OFFSET
1,3
LINKS
D. Xu, Generalizations of two-stack-sortable permutations, PhD thesis, arXiv:math/0209313 [math.CO], 2002.
FORMULA
T(r,n) = 2(r+1) * ((2r+1)n)!/[n!*(2rn+2)! ].
MATHEMATICA
T[r_, n_] := 2(r+1) * ((2r+1)n)!/(n!*(2r n+2)!); Table[T[r-n+1, n], {r, 1, 10}, {n, 1, r}] // Flatten (* Jean-François Alcover, Feb 18 2019 *)
PROG
(PARI) T(r, n)=2*(r+1)*((2*r+1)*n)!/(n!*(2*r*n+2)!)
CROSSREFS
KEYWORD
nonn,tabl
AUTHOR
Ralf Stephan, Apr 26 2004
STATUS
approved