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 A090375 Number of unrooted Eulerian maps with bicolored faces which are self-isomorphic under reversing the colors. 1
 1, 1, 2, 4, 8, 17, 40, 93, 224, 538, 1344, 3352, 8448, 21573, 54912, 143037, 366080, 968083, 2489344, 6664856, 17199104, 46515759, 120393728, 328382874, 852017152, 2340706462, 6085836800, 16822999572, 43818024960, 121777594508, 317680680960, 887053276477 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS M. Deryagina, On the enumeration of hypermaps which are self-equivalent with respect to reversing the colors of vertices, Preprint 2016. V. Liskovets, Some easily derivable integer sequences, J. Integer Seq., v.3 (2000), Article 00.2.2. V. A. Liskovets, Enumerative formulas for unrooted planar maps: a pattern, Electron. J. Combin., 11:1 (2004), R88. FORMULA a(n) = 2*A069727(n) - A090371(n). a(2k+1) = 2^k*Catalan(k) = A052701(k+1). MATHEMATICA A069727[n_] := (1/(2n)) (3*2^(n - 1) Binomial[2 n, n]/((n + 1) (n + 2)) + Sum[EulerPhi[n/k] d[n/k] 2^(k - 2) Binomial[2 k, k], {k, Most[Divisors[n]]}]) + q[n]; A069727[0] = 1; q[n_?EvenQ] := 2^((n - 4)/2) Binomial[n, n/2]/(n + 2); q[n_?OddQ] := 2^((n - 1)/2) Binomial[(n - 1), (n - 1)/2]/(n + 1); d[n_] := 4 - Mod[n, 2]; h0[n_] := 3*2^(n - 1) Binomial[2n, n]/((n + 1)(n + 2)); A090371[n_] := (h0[n] + DivisorSum[n, If[# > 1, EulerPhi[#]*Binomial[n/# + 2, 2] h0[n/#], 0] &])/n; a[n_] := 2 A069727[n] - A090371[n]; Array[a, 32] (* Jean-François Alcover, Aug 28 2019 *) PROG (PARI) h0(n) = 3*2^(n-1)*binomial(2*n, n)/((n+1)*(n+2)); a090371(n) = (h0(n) + sumdiv(n, d, (d>1)*eulerphi(d)*binomial(n/d+2, 2)*h0(n/d)))/n; d(n) = if (n%2, 3, 4); q(n) = if (n%2, 2^((n-1)/2)*binomial(n-1, (n-1)/2)/(n+1), 2^((n-4)/2)*binomial(n, n/2)/(n+2)); a069727(n) = if (n==0, 1, q(n) + (3*2^(n-1)*binomial(2*n, n)/((n+1)*(n+2)) + sumdiv(n, k, (k!=n)*eulerphi(n/k)*d(n/k)*2^(k-2)*binomial(2*k, k)))/(2*n)); a(n) = 2*a069727(n) - a090371(n); \\ Michel Marcus, Dec 11 2014 CROSSREFS Cf. A052701, A069727, A090371. Sequence in context: A287776 A137856 A304970 * A210540 A332398 A307555 Adjacent sequences:  A090372 A090373 A090374 * A090376 A090377 A090378 KEYWORD easy,nonn AUTHOR Valery A. Liskovets, Dec 01 2003 EXTENSIONS More terms from Michel Marcus, Dec 11 2014 STATUS approved

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Last modified August 13 19:30 EDT 2020. Contains 336451 sequences. (Running on oeis4.)