As in the case of programs such algorithms are typically created for solving not a particular task, but a group of tasks. The input, output and other related objects - e.g. the algorithm itself - should be defined and used in some general way. To do this we need some basic concepts and results in the theory of formal languages.

We start with some definitions related to functions.

**Definition
2.1.**

Let and be two arbitrary non-empty sets. The Cartesian product of the two sets is the set containing pairs from the base sets. If , then we may use the notation. Let . The set is called a relation defined on . If for some and we have , then we say that and are in relation by. Let be a relation. If and the property implies , then is calleda (partial) function and is denoted by. If , then we use the traditional notation . If such that , then it is called atotal function.Let be a total function. If such that , then is called a surjective function. If implies , then is called aninjektive or one-to-one function.If is both surjective and injective then it is calledbijektive.

**Remark 2.2.**

The traditional notation for the relation is , as we are used with e.g. the relations and . If and are two function, then the union of them can be defined as the union of relations. This, however, is not necessarily a function. If we assume, that , then is a function and .

Since in the following parts we want to prove that the mathematical concept of algorithm we construct can be precisely specified, thus the detailed definition of the alphabet, word and language are necessary. There is a particular need to find place of the empty symbol in the structure.

**Definition 2.3.**

The finite, non-empty set is called an alphabet. The elements of are called letters (somtemies characters or symbols). If , then we assume for that . (The symbol is called the empty character.)

**Definition 2.4.**

Let be a nonepty set and let be a map. Assume that 1. ; 2. sympols such that ; 3. and implies if and only if and ; 4. and , such that ; 5. (Induction) Let a statement on the elements of . If , holds and implies , then holds, too; 6. If , then . Then is called the set of finite words over the alphabet and is called the empty word. As it is usual, we will use the notation and .

**Remark 2.5.**

1. Let . The map can be viewed as a transformation, which extends the word by the symbol . 2. One can define a natural embedding between and , by . Thus the elements of can be regarded as finite sequences of elements of . 3. By 6. of Definition 2.4 , which means that and are different, but they are corresponding to each other in the two sets. 4. By 6. of Definition 2.4 the 3. can be extended to the cases és . 5. The 5. pf Definition 2.4 means that every word can be created from the empty word by adding symbols to it. Thus, if , where , then we may denote the word by its traditional notation . 6. By point 3. the above represenatation is unique. 7. The previous statement can be extended to the empty symbol with the property .

**Definition 2.6.**

Let be a function with properties: 1. ; 2. , and . Then is called the length of the word .

**Theorem 2.7.**

the length is well defined.

**Proof**

By definition, the length of a word means the number of steps the map applied on the empty word to get . Since number and the order of the extension steps are fixed for the creation of , the length is unique. ✓

**Remark 2.8.**

If a word is regarded as a finite sequence of symbols, then the length is equal to the number of symbols in it.

**Definition 2.9.**

Let be a binary operation on , i.e. ⋅: with the properties: 1. let ; 2. and implies . Then the operation is called the concatenation. Traditionally we use the simpler notation instead of .

The operation of concatenation has the following property.

**Theorem 2.10.**

if and , then .

**Proof**

The proof is by induction on .

a.) By definition, if , then , which means that the statement holds.

b.) Assume that for a given for all words if , then the

statement holds.

Let be a word of length . If the is the last letter of , then for some , where .

By the inductive assumption . Applying the definition of the concatenation,

which means that the statement holds for , too.

Form a.) and b.) by induction the theorem follows. ✓

**Theorem 2.11.**

with the binary operation of concatenation is a monoid (semigroup with a unit element).

**Proof**

The proof is again by induction.

1. The concatenation is associative:

a.)By definition .

b.) Assume, that for a given for all words , if , then the associativity holds. Let is such that . Then and satisfying and . By the inductive assumption, for all words the equality holds. Hence

,

which means that the statement is true for , too.

From a.) and b.) by induction the associativity follows.

2. is a unit element.

By definition for all .

The equality can be proven by induction.

a.) By the definition .

b.) Assume for all that if , then . Let be a word of length . Then and , such that and . By the inductive assumption

,

which means that the statement holds for , too.

From a.) and b.) by induction follows.

With this, the proof is complete. ✓**
**

**Theorem 2.12.**

The operation of concatenation satisfies the simplification rule, i.e. the eqality implies . Similarly, the equality implies .

**Proof**

As before, the proof can be done by induction. In the firs case the induction is on , while in then second case the induction is on and , simultaneously. ✓

The following definitions are useful for the description of the structure of word.

**Definition 2.13.**

Let be such that . Then is called a prefix, while is called a postfix of .

**Remark 2.14.**

A word is a so called trivial pre- and postfix of itself. The empty word is a trivial pre- and postfix of any word.

One may define transformations in the form on the set of finite words. These transformation can have some good properties. Some of the most important ones are given in the following.

**Definition 2.15.**

A transformation is called length preserving, if the equality holds.

**Remark 2.16.**

Length preserving transformation e.g. the mirror image, permutation of characters, cyclic permutation, e.t.c.

**Definition 2.17.**

A transformation is called prefix preserving, if words , such that .

**Remark 2.18.**

In the examples of Remark 2.16, the mirror image and cyclic permutation is not, but the character permutation is prefix preserving transformation. Prefix preserving, but not length preserving, however, the following transformation: . (Repetition.)

Using the concept of words, we arrive to the definition of an important object of the theory of algorithms.

**Definition 2.19.**

Let be a finite alphabet. The set is called a (formal) language over .

It is natural idea to define the usual set operations on languages. E.g. if, are two languages, then and are languages, too. The language is called the complement of .

Other important operations on languages are the following.

**Definition 2.20.**

Let be two languages. The language is called the concatenation of the languages and denoted by .

**Remark 2.21.**

Concatenating by itself, we may use the following simplified notation és , ha .

**Definition 2.22.**

Let be a language. The language is called the iteration - or the closure for the concatenation - of .

**Remark
2.23.**

The name closure comes from the fact, that is exactly the smallest language, such that is a subset of it and closed under concatenation.

**Definition 2.24.**

Let be a class of languages. Then the class . Here denites the complement of .

**Remark 2.25.**

The class is in general not the complement of . Actually is not necessarily empty. If e.g. is the class of all languages, then .

**Theorem 2.26.**

Let and be two class of languages over the same alphabet. Then 1. ; 2. ; 3. ; 4. if and omly if .

**Proof**

We apply the usual set theoretic proofs for equalities.

1.We may write the following chain of equivalences

or or .

2. Similarly, we may write the following chain of equivalences:

and and .

3. In this case we may write:

.

4. By definition if and only if and if and only if . Again by definition if and only if implies . This, however, stands exactly when implies . ✓