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 A006421 M5211 #21 Dec 29 2024 03:40:55
%S A006421 1,30,449,4795,41850,319320,2213665,14283280,87169790,508887860,
%T A006421 2865204762,15654301865,83388235348,434685964540,2223970137825,
%U A006421 11194499812388,55546566721430,272142754971892,1318317357277470,6321681903231990,30037740651227756,141545610360126400
%N A006421 Number of rooted planar maps with 4 vertices and n faces and no isthmuses.
%D A006421 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A006421 Andrew Howroyd, <a href="/A006421/b006421.txt">Table of n, a(n) for n = 2..500</a>
%H A006421 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 A006421 G.f.: x^2*(1 + 9*g - 9*g^2 - 20*g^3 + 20*g^4)/((1 - g)^5*(1 - 2*g)^8) where g/x is the g.f. of A000108. - _Andrew Howroyd_, Apr 02 2021
%o A006421 (PARI) seq(n)={my(g=x*(1-sqrt(1-4*x + O(x^n)))/(2*x)); Vec((1 + 9*g - 9*g^2 - 20*g^3 + 20*g^4)/((1 - g)^5*(1 - 2*g)^8))} \\ _Andrew Howroyd_, Apr 02 2021
%Y A006421 A diagonal of A342981.
%K A006421 nonn
%O A006421 2,2
%A A006421 _N. J. A. Sloane_
%E A006421 a(13) and title improved by _Sean A. Irvine_, Apr 06 2017
%E A006421 Terms a(14) and beyond from _Andrew Howroyd_, Apr 02 2021