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 A113183 #14 Aug 24 2023 02:31:39
%S A113183 1,1,2,3,8,18,58,155,546,1592,5774,17798,65676,210362,785248,2588155,
%T A113183 9743348,32832290,124416022,426685544,1625465732,5654938190,
%U A113183 21636274202,76171463926,292498386900,1040120036300,4006388161846,14369121494126
%N A113183 Number of unrooted two-face maps in the plane (considered up to orientation-preserving homeomorphism) with the faces of equal degree n: planar maps with a distinguished outside face.
%H A113183 M. Bousquet, G. Labelle and P. Leroux, <a href="http://dx.doi.org/10.1016/S0012-365X(99)00406-9">Enumeration of planar two-face maps</a>, Discrete Math., vol. 222 (2000), 1-25.
%F A113183 a(n) = (1/n) Sum_{k|n} phi(k) C((n/k)-1,floor(n/(2k)))^2 where phi(k) is the Euler function A000010.
%e A113183 There exist 2 maps in the plane with two triangular faces: a triangle and a map consisting of a 2-path and a loop in its middle vertex that separates both ends. Therefore a(3) = 2.
%t A113183 a[n_] := DivisorSum[n, EulerPhi[#] * Binomial[n/# - 1, Floor[n/(2*#)]]^2 &] / n; Array[a, 30] (* _Amiram Eldar_, Aug 24 2023 *)
%o A113183 (PARI) a(n) = sumdiv(n, k, eulerphi(k)*binomial(n/k - 1, n\(2*k))^2)/n; \\ _Michel Marcus_, Oct 14 2015
%Y A113183 Cf. A000010, A060404, A113181.
%K A113183 nonn
%O A113183 1,3
%A A113183 _Valery A. Liskovets_, Oct 19 2005