OFFSET
1,2
LINKS
S. Fishel, E. Milicevic, R. Patrias, and B. E. Tenner, Enumerations relating braid and commutation classes, arXiv:1708.04372 [math.CO], 2017.
FORMULA
a(1) = 1, a(2) = 2, and a(n) = C(n) + (n^3 - 3n^2 + 8n - 21)/3 for n > 2, where C(n) is the n-th Catalan number.
EXAMPLE
a(3) = 6 because all six permutations in S_3 have this property.
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Bridget Tenner, Aug 15 2017
STATUS
approved