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 A257711 #19 Apr 28 2020 07:48:42
%S A257711 210,3486,51681,883785,13125126,224476266,3333728685,57016086141,
%T A257711 846753959226,14481861401910,215072171913081,3678335779997361,
%U A257711 54627484911961710,934282806257926146,13875166095466359621,237304154453733242085,3524237560763543380386
%N A257711 Triangular numbers (A000217) that are the sum of seven consecutive triangular numbers.
%H A257711 Colin Barker, <a href="/A257711/b257711.txt">Table of n, a(n) for n = 1..830</a>
%H A257711 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,254,-254,-1,1).
%F A257711 G.f.: -21*x*(x^4-245*x^2+156*x+10) / ((x-1)*(x^2-16*x+1)*(x^2+16*x+1)).
%F A257711 16*a(n) = 104 +225*A157456(n+1) +7*(-1)^n*A159678(n+1). - _R. J. Mathar_, Apr 28 2020
%e A257711 210 is in the sequence because T(20) = 210 = 10+15+21+28+36+45+55 = T(4)+ ... +T(10).
%t A257711 LinearRecurrence[{1, 254, -254, -1, 1}, {210, 3486, 51681, 883785, 13125126}, 30] (* _Vincenzo Librandi_, Jun 27 2015 *)
%o A257711 (PARI) Vec(-21*x*(x^4-245*x^2+156*x+10) / ((x-1)*(x^2-16*x+1)*(x^2+16*x+1)) + O(x^100))
%Y A257711 Cf. A000217, A001110, A129803, A131557, A257712, A257713, A259413, A259414, A259415.
%K A257711 nonn,easy
%O A257711 1,1
%A A257711 _Colin Barker_, May 05 2015