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 A253446 #15 Oct 04 2023 15:21:10
%S A253446 1,16,465,13920,417121,12499696,374573745,11224712640,336366805441,
%T A253446 10079779450576,302057016711825,9051630721904160,271246864640412961,
%U A253446 8128354308490484656,243579382390074126705,7299253117393733316480,218734014139421925367681
%N A253446 Indices of centered heptagonal numbers (A069099) which are also centered octagonal numbers (A016754).
%C A253446 Also positive integers x in the solutions to 7*x^2 - 8*y^2 - 7*x + 8*y = 0, the corresponding values of y being A253447.
%H A253446 Colin Barker, <a href="/A253446/b253446.txt">Table of n, a(n) for n = 1..678</a>
%H A253446 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (31,-31,1).
%F A253446 a(n) = 31*a(n-1)-31*a(n-2)+a(n-3).
%F A253446 G.f.: x*(15*x-1) / ((x-1)*(x^2-30*x+1)).
%F A253446 a(n) = sqrt((-2-(15-4*sqrt(14))^n-(15+4*sqrt(14))^n)*(2-(15-4*sqrt(14))^(1+n)-(15+4*sqrt(14))^(1+n)))/(4*sqrt(7)). - _Gerry Martens_, Jun 04 2015
%e A253446 16 is in the sequence because the 16th centered heptagonal number is 841, which is also the 15th centered octagonal number.
%t A253446 LinearRecurrence[{31,-31,1},{1,16,465},20] (* _Harvey P. Dale_, Oct 04 2023 *)
%o A253446 (PARI) Vec(x*(15*x-1)/((x-1)*(x^2-30*x+1)) + O(x^100))
%Y A253446 Cf. A016754, A069099, A253447, A253514.
%K A253446 nonn,easy
%O A253446 1,2
%A A253446 _Colin Barker_, Jan 01 2015