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 A380238 #8 Jan 21 2025 21:29:47
%S A380238 1,2,5,12,28,68,157,372,845,1949,4367,9880,21858,48679,106612,234546,
%T A380238 509246,1109352,2391299,5167423,11070598,23762557,50641725,108085708,
%U A380238 229303142,487039228,1029167119,2176808877,4583856878,9660020146,20279242545,42599286814
%N A380238 Number of achiral planar maps with n vertices and 2 faces.
%C A380238 Also by duality the number of achiral planar maps with n faces and 2 vertices.
%C A380238 The number of edges is n.
%H A380238 Andrew Howroyd, <a href="/A380238/b380238.txt">Table of n, a(n) for n = 1..1000</a>
%o A380238 (PARI)
%o A380238 G1(n)={my(g=(1-sqrt(1-4*x^2 + O(x*x^n)))/(2*x^2)); ((1 + x/(1-x-x^2*g)^2)^2/(1 - x^2*g^2) - 1)/2}
%o A380238 G2(n)={my(c(d)=(1-sqrt(1-4*x^d + O(x*x^(n+d))))/(2*x^d)); sum(k=1, n, my(m=1+k%2); -log(1 - x^k*c(m*k)^(2/m))*eulerphi(k)/k, O(x*x^n))}
%o A380238 seq(n)={Vec(G1(n)+G2(n))/2}
%Y A380238 Column 2 of A379431.
%Y A380238 Cf. A380237 (sensed), A380239 (unsensed).
%K A380238 nonn
%O A380238 1,2
%A A380238 _Andrew Howroyd_, Jan 19 2025