OFFSET
0,6
COMMENTS
A permutation is Grassmann if it has at most one descent. A closed-form formula was proved by J. B. Gil and J. A. Tomasko.
LINKS
Juan B. Gil and Jessica A. Tomasko, Restricted Grassmannian permutations, arXiv:2112.03338 [math.CO], 2021.
Juan B. Gil and Jessica A. Tomasko, Restricted Grassmannian permutations, Enum. Combin. Appl. 2 (2022), no. 4, Article #S4PP6.
Index entries for linear recurrences with constant coefficients, signature (2,1,-4,2).
FORMULA
EXAMPLE
For n=3, 123, 231, 312 are even Grassmann permutations, and 132, 213 are the odd ones. Hence a(3) = 1.
MATHEMATICA
Table[2^Floor[1 + (n - 1)/2] - n, {n, 1, 80}]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Per W. Alexandersson, Jul 28 2022
STATUS
approved