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 A069725 Number of nonisomorphic unrooted unicursal planar maps with n edges and two vertices of valency 1 (unicursal means that exactly two vertices are of odd valency; there is an Eulerian path). 2
 1, 1, 3, 11, 62, 342, 2152, 13768, 91800, 622616, 4301792, 30100448, 213019072, 1521473984, 10954616064, 79420280064, 579300888960, 4248201302400, 31302536066560, 231638727063040 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS There is an easy formula. LINKS V. A. Liskovets and T. R. S. Walsh, Enumeration of Eulerian and unicursal planar maps, Discr. Math., 282 (2004), 209-221. MATHEMATICA a[1] = a[2] = 1; a[n_] := With[{m = Floor[(n+1)/2]}, 1/n 2^(n-3) Binomial[2n-2, n-1] + 2^(m-3) Binomial[2m-2, m-1]]; Array[a, 20] (* Jean-François Alcover, Aug 28 2019 *) CROSSREFS Cf. A069727, A003645, A069732. Sequence in context: A203007 A296321 A203768 * A291287 A193499 A261919 Adjacent sequences:  A069722 A069723 A069724 * A069726 A069727 A069728 KEYWORD easy,nonn AUTHOR Valery A. Liskovets, Apr 07 2002 STATUS approved

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Last modified April 7 04:20 EDT 2020. Contains 333292 sequences. (Running on oeis4.)