# Formal language

*in mathematical linguistics*

An arbitrary set of chains (that is, words, cf. Word) over some (finite or infinite) alphabet (sometimes also called a dictionary), that is, a set of expressions of the form , where ; the number , usually denoted by , is the length of the chain . One also considers the empty chain, denoted by ; one sets . One often speaks of a language over the alphabet , omitting the word "formal" . In mathematical linguistics and the theory of automata (cf. Automata, theory of) one considers various effective ways of specifying a formal language, principally by means of formal grammars (cf. Grammar, formal) and automata of various types, which can, in the majority of cases, be described as modifications of non-deterministic Turing machines (cf. Turing machine), often multi-tape, with some restrictions on the ways the machine works on each tape.

## Operations on formal languages.

In addition to the usual set-theoretic operations on formal languages, one carries out: multiplication (or direct multiplication, or concatenation):

left division:

right division is defined by analogy with left division; iteration:

where denotes and (in particular, the set of all chains in is ); truncated iteration:

and substitution: If is a language over the finite alphabet and are arbitrary languages, then

if each of the languages , , consists of one chain , a substitution is called a homomorphism; if all the are non-empty, one speaks of an unabbreviated homomorphism. If the language consists of one chain , then instead of , , etc., one writes , , etc., respectively.

A variety of languages is an ordered pair (or , if is taken for granted), where is an infinite alphabet and is a class of languages such that: 1) for every there is a finite alphabet such that ; 2) for some ; and 3) is closed relative to union, multiplication, intersection with regular sets, truncated iteration, unabbreviated homomorphisms, and inverses of arbitrary homomorphisms. A variety that is closed relative to arbitrary homomorphisms is called complete.

#### References

[1] | A.V. Gladkii, "Formal grammars and languages" , Moscow (1973) (In Russian) |

[2] | S. Ginsburg, S. Greibach, Y. Hopcroft, "Studies in abstract families of languages" Mem. Amer. Math. Soc. , 87 (1969) pp. 1–32 |

#### Comments

For more details see Formal languages and automata.

**How to Cite This Entry:**

Formal language.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Formal_language&oldid=18696