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 A248091 #14 Jun 13 2015 00:55:16
%S A248091 0,0,0,1,0,1,1,3,6,16,39,101,259,670,1732,4485,11613,30079,77910,
%T A248091 201812,522763,1354153,3507775,9086502,23537592,60971593,157940361,
%U A248091 409127579,1059801190,2745301576,7111410031,18421346973
%N A248091 Expansion of x^3*(1-2x-x^2-x^3+x^4+x^5)/((1+x)*(1-3x+x^2-x^3+3x^4)).
%C A248091 a(n) = number of nonisomorphic n-element posets P such that if 1<=i<=n-1, then P has exactly 3 order ideals of cardinality i.
%D A248091 R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 1, 2nd ed., 2012, Exercise 3.35c, p. 359.
%H A248091 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (2,2,0,-2,-3).
%p A248091 g := x^3*(x^5+x^4-x^3-x^2-2*x+1)/((x+1)*(3*x^4-x^3+x^2-3*x+1)): gser := series(g, x = 0, 45): seq(coeff(gser, x, n), n = 0 .. 40);
%t A248091 CoefficientList[Series[x^3*(1 - 2 x - x^2 - x^3 + x^4 + x^5)/((1 + x) (1 - 3 x + x^2 - x^3 + 3 x^4)), {x, 0, 30}], x] (* _Wesley Ivan Hurt_, Oct 28 2014 *)
%K A248091 nonn
%O A248091 0,8
%A A248091 _Emeric Deutsch_, Oct 28 2014