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 A254285 #6 Jun 13 2015 00:55:23 %S A254285 1,1891,26335,71156485,991081981,2677903145191,37298379237211, %T A254285 100780206894952201,1403687203222107385,3792762303606727977835, %U A254285 52826364168762410080471,142736816433155393822880781,1988067387723517337746328821,5371757345852607787523567324911 %N A254285 Hexagonal numbers (A000384) which are also centered triangular numbers (A005448). %H A254285 Colin Barker, <a href="/A254285/b254285.txt">Table of n, a(n) for n = 1..437</a> %H A254285 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,37634,-37634,-1,1). %F A254285 a(n) = a(n-1)+37634*a(n-2)-37634*a(n-3)-a(n-4)+a(n-5). %F A254285 G.f.: -x*(x^4+1890*x^3-13190*x^2+1890*x+1) / ((x-1)*(x^2-194*x+1)*(x^2+194*x+1)). %e A254285 1891 is in the sequence because it is the 31st hexagonal number and the 36th centered triangular number. %o A254285 (PARI) Vec(-x*(x^4+1890*x^3-13190*x^2+1890*x+1)/((x-1)*(x^2-194*x+1)*(x^2+194*x+1)) + O(x^100)) %Y A254285 Cf. A000384, A005448, A254283, A254284. %K A254285 nonn,easy %O A254285 1,2 %A A254285 _Colin Barker_, Jan 28 2015