|
Search: id:A145223
|
|
|
| A145223 |
|
a(n) is the number of odd permutations (of an n-set) with exactly 2 fixed points. |
|
+0
2
|
|
| 0, 0, 6, 0, 90, 420, 3780, 33264, 333900, 3670920, 44054010, 572697840, 8017775766
(list; graph; listen)
|
|
|
OFFSET
|
2,3
|
|
|
REFERENCES
|
Ali, Bashir and Umar, A., "Some combinatorial properties of the alternating group". Southeast Asian Bulletin Math. 32 (2008), 823-830.
|
|
FORMULA
|
a(n)=(n(n-1)/2)*A145221(n-2), (n > 1)
Egf.: ((x^4)e^(-x))/4(1-x)
|
|
EXAMPLE
|
a(4) = 6 because there are exactly 6 odd permutations (of a 4-set) having 2 fixed points, namely: (12), (13), (14), (23), (24), (34).
|
|
CROSSREFS
|
A145221
Sequence in context: A051767 A156488 A057399 this_sequence A072129 A085511 A005212
Adjacent sequences: A145220 A145221 A145222 this_sequence A145224 A145225 A145226
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
A. Umar (aumarh(AT)squ.edu.om), Oct 09 2008
|
|
|
Search completed in 0.002 seconds
|
