|
|
A361271
|
|
Number of 1342-avoiding odd Grassmannian permutations of size n.
|
|
3
|
|
|
0, 0, 1, 2, 6, 9, 19, 25, 44, 54, 85, 100, 146, 167, 231, 259, 344, 380, 489, 534, 670, 725, 891, 957, 1156, 1234, 1469, 1560, 1834, 1939, 2255, 2375, 2736, 2872, 3281, 3434, 3894, 4065, 4579, 4769, 5340, 5550, 6181, 6412, 7106, 7359, 8119, 8395, 9224, 9524, 10425
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
0,4
|
|
COMMENTS
|
A permutation is said to be Grassmannian if it has at most one descent. A permutation is odd if it has an odd number of inversions.
a(n) is also the number of 3124-avoiding odd Grassmannian permutations of size n.
|
|
LINKS
|
|
|
FORMULA
|
G.f.: x^2*(x^4+x^2+x+1)/((1+x)^3*(1-x)^4).
|
|
EXAMPLE
|
For n=4 the a(4)=6 permutations are 1243, 1324, 2134, 2341, 2413, 4123.
|
|
PROG
|
(PARI) seq(n) = Vec(x^2*(x^4+x^2+x+1)/((1+x)^3*(1-x)^4) + O(x*x^n), -n-1) \\ Andrew Howroyd, Mar 07 2023
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn,easy
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|