OFFSET
1,1
REFERENCES
V. A. Liskovets and T. R. Walsh, Enumeration of unrooted maps on the plane, Rapport technique, UQAM, No. 2005-01, Montreal, Canada, 2005.
LINKS
V. A. Liskovets and T. R. Walsh, Counting unrooted maps on the plane, Advances in Applied Math., 36, No.4 (2006), 364-387.
FORMULA
a(n) = (n+2)*A000139(n)/2.
From Peter Bala, Feb 07 2022: (Start)
a(n) = (3*n+6)*(3*n-4)*(3*n-5)/(n*(2*n-1)*(2*n+2))*a(n-1).
O.g.f.: A(x) = x*T(x)*(4 - T(x)), where T(x) = 1 + x*T(x)^3 is the g.f. of A001764. (End)
MATHEMATICA
a[n_] := (n+2)(3(n-1))!/((2n-1)! n!);
Array[a, 30] (* Jean-François Alcover, Aug 29 2019 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Valery A. Liskovets, Mar 17 2005
EXTENSIONS
More terms from Jean-François Alcover, Aug 29 2019
STATUS
approved