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 and a(n) = C(n) + n - 2 for n > 1, where C(n) is the n-th Catalan number.
EXAMPLE
a(3) = 6 because all six permutations in S_3 have this property.
MATHEMATICA
Join[{1}, Table[CatalanNumber[n] + n - 2, {n, 2, 30}]] (* Vincenzo Librandi, Aug 16 2017 *)
PROG
(Magma) [1] cat [Catalan(n) + n - 2: n in [2..40]]; // Vincenzo Librandi, Aug 16 2017
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Bridget Tenner, Aug 15 2017
STATUS
approved