|
Search: id:A145844
|
|
|
| A145844 |
|
Number of permutations of length 2n which are invariant under the reverse-complement map and have no decreasing subsequences of length 5. |
|
+0
1
|
|
| 1, 2, 8, 46, 332, 2784, 25888, 259382, 2749244, 30449416, 349379648
(list; graph; listen)
|
|
|
OFFSET
|
0,2
|
|
|
FORMULA
|
a(n) = sum(j=0, n, A000108(j)*A000108(n-j)*C(n, j)^2 ) where
A000108(n)=Catalan(n)=(2n)!/(n!(n+1)!) and C(n, j)=n!/(k!(n-j)!)
|
|
EXAMPLE
|
a(4) = 1*1*14 + 16*1*5 + 36*2*2 + 16*5*1 + 1*14*1 = 332
|
|
MATHEMATICA
|
Table[Sum[ Binomial[n, j]^2*Binomial[2*j, j]* Binomial[2*(n - j), n - j]/((n - j + 1)*(j + 1)), {j, 0, n}], {n, 0, 20}]
|
|
CROSSREFS
|
Sequence in context: A119501 A006664 A141117 this_sequence A005840 A161881 A088791
Adjacent sequences: A145841 A145842 A145843 this_sequence A145845 A145846 A145847
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
Eric Egge (eegge(AT)carleton.edu), Oct 21 2008
|
|
|
Search completed in 0.002 seconds
|
