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 A253459 #10 Jun 13 2015 00:55:21 %S A253459 1,21,148,5208,37465,1322685,9515836,335956656,2416984753,85331667813, %T A253459 613904611300,21673907667720,155929354285321,5505087215932941, %U A253459 39605442083860108,1398270478939299168,10059626359946181985,355155196563366055605,2555105489984246363956 %N A253459 Indices of centered square numbers (A001844) which are also centered heptagonal numbers (A069099). %C A253459 Also positive integers x in the solutions to 4*x^2 - 7*y^2 - 4*x + 7*y = 0, the corresponding values of y being A253460. %H A253459 Colin Barker, <a href="/A253459/b253459.txt">Table of n, a(n) for n = 1..832</a> %H A253459 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,254,-254,-1,1). %F A253459 a(n) = a(n-1)+254*a(n-2)-254*a(n-3)-a(n-4)+a(n-5). %F A253459 G.f.: x*(20*x^3+127*x^2-20*x-1) / ((x-1)*(x^2-16*x+1)*(x^2+16*x+1)). %e A253459 21 is in the sequence because the 21st centered square number is 841, which is also the 16th centered heptagonal number. %o A253459 (PARI) Vec(x*(20*x^3+127*x^2-20*x-1)/((x-1)*(x^2-16*x+1)*(x^2+16*x+1)) + O(x^100)) %Y A253459 Cf. A001844, A069099, A253460, A253599. %K A253459 nonn,easy %O A253459 1,2 %A A253459 _Colin Barker_, Jan 01 2015