OFFSET
1,2
COMMENTS
No formula is presently known.
From Colin Defant, Aug 16 2016: (Start)
For each subset W={w_1,w_2,...,w_k} of {n+1,n+2,...,2n} satisfying w_1<w_2<...<w_k, let H(W) be the number of sequences of integers i_1,i_2,...,i_k such that i_1<i_2<...<i_k and w_j-n+j<=i_j<=w_j-1 for all j. We have a(n)=Sum(H(W)), where the sum ranges over all subsets W of {n+1,n+2,...,2n}.
3 + sqrt(8) <= lim_(n->oo)a(n)^(1/n) <= 27/4. (End)
LINKS
C. Defant, Some Poset Pattern-Avoidance Problems Posed by Yakoubov, arXiv:1608.03951 [math.CO], 2016.
S. Yakoubov, Pattern Avoidance in Extensions of Comb-Like Posets, arXiv preprint arXiv:1310.2979 [math.CO], 2013.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
N. J. A. Sloane, Dec 28 2013
EXTENSIONS
a(7)-a(13) from Colin Defant, Aug 16 2016
STATUS
approved