|
Search: id:A137782
|
|
|
| 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
|
| |
|
|
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
|
|
|
Search completed in 0.002 seconds
|
