Define string, substring, empty string, and empty language over alphabet {a,b}.

A string (also called a word) is a finite sequence of symbols taken from an alphabet. Over the alphabet Σ = {a, b}, a string is any finite arrangement of the symbols a and b. Examples over {a, b}: - a - b - ab - aab - bba - abab Formally, if Σ = {a, b}, then Σ (Kleene closure) gives the set of al...