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 A112944 Number of unrooted regular odd-valent planar maps with 2 vertices; maps are considered up to orientation-preserving homeomorphisms and the vertices are of valency 2n+1. 4
 1, 2, 7, 39, 308, 3013, 33300, 394340, 4878109, 62232321, 812825244, 10818489817, 146250545528, 2003199281223, 27747288947266, 388087900316025, 5474206895126243, 77795972452841542, 1112947041203866164, 16016508647052018408, 231727628211887783830, 3368855109532696440867 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS M. Bousquet, G. Labelle and P. Leroux, Enumeration of planar two-face maps, Discrete Math., vol. 222 (2000), 1-25. Z. C. Gao, V. A. Liskovets and N. C. Wormald, Enumeration of unrooted odd-valent regular planar maps, Preprint, 2005. FORMULA a(n) = (1/2)binomial(2n, n) + (1/(4n+2))sum_{k|(2n+1)}phi(k)* binomial(2*floor(n/k), floor(n/k))^2, where phi(k) is the Euler function A000010. EXAMPLE There exist 2 planar maps with two 3-valent vertices: a map with three parallel edges and a map with one loop in each vertex and a link. Therefore a(1)=2. MATHEMATICA a[n_] := (1/2) Binomial[2n, n] + (1/(4n+2)) Sum[EulerPhi[k] Binomial[2 Floor[n/k], Floor[n/k]]^2, {k, Divisors[2n+1]}]; Table[a[n], {n, 0, 21}] (* Jean-François Alcover, Jul 24 2018 *) PROG (PARI) a(n) = binomial(2*n, n)/2 + sumdiv(2*n+1, k, eulerphi(k)* binomial(2*(n\k), (n\k))^2)/(4*n+2); \\ Michel Marcus, Oct 14 2015 CROSSREFS Cf. A005470, A112945, A113181, A113182. Sequence in context: A266310 A032118 A125660 * A060073 A322152 A336185 Adjacent sequences:  A112941 A112942 A112943 * A112945 A112946 A112947 KEYWORD nonn AUTHOR Valery A. Liskovets, Oct 10 2005 EXTENSIONS More terms from Michel Marcus, Oct 14 2015 STATUS approved

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Last modified April 17 09:13 EDT 2021. Contains 343064 sequences. (Running on oeis4.)