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 A259210 #9 Aug 16 2015 12:04:01 %S A259210 15,567645,21362755051,803965923024825,30256453525753512135, %T A259210 1138671371184241752666901,42852758352891300594112643235, %U A259210 1612720706714039835374593462842225,60693131033623416811596149786491655551,2284125291706662961573569665690233502167245 %N A259210 Positive hexagonal numbers (A000384) that are other hexagonal numbers divided by 3. %H A259210 Colin Barker, <a href="/A259210/b259210.txt">Table of n, a(n) for n = 1..219</a> %H A259210 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (37635,-37635,1). %F A259210 a(n) = 37635*a(n-1)-37635*a(n-2)+a(n-3). %F A259210 G.f.: -x*(x^2+3120*x+15) / ((x-1)*(x^2-37634*x+1)). %e A259210 15 is in the sequence because 15 is the 3rd hexagonal number, and 3*15 is the 5th hexagonal number. %o A259210 (PARI) Vec(-x*(x^2+3120*x+15)/((x-1)*(x^2-37634*x+1)) + O(x^20)) %Y A259210 Cf. A000384, A076139, A121338, A259211. %K A259210 nonn,easy %O A259210 1,1 %A A259210 _Colin Barker_, Jun 21 2015