OFFSET
1,2
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..1000
V. A. Liskovets and T. R. S. Walsh, Enumeration of Eulerian and unicursal planar maps, Discr. Math., 282 (2004), 209-221.
FORMULA
There is an easy formula.
a(n) ~ 8^(n-1) / (sqrt(Pi) * n^(3/2)). - Vaclav Kotesovec, Aug 28 2019
MATHEMATICA
a[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]]; Array[a, 21] (* Jean-François Alcover, Sep 18 2016 *)
CROSSREFS
KEYWORD
easy,nice,nonn
AUTHOR
Valery A. Liskovets, Apr 07 2002
STATUS
approved