Given a sequence
then given any strictly increasing map
σ
:
N
1
→
N
1
we say that
(
a
σ
(
n
)
)
is a subsequence.
Y
and a function
σ
:
X
→
X
which is
order preserving. Then we notate
(
a
σ
(
n
)
)
then the
x
-th term of
(
a
σ
(
n
)
)
is-->
−
><
!
−
−
a
σ
(
x
)
−
−
><
!
−
−
-->