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A137782 a(n) = the number of permutations (p(1),p(2),...,p(n)) of (1,2,...,n) where, for each k (2<=k<=n), the sign of (p(k) - p(k-1)) equals the sign of (p(n+2-k) - p(n+1-k)). +0
2
1, 2, 2, 12, 24, 200, 540, 6160 (list; graph; listen)
OFFSET

1,2

COMMENT

First 8 terms calculated by Olivier Gerard.

EXAMPLE

Consider the permutation (for n = 7):

3,6,7,5,1,2,4

The signs of the differences between adjacent terms forms the sequence: ++--++, which has

reflective symmetry. So this permutation, among others, is counted when n = 7.

CROSSREFS

Cf. A137783.

Adjacent sequences: A137779 A137780 A137781 this_sequence A137783 A137784 A137785

Sequence in context: A140431 A092900 A122007 this_sequence A131384 A052612 A130306

KEYWORD

more,nonn

AUTHOR

Leroy Quet (qq-quet(AT)mindspring.com), Feb 10 2008

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Last modified October 11 13:47 EDT 2008. Contains 144830 sequences.


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