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 A006420 M5012 #23 Dec 29 2024 03:40:52
%S A006420 1,16,150,1104,7077,41504,228810,1205520,6135690,30391520,147277676,
%T A006420 700990752,3286733805,15215673408,69675615234,316058238864,
%U A006420 1421891923038,6350464644960,28179908990772,124327908683616,545691921346146,2383936774151616,10370479696102500
%N A006420 Number of rooted planar maps with 3 vertices and n faces and no isthmuses.
%D A006420 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A006420 Andrew Howroyd, <a href="/A006420/b006420.txt">Table of n, a(n) for n = 2..500</a>
%H A006420 T. R. S. Walsh and 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 A006420 G.f.: x^2*(1 + 2*g - 4*g^2)/((1 - g)^4*(1 - 2*g)^5) where g/x is the g.f. of A000108.
%o A006420 (PARI) seq(n)={my(g=x*(1-sqrt(1-4*x + O(x^n)))/(2*x)); Vec((1 + 2*g - 4*g^2)/((1 - g)^4*(1 - 2*g)^5))} \\ _Andrew Howroyd_, Apr 06 2021
%Y A006420 A diagonal of A342981.
%K A006420 nonn
%O A006420 2,2
%A A006420 _N. J. A. Sloane_
%E A006420 a(14) and a(15) from _Sean A. Irvine_, Apr 05 2017
%E A006420 Terms a(16) and beyond from _Andrew Howroyd_, Apr 02 2021