<s>
In	O
mathematics	O
,	O
a	O
vexillary	B-Algorithm
permutation	I-Algorithm
is	O
a	O
permutation	B-Algorithm
μ	O
of	O
the	O
positive	O
integers	O
containing	O
no	O
subpermutation	B-Algorithm
isomorphic	O
to	O
the	O
permutation	B-Algorithm
(	O
2143	O
)	O
;	O
in	O
other	O
words	O
,	O
there	O
do	O
not	O
exist	O
four	O
numbers	O
i	O
<	O
j	O
<	O
k	O
<	O
l	O
with	O
μ(j )	O
<	O
μ(i )	O
<	O
μ(l )	O
<	O
μ(k )	O
.	O
</s>
<s>
The	O
word	O
"	O
vexillary	O
"	O
means	O
flag-like	O
,	O
and	O
comes	O
from	O
the	O
fact	O
that	O
vexillary	B-Algorithm
permutations	I-Algorithm
are	O
related	O
to	O
flags	O
of	O
modules	O
.	O
</s>
<s>
showed	O
that	O
vexillary	B-Algorithm
involutions	I-Algorithm
are	O
enumerated	O
by	O
Motzkin	O
numbers	O
.	O
</s>
