

A093346


Array read by antidiagonals: T(r,n) = number of twostack sortable rpermutations.


0



1, 1, 2, 1, 3, 6, 1, 4, 15, 22, 1, 5, 28, 95, 91, 1, 6, 45, 252, 690, 408, 1, 7, 66, 525, 2618, 5481, 1938, 1, 8, 91, 946, 7095, 29848, 46376, 9614, 1, 9, 120, 1547, 15741, 105417, 363216, 411255, 49335, 1, 10, 153, 2360, 30576, 288288, 1673535, 4638348, 3781635, 260130
(list;
table;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

1,3


LINKS

Table of n, a(n) for n=1..55.
D. Xu, Generalizations of twostacksortable 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[rn+1, n], {r, 1, 10}, {n, 1, r}] // Flatten (* JeanFrançois Alcover, Feb 18 2019 *)


PROG

(PARI) T(r, n)=2*(r+1)*((2*r+1)*n)!/(n!*(2*r*n+2)!)


CROSSREFS

Cf. A000139.
Sequence in context: A213941 A181511 A115196 * A115597 A325007 A103371
Adjacent sequences: A093343 A093344 A093345 * A093347 A093348 A093349


KEYWORD

nonn,tabl


AUTHOR

Ralf Stephan, Apr 26 2004


STATUS

approved



