Subsequences of String

#1

Are subsequences substring ?

#2

Subsequence is what we call subset in mathematics. So, if the string has n characters, it has 2^n(including NULL) subsets(subsequences).

Substring is when we maintain the continuity of characters of original string.
Example:
String = “abc”
subsequence={‘a’,‘b’,‘c’,‘ab’,‘ac’,‘bc’,‘abc’,’’}; \ last element is NULL
substring = {a,b,c,ab,bc,abc}