Length 0 string is the empty string. The empty string is a subsequence of all sequences. Hence, substrings are subsequences but the converse may not be true. The interval contains all indices between i and j including i and j. Concatenation of two strings s and t is denoted by st and is given by placing characters of string t in sequence aft er those of s.
Sequence alignment is a generic problem in bioinformatics research.
Let us consider the following examples. We would like to locate the positions where the differences occur. We have two sequences over the same alphabet with a few hundred characters each. We want to know whether there is a prefix of one which is similar to a suffix of the other. If so, we should obtain an alignment of the appropriate prefix-suffix combination.