**Define the following terms :**- Concatenation of strings
- Language
- Problems

__Let x and y be strings. Then xy denotes the concatenation of x and y, that is , the string formed by making a copy of x and following it by a copy of y. More precisely, if x is the string composed of i symbol ; x = a1 a2.....ai and y is the string composed of j symbols y = b1 b2..........bi, the xy is the string of length, i+j : xy = a1 a2.........ai b1 b2.........bi.__

**Concatenation of strings :**

**let x = 01101 and y = 110**

__Example :__
Then, xy = 01101110

And, yx = 11001101

__A set of string all of which are chosen from some summession symbol, where summession symbol is a particular alphabet, is called Language. If summession symbol is an alphabet, The L is a language over summession symbol. The set of palindromes over the alphabet {0, 1} is an infinite language is the set of all strings over a fixed alphabet summession symbol. We denoted this language by summession symbol.__

**Languages :****If summession symbol = {a} then summession symbol = {E, a, aa, aaa.......}**

__Example :__
L = {0, 1}, then L = {E, 0, 1, 00, 01, 10, 11, 000.......}.

__In automata theory a problem is the question of deciding whether a given string is a number of some particular languages. More precisely, if summession symbol is an alphabet, and L is a language over summession symbol, then the problem is given a string W in summession symbol, decide whether or not W is in L.__

**Problems :**