|
Search: id:A133221
|
|
| |
|
| 1, 1, 1, 1, 3, 3, 15, 15, 105, 105, 945, 945, 10395, 10395, 135135, 135135, 2027025, 2027025, 34459425, 34459425, 654729075, 654729075, 13749310575, 13749310575, 316234143225, 316234143225, 7905853580625, 7905853580625, 213458046676875, 213458046676875
(list; graph; listen)
|
|
|
OFFSET
|
0,5
|
|
|
COMMENT
|
Normally such sequences are excluded from the OEIS, but I have made an exception for this one because so many variants of it have occurred in recent submissions.
For n>=2, a(n) = product of odd positive integers <=(n-1). [From Jaroslav Krizek (jaroslav.krizek(AT)atlas.cz), Mar 21 2009]
Contribution from Karl-Dieter Crisman (karl.crisman(AT)gordon.edu), Nov 03 2009: (Start)
a(n) is, for n>=3, the number of way to choose floor((n-1)/2) disjoint
pairs of items from n-1 items. It is then a fortiori the size of the
conjugacy class of the reversal permutation [n-1,n-2,n-3,...,3,2,1]=
(1 n-1)(2 n-2) (3 n-3)... in the symmetric group on n-1 elements. (End)
|
|
CROSSREFS
|
Contribution from Johannes W. Meijer (meijgia(AT)hotmail.com), Jun 18 2009: (Start)
Appears in A161736.
(End)
Sequence in context: A100735 A129356 A055634 this_sequence A110096 A157526 A153512
Adjacent sequences: A133218 A133219 A133220 this_sequence A133222 A133223 A133224
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
N. J. A. Sloane (njas(AT)research.att.com), Oct 13 2007
|
|
|
Search completed in 0.002 seconds
|
