OFFSET
0,3
COMMENTS
Take the smallest element from each block of the set partitions of {1,2,...,n+1} into an odd number of blocks. Form a "zag" permutation a[1],a[2],...,a[k] such that a[1] < a[2] > a[3] < ... > a[k]. a(n) is the number of ways to order the blocks in accordance with such "zag" permutations. - Geoffrey Critzer, Nov 23 2012
LINKS
Muniru A Asiru, Table of n, a(n) for n = 0..100
P. Flajolet and R. Sedgewick, Analytic Combinatorics, 2009; see page 144
FORMULA
E.g.f.: exp(x) / (cos(exp(x) - 1))^2.
The sequence 0, 1, 1, 3, ... has e.g.f. tan(exp(x)-1). It has general term sum{k=0..n, S2(n, k) A009006(k)} for n>1 (S2(n, k) Stirling numbers of second kind). - Paul Barry, Apr 20 2005
a(n) ~ 2*n * n! / ((2+Pi) * (log(1+Pi/2))^(n+2)). - Vaclav Kotesovec, Jul 28 2018
MAPLE
seq(coeff(series(factorial(n)*exp(x)*(sec(exp(x)-1))^2, x, n+1), x, n), n=0..25); # Muniru A Asiru, Jul 28 2018
MATHEMATICA
nn=21; t=Sum[n^(n-1)x^n/n!, {n, 1, nn}]; Drop[Range[0, nn]!CoefficientList[ Series[Tan[Exp[x]-1], {x, 0, nn}], x], 1] (* Geoffrey Critzer, Nov 23 2012 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Emanuele Munarini, Mar 28 2003
STATUS
approved