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 A069724 #16 Aug 28 2019 10:00:50 %S A069724 1,2,9,38,214,1253,7925,51620,346307,2365886,16421359,115384738, %T A069724 819276830,5868540399,42357643916,307753571520,2249048959624, %U A069724 16520782751969,121915128678131,903391034923548,6719098772562182 %N A069724 Number of nonisomorphic unrooted unicursal planar maps with n edges (unicursal means that exactly two vertices are of odd valency; there is an Eulerian path). %H A069724 G. C. Greubel, <a href="/A069724/b069724.txt">Table of n, a(n) for n = 1..1000</a> %H A069724 V. A. Liskovets and T. R. S. Walsh, <a href="http://dx.doi.org/10.1016/j.disc.2003.09.015">Enumeration of Eulerian and unicursal planar maps</a>, Discr. Math., 282 (2004), 209-221. %F A069724 There is an easy formula. %F A069724 a(n) ~ 8^(n-1) / (sqrt(Pi) * n^(3/2)). - _Vaclav Kotesovec_, Aug 28 2019 %t A069724 a[n_] := 1/(2 n) DivisorSum[n, If[OddQ[n/#], EulerPhi[n/#] 2^(#-2) Binomial[2 #, #], 0]&] + If[OddQ[n], 2^((n-3)/2) Binomial[n-1, (n-1)/2], 2^((n-6)/2) Binomial[n, n/2]]; Array[a, 21] (* _Jean-François Alcover_, Sep 18 2016 *) %Y A069724 Cf. A069720, A069727, A005470. %K A069724 easy,nice,nonn %O A069724 1,2 %A A069724 _Valery A. Liskovets_, Apr 07 2002