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 A253879 #9 Mar 04 2016 08:31:26 %S A253879 1,9,136,2160,34417,548505,8741656,139317984,2220346081,35386219305, %T A253879 563959162792,8987960385360,143243407002961,2282906551662009, %U A253879 36383261419589176,579849276161764800,9241205157168647617,147279433238536597065,2347229726659416905416 %N A253879 Indices of centered heptagonal numbers (A069099) which are also triangular numbers (A000217). %C A253879 Also positive integers y in the solutions to x^2 - 7*y^2 + x + 7*y - 2 = 0, the corresponding values of x being A253878. %H A253879 Colin Barker, <a href="/A253879/b253879.txt">Table of n, a(n) for n = 1..832</a> %H A253879 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (17,-17,1). %F A253879 a(n) = 17*a(n-1)-17*a(n-2)+a(n-3). %F A253879 G.f.: x*(8*x-1) / ((x-1)*(x^2-16*x+1)). %F A253879 a(n) = (14-(8-3*sqrt(7))^n*(7+3*sqrt(7))+(-7+3*sqrt(7))*(8+3*sqrt(7))^n)/28. - _Colin Barker_, Mar 04 2016 %e A253879 9 is in the sequence because the 9th centered heptagonal number is 253, which is also the 22nd triangular number. %o A253879 (PARI) Vec(x*(8*x-1)/((x-1)*(x^2-16*x+1)) + O(x^100)) %Y A253879 Cf. A000217, A069099, A253878, A253880. %K A253879 nonn,easy %O A253879 1,2 %A A253879 _Colin Barker_, Jan 17 2015