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A234282
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Number of 321-avoiding extensions of comb K_{s,2}^{alpha}.
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0
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1, 3, 13, 67, 378, 2244, 13737, 85767, 542685, 3466515, 22298796, 144210388, 936575968
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OFFSET
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1,2
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COMMENTS
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No formula is presently known.
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)
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LINKS
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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