OFFSET
1,2
LINKS
V. A. Liskovets, Enumerative formulas for unrooted planar maps: a pattern, Electron. J. Combin., 11:1 (2004), R88.
J. Malenfant, On the Matrix-Element Expansion of a Circulant Determinant, arXiv preprint arXiv:1502.06012 [math.NT], 2015.
FORMULA
a(n) ~ 2^(2*n-1) / sqrt(Pi*n). - Vaclav Kotesovec, Jun 08 2019
From Richard L. Ollerton, May 07 2021: (Start)
a(n) = Sum_{k=1..n} C(2*gcd(n,k),gcd(n,k))/2.
a(n) = Sum_{k=1..n} A088218(gcd(n,k)). (End)
EXAMPLE
G.f. = x + 4*x^2 + 12*x^3 + 40*x^4 + 130*x^5 + 480*x^6 + 1722*x^7 + ...
MATHEMATICA
Table[Fold[ #1+EulerPhi[n/#2]*Binomial[2#2, #2]/2&, 0, Divisors[n]], {n, 1, 32}]
a[ n_] := If[ n < 0, 0, Sum[ Binomial[2 d, d] EulerPhi[n / d], {d, Divisors @ n}] / 2]; (* Michael Somos, Nov 01 2014 *)
PROG
(PARI) {a(n) = if( n<1, 0, sumdiv(n, d, binomial(2*d, d) * eulerphi(n/d)) / 2)}; /* Michael Somos, Nov 01 2014 */
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Wouter Meeussen, Apr 12 2003
STATUS
approved