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 A280070 #12 Dec 25 2016 11:51:37 %S A280070 1,11,191,3421,61381,1101431,19764371,354657241,6364065961, %T A280070 114198530051,2049209474951,36771572019061,659839086868141, %U A280070 11840331991607471,212466136762066331,3812550129725586481,68413436198298490321,1227629301439647239291,22028913989715351816911 %N A280070 Indices of 10-gonal numbers (A001107) that are also centered 10-gonal numbers (A062786). %C A280070 Also positive integers x in the solutions to 4*x^2 - 5*y^2 - 3*x + 5*y - 1 = 0, the corresponding values of y being A133273. %H A280070 Colin Barker, <a href="/A280070/b280070.txt">Table of n, a(n) for n = 1..750</a> %H A280070 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (19,-19,1). %F A280070 a(n) = (6 + (5+2*sqrt(5))*(9+4*sqrt(5))^(-n) + (5-2*sqrt(5))*(9+4*sqrt(5))^n)/16. %F A280070 a(n) = 19*a(n-1) - 19*a(n-2) + a(n-3) for n>3. %F A280070 G.f.: x*(1 - 8*x + x^2) / ((1 - x)*(1 - 18*x + x^2)). %e A280070 11 is in the sequence because the 11th 10-gonal number is 451, which is also the 10th centered 10-gonal number. %o A280070 (PARI) Vec(x*(1 - 8*x + x^2) / ((1 - x)*(1 - 18*x + x^2)) + O(x^30)) %Y A280070 Cf. A001107, A062786, A128922, A133273. %K A280070 nonn,easy %O A280070 1,2 %A A280070 _Colin Barker_, Dec 25 2016