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 A003584 #14 Aug 03 2017 01:56:28
%S A003584 1,2,9,52,336,2304,16368,118976,878592,6562816,49447424,375072768,
%T A003584 2860343296,21909012480,168425533440,1298753372160,10041201131520,
%U A003584 77809145610240,604138825973760,4698956908462080,36604934482821120
%N A003584 Unicursal (i.e., possessing an Eulerian path) planar rooted maps with n edges.
%C A003584 Unicursal (in a broad sense) means that no more than two vertices are of odd valency (that is maps possessing an Eulerian path or cycle).
%H A003584 V. A. Liskovets and T. R. S. Walsh, <a href="http://dx.doi.org/10.1016/j.disc.2003.09.015">Enumeration of Eulerian and unicursal planar maps</a>, Discr. Math., 282 (2004), 209-221.
%F A003584 a(n) = A000257(n) + A069720(n).
%t A003584 a[n_] := 2^(n-1)*(3*Binomial[2*n, n]/((n+1)*(n+2))+Binomial[2*n-1, n]); a[0]=1; Table[a[n], {n, 0, 20}] (* _Jean-François Alcover_, Dec 11 2014 *)
%Y A003584 Cf. A000257, A005470, A069720.
%K A003584 nonn,nice
%O A003584 0,2
%A A003584 _Valery A. Liskovets_
%E A003584 More terms from _Valery A. Liskovets_, Apr 07 2002