OFFSET
0,2
COMMENTS
Number of {12,2*1}-avoiding signed permutations in the hyperoctahedral group B_n.
LINKS
D. Daly and L. Pudwell, Pattern avoidance in rook monoids, 2013. - From N. J. A. Sloane, Feb 03 2013
T. Mansour and J. West, Avoiding 2-letter signed patterns, arXiv:math/0207204 [math.CO], 2002.
FORMULA
E.g.f.: (exp(1)*(Ei(1, 1-x)-Ei(1, 1))+1)/(1-x). a(n) = n!*(1+Sum(A000522(i-1)/i!, i =1..n)). - Vladeta Jovovic, Apr 27 2004
Conjecture: a(n) -2*n*a(n-1) +(n^2-2)*a(n-2) -(n-2)^2*a(n-3)=0. - R. J. Mathar, May 30 2014
MATHEMATICA
a[n_] := n! (1+Sum[1/i Sum[1/j!, {j, 0, i-1}], {i, 1, n}])
Table[a[n], {n, 0, 19}] (* Jean-François Alcover, Oct 05 2018 *)
PROG
(PARI) a(n)=n!+n!*sum(i=1, n, 1/i*sum(j=0, i-1, 1/j!))
CROSSREFS
KEYWORD
nonn
AUTHOR
Ralf Stephan, Apr 26 2004
STATUS
approved