A069730 - OEIS

A069730

Number of nonisomorphic unrooted unicursal planar maps with n edges.

1, 2, 4, 13, 50, 248, 1407, 8600, 55154, 365292, 2473956, 17053468, 119191992, 842688120, 6015275094, 43292026736, 313788095994, 2288506113056, 16781638172458, 123656774440396, 915123392599456

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OFFSET

0,2

COMMENTS

Unicursal (in a broad sense) means that no more than two vertices are of odd valency (that is, maps possessing an Eulerian path).

LINKS

Table of n, a(n) for n=0..20.

V. A. Liskovets and T. R. S. Walsh, Enumeration of Eulerian and unicursal planar maps, Discr. Math., 282 (2004), 209-221.

FORMULA

a(n) = A069727(n) + A069724(n).

MATHEMATICA

A069724[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]];

A069727[n_] := (1/(2 n))*(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];

a[n_] := A069727[n] + A069724[n];

Table[a[n], {n, 0, 20}] (* Jean-François Alcover, Aug 28 2019 *)

CROSSREFS

Cf. A069727, A069724.

Sequence in context: A341378 A058134 A246012 * A072605 A330344 A161905

Adjacent sequences: A069727 A069728 A069729 * A069731 A069732 A069733

KEYWORD

easy,nonn

AUTHOR

Valery A. Liskovets, Apr 07 2002

STATUS

approved