|
|
A201168
|
|
The number of permutations avoiding the "boxed" pattern 123.
|
|
0
|
|
|
1, 1, 2, 5, 15, 51, 194, 810, 3675, 17935, 93481, 517129, 3021133
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
0,3
|
|
COMMENTS
|
The statement in the Avgustinovich, Kitaev and Valyuzhenich paper that a(6) is greater than 303 is easily seen to be wrong, since that would require (among other constraints) that no more than 2 boxed-123-avoiding permutations of length 5 end in an ascent. - Peter J. Taylor, Apr 27 2015
|
|
LINKS
|
|
|
MATHEMATICA
|
valid[l_] := valid[l] = Which[Length@l<3, True, Length@l==3, !Less@@l, True, valid[Most@l]&&valid[Rest@l]&&valid[DeleteCases[l, Min@l]]&&valid[DeleteCases[l, Max@l]]]; Length@Select[Permutations@Range@#, valid] & /@ Range[0, 9]
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn,more
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|