OFFSET
0,3
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
Andrew Howroyd, Table of n, a(n) for n = 0..500
V. A. Liskovets and T. R. Walsh, Counting unrooted maps on the plane, Advances in Applied Math., 36, No.4 (2006), 364-387.
FORMULA
For n > 0, a(n) = (1/(2n))*(2^n*binomial(2n, n)/(n+1) + Sum_{0<k<n, k|n} phi(n/k)*2^k*binomial(2k, k)) where phi is the Euler function A000010.
MATHEMATICA
a[n_] := (1/(2n)) (2^n Binomial[2n, n]/(n+1) + Sum[Boole[0<k<n] EulerPhi[ n/k] 2^k Binomial[2k, k], {k, Divisors[n]}]);
Array[a, 20] (* Jean-François Alcover, Aug 28 2019 *)
PROG
(PARI) a(n)={if(n==0, 1, sumdiv(n, d, if(d<n, 1, 1/(n+1)) * eulerphi(n/d) * 2^d * binomial(2*d, d))/(2*n))} \\ Andrew Howroyd, Mar 29 2021
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Valery A. Liskovets, Mar 17 2005
EXTENSIONS
a(0)=1 prepended and terms a(21) and beyond from Andrew Howroyd, Mar 29 2021
STATUS
approved