2. Sequence

    는 Definition을 나타내고,      는  Theorem을 나타낸다.

ordered pair

we call (x, y) on ordered pair.

(x1, x2)=(y1, y2) if and only if x1=y1, and x2=y2.

cartesian product

X x Y = { (x, y) | x∈X and y∈Y} 

sequence

A sequence is an ordered list.

S or {Sn} is a sequence named S, the entire sequence.

Sn is single element of the sequence S at index n.

sub-sequence

A sub-sequence {Snk} is a sequence that consist of certain element of {Sn} retained in the original order in S.

string

A string over a set X is a finite sequence of elements from X.