oct 19 2008