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A109468
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a(n) is the number of permutations of (1,2,3,...,n) written in binary such that no adjacent elements share a common 1-bit.
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0
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1, 2, 0, 4, 2, 0, 0, 0, 8, 32, 0, 8, 0, 0, 0, 0, 0, 64, 0, 1968, 508, 0, 0, 0, 16
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| In other words, if b(m) and b(m+1) are adjacent elements written in binary, then (b(m) AND b(m+1)) = 0 for 1 <= m <= n-1. (If a logical AND is applied to each pair of adjacent terms, the result is zero.)
Let 2^k be the largest power of 2 <= n. Note that element 2^k-1 can be adjacent only to 2^k. So 2^k-1 must be at the beginning or the end of the permutation while 2^k must be next to 2^k-1. The elements 2^k-1-2^i (i=1,...,k-1) can be adjacent only to 2^i, 2^k and 2^k+2^i implying that n must be >=2^k+2^(k-3) to yield a nonzero number of permutations.
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CROSSREFS
| Sequence in context: A168036 A177256 A199891 * A185879 A081880 A144289
Adjacent sequences: A109465 A109466 A109467 * A109469 A109470 A109471
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KEYWORD
| nonn
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), based on a suggestion from Leroy Quet, Aug 21 2005
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EXTENSIONS
| More terms from Max Alekseyev (maxale(AT)gmail.com), Aug 28 2005
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