Recursive language
In mathematics, logic and computer science, a formal language a set of finite sequences of symbols taken from a fixed alphabet is called recursive if it is a recursive subset of the set of all possible finite sequences over the alphabet of the language Equivalently, a formal language is recursive if there exists a total Turing machine a Turing machine that halts for every given input that, when given a finite sequence of symbols as input, accepts it if belongs to the language and rejects it otherwise Recursive languages are also called decidable
The concept of decidability may be extended to other models of computation For example one may speak of languages decidable on a nondeterministic Turing machine Therefore, whenever an ambiguity is possible, the synonym for "recursive language" used is Turingdecidable language, rather than simply decidable
The class of all recursive languages is often called R, although this name is also used for the class RP
This type of language was not defined in the Chomsky hierarchy of Chomsky 1959 All recursive languages are also recursively enumerable All regular, contextfree and contextsensitive languages are recursive
Contents
 1 Definitions
 2 Examples
 3 Closure properties
 4 See also
 5 References
Definitionsedit
There are two equivalent major definitions for the concept of a recursive language:
 A recursive formal language is a recursive subset in the set of all possible words over the alphabet of the language
 A recursive language is a formal language for which there exists a Turing machine that, when presented with any finite input string, halts and accepts if the string is in the language, and halts and rejects otherwise The Turing machine always halts: it is known as a decider and is said to decide the recursive language
By the second definition, any decision problem can be shown to be decidable by exhibiting an algorithm for it that terminates on all inputs An undecidable problem is a problem that is not decidable
Examplesedit
As noted above, every contextsensitive language is recursive Thus, a simple example of a recursive language is the set L=; more formally, the set
L = ∗ ∣ w = a n b n c n for some n ≥ 1 ^\mid w=a^b^c^n\geq 1\,\is contextsensitive and therefore recursive
Examples of decidable languages that are not contextsensitive are more difficult to describe For one such example, some familiarity with mathematical logic is required: Presburger arithmetic is the firstorder theory of the natural numbers with addition but without multiplication While the set of wellformed formulas in Presburger arithmetic is contextfree, every deterministic Turing machine accepting the set of true statements in Presburger arithmetic has a worstcase runtime of at least 2 2 c n , for some constant c>0 Fischer & Rabin 1974 Here, n denotes the length of the given formula Since every contextsensitive language can be accepted by a linear bounded automaton, and such an automaton can be simulated by a deterministic Turing machine with worstcase running time at most c n for some constant ccitation needed, the set of valid formulas in Presburger arithmetic is not contextsensitive On positive side, it is known that there is a deterministic Turing machine running in time at most triply exponential in n that decides the set of true formulas in Presburger arithmetic Oppen 1978 Thus, this is an example of a language that is decidable but not contextsensitive
Closure propertiesedit
Recursive languages are closed under the following operations That is, if L and P are two recursive languages, then the following languages are recursive as well:
 The Kleene star L ∗
 The image φL under an efree homomorphism φ
 The concatenation L ∘ P
 The union L ∪ P
 The intersection L ∩ P
 The complement of L
 The set difference L − P
The last property follows from the fact that the set difference can be expressed in terms of intersection and complement
See alsoedit
 Recursively enumerable language
 Recursion
Referencesedit
 Michael Sipser 1997 "Decidability" Introduction to the Theory of Computation PWS Publishing pp 151–170 ISBN 053494728X
 Chomsky, Noam 1959 "On certain formal properties of grammars" Information and Control 2 2: 137–167 doi:101016/S0019995859903626
 Fischer, Michael J; Rabin, Michael O 1974 "SuperExponential Complexity of Presburger Arithmetic" Proceedings of the SIAMAMS Symposium in Applied Mathematics 7: 27–41
 Oppen, Derek C 1978 "A 222pn Upper Bound on the Complexity of Presburger Arithmetic" PDF J Comput Syst Sci 16 3: 323–332 doi:101016/0022000078900211





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