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A211309
a(n) = number |fdw(P,(n))| of entangled P-words with s=2.
0
1, 4, 60, 1776, 84720, 5876640, 556466400, 68882446080
OFFSET
1,2
COMMENTS
See Jenca and Sarkoci for the precise definition.
LINKS
Gejza Jenca and Peter Sarkoci, Linear extensions and order-preserving poset partitions, arXiv preprint arXiv:1112.5782, 2011
FORMULA
From Peter Bala, Sep 05 2012: (Start)
Conjectural e.g.f.: 2 - 1/A(x), where A(x) = sum {n = 0..inf} (2*n)!/2^n*x^n/n! is the e.g.f. for A000680 (also the o.g.f. for A001147).
If true, this gives a(n) = n!*A000698(n) and leads to the recurrence equation: a(n) = (2*n)!/2^n - sum {k = 1..n-1} (2*k)!/2^k*binomial(n,k)*a(n-k) with a(1) = 1.
(End)
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Apr 08 2012
STATUS
approved