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A080832 Expansion of e.g.f. exp(x) * (sec(exp(x) - 1))^2. 6
1, 1, 3, 13, 67, 421, 3115, 26349, 250867, 2655541, 30929019, 393019837, 5410699075, 80221867909, 1274393162827, 21594697199757, 388796268801427, 7411769447027413, 149143210226032923, 3159088788867736669 (list; graph; refs; listen; history; text; internal format)
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

Cf. A000182, A219613.

Sequence in context: A295226 A028418 A180191 * A194019 A020017 A060014

Adjacent sequences:  A080829 A080830 A080831 * A080833 A080834 A080835

KEYWORD

easy,nonn

AUTHOR

Emanuele Munarini, Mar 28 2003

STATUS

approved

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Last modified April 20 16:17 EDT 2019. Contains 322310 sequences. (Running on oeis4.)