OFFSET
1,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..246
Guo-Niu Han, Enumeration of Standard Puzzles, 2011. [Cached copy]
Guo-Niu Han, Enumeration of Standard Puzzles, arXiv:2006.14070 [math.CO], 2020.
D. E. Knuth, Whirlpool Permutations, May 05 2020
M. La Croix, A combinatorial proof of a result of Gessel and Greene, Discr. Math., 306 (2006), 2251-2256.
Jiaxi Lu and Yuanzhe Ding, A skeleton model to enumerate standard puzzle sequences, arXiv:2106.09471 [math.CO], 2021.
FORMULA
a(n) = A113583(n)/2^(n-1).
MAPLE
egf:= (x->1/(1-x*tanh(x))-1)(z/sqrt(2)):
a:= n-> (2*n)!/n*coeff(series(egf, z, 2*n+1), z, 2*n):
seq(a(n), n=1..18); # Alois P. Heinz, Oct 05 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Sep 21 2006
STATUS
approved