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 A047009 #17 May 11 2020 05:42:44 %S A047009 1,2,4,14,60,286,1456,7752,42636,240350,1381380,8064030,47682960, %T A047009 284997384,1719031840,10450528048,63967345068,393895175646, %U A047009 2438398706380,15166284983850,94730949283740,593966352738990 %N A047009 T(2n,n), array T given by A047000. %C A047009 Also: number of nonseparable planar rooted maps with n edges. - _R. J. Mathar_, Aug 18 2006 %H A047009 T. R. S. Walsh, A. B. Lehman, <a href="http://dx.doi.org/10.1016/0095-8956(75)90050-7">Counting rooted maps by genus. III: Nonseparable maps</a>, J. Combinatorial Theory Ser. B 18 (1975), 222-259. %F A047009 a(n) = 2*A006013(n-1) = 2*(3n-2)!/(n!*(2*n-1)!). - _R. J. Mathar_, Aug 18 2006 %F A047009 G.f.: 1+2*g where g*(1-g)^2 = x. - _Mark van Hoeij_, Nov 10 2011 %K A047009 nonn %O A047009 0,2 %A A047009 _Clark Kimberling_