 Theory of computation

In theoretical computer science, the theory of computation is the branch that deals with whether and how efficiently problems can be solved on a model of computation, using an algorithm. The field is divided into three major branches: automata theory, computability theory and computational complexity theory.^{[1]}
In order to perform a rigorous study of computation, computer scientists work with a mathematical abstraction of computers called a model of computation. There are several models in use, but the most commonly examined is the Turing machine. Computer scientists study the Turing machine because it is simple to formulate, can be analyzed and used to prove results, and because it represents what many consider the most powerful possible "reasonable" model of computation.^{[citation needed]} It might seem that the potentially infinite memory capacity is an unrealizable attribute, but any decidable problem solved by a Turing machine will always require only a finite amount of memory. So in principle, any problem that can be solved (decided) by a Turing machine can be solved by a computer that has a bounded amount of memory.
Contents
History
Main article: History of theory of computationThe theory of computation can be considered the creation of models of all kinds in the field of computer science. Therefore mathematics and logic are used. In the last century it became an independent academic discipline and was separated from mathematics.
Some pioneers of the theory of computation were Alonzo Church, Alan Turing, Stephen Kleene, John von Neumann, Claude Shannon, and Noam Chomsky.
Branches
Automata theory
Main article: Automata theoryAutomata theory is the study of abstract machines (or more appropriately, abstract 'mathematical' machines or systems) and the computational problems that can be solved using these machines. These abstract machines are called automata. Automata comes from the Greek word (Αυτόματα) which means that something is doing something by itself. Automata theory is also closely related to formal language theory, as the automata are often classified by the class of formal languages they are able to recognize. An automaton can be a finite representation of a formal language that may be an infinite set.
Computability theory
Main article: Computability theoryComputability theory deals primarily with the question of the extent to which a problem is solvable on a computer. The statement that the halting problem cannot be solved by a Turing machine is one of the most important results in computability theory, as it is an example of a concrete problem that is both easy to formulate and impossible to solve using a Turing machine. Much of computability theory builds on the halting problem result.
Another important step in computability theory was Rice's theorem, which states that for all nontrivial properties of partial functions, it is undecidable whether a Turing machine computes a partial function with that property.
Computability theory is closely related to the branch of mathematical logic called recursion theory, which removes the restriction of studying only models of computation which are reducible to the Turing model. Many mathematicians and computational theorists who study recursion theory will refer to it as computability theory.
Computational complexity theory
Main article: Computational complexity theoryComplexity theory considers not only whether a problem can be solved at all on a computer, but also how efficiently the problem can be solved. Two major aspects are considered: time complexity and space complexity, which are respectively how many steps does it take to perform a computation, and how much memory is required to perform that computation.
In order to analyze how much time and space a given algorithm requires, computer scientists express the time or space required to solve the problem as a function of the size of the input problem. For example, finding a particular number in a long list of numbers becomes harder as the list of numbers grows larger. If we say there are n numbers in the list, then if the list is not sorted or indexed in any way we may have to look at every number in order to find the number we're seeking. We thus say that in order to solve this problem, the computer needs to perform a number of steps that grows linearly in the size of the problem.
To simplify this problem, computer scientists have adopted Big O notation, which allows functions to be compared in a way that ensures that particular aspects of a machine's construction do not need to be considered, but rather only the asymptotic behavior as problems become large. So in our previous example we might say that the problem requires O(n) steps to solve.
Perhaps the most important open problem in all of computer science is the question of whether a certain broad class of problems denoted NP can be solved efficiently. This is discussed further at Complexity classes P and NP.
Models of computation
Main article: Model of computationAside from a Turing machine, other equivalent (See: Church–Turing thesis) models of computation are in use.
 Lambda calculus
 A computation consists of an initial lambda expression (or two if you want to separate the function and its input) plus a finite sequence of lambda terms, each deduced from the preceding term by one application of Beta reduction.
 Combinatory logic
 is a concept which has many similarities to λcalculus, but also important differences exist (e.g. fixed point combinator Y has normal form in combinatory logic but not in λcalculus). Combinatory logic was developed with great ambitions: understanding the nature of paradoxes, making foundations of mathematics more economic (conceptually), eliminating the notion of variables (thus clarifying their role in mathematics).
 murecursive functions
 a computation consists of a murecursive function, i.e. its defining sequence, any input value(s) and a sequence of recursive functions appearing in the defining sequence with inputs and outputs. Thus, if in the defining sequence of a recursive function f(x) the functions g(x) and h(x,y) appear, then terms of the form 'g(5)=7' or 'h(3,2)=10' might appear. Each entry in this sequence needs to be an application of a basic function or follow from the entries above by using composition, primitive recursion or mu recursion. For instance if f(x) = h(x,g(x)), then for 'f(5)=3' to appear, terms like 'g(5)=6' and 'h(5,6)=3' must occur above. The computation terminates only if the final term gives the value of the recursive function applied to the inputs.
 Markov algorithm
 a string rewriting system that uses grammarlike rules to operate on strings of symbols.
 Register machine
 is a theoretically interesting idealization of a computer. There are several variants. In most of them, each register can hold a natural number (of unlimited size), and the instructions are simple (and few in number), e.g. only decrementation (combined with conditional jump) and incrementation exist (and halting). The lack of the infinite (or dynamically growing) external store (seen at Turing machines) can be understood by replacing its role with Gödel numbering techniques: the fact that each register holds a natural number allows the possibility of representing a complicated thing (e.g. a sequence, or a matrix etc.) by an appropriate huge natural number — unambiguity of both representation and interpretation can be established by number theoretical foundations of these techniques.
 P′′
 Like Turing machines, P′′ uses an infinite tape of symbols (without random access), and a rather minimalistic set of instructions. But these instructions are very different, thus, unlike Turing machines, P′′ does not need to maintain a distinct state, because all “memorylike” functionality can be provided only by the tape. Instead of rewriting the current symbol, it can perform a modular arithmetic incrementation on it. P′′ has also a pair of instructions for a cycle, inspecting the blank symbol. Despite its minimalistic nature, it has become the parental formal language of an implemented and (for entertainment) used programming language called Brainfuck.
In addition to the general computational models, some simpler computational models are useful for special, restricted applications. Regular expressions, for example, specify string patterns in many contexts, from office productivity software to programming languages. Another formalism mathematically equivalent to regular expressions, Finite automata are used in circuit design and in some kinds of problemsolving. Contextfree grammars specify programming language syntax. Nondeterministic pushdown automata are another formalism equivalent to contextfree grammars. Primitive recursive functions are a defined subclass of the recursive functions.
Different models of computation have the ability to do different tasks. One way to measure the power of a computational model is to study the class of formal languages that the model can generate; in such a way to the Chomsky hierarchy of languages is obtained.
References
 ^ Sipser
Further reading
 Textbooks aimed at computer scientists
(There are many textbooks in this area; this list is by necessity incomplete.)
 Hopcroft, John E., and Jeffrey D. Ullman (2006). Introduction to Automata Theory, Languages, and Computation. 3rd ed Reading, MA: AddisonWesley. ISBN 9780321455369 One of the standard references in the field.
 Michael Sipser (2006). Introduction to the Theory of Computation 2ed. PWS Publishing. ISBN 0534950973.
 Eitan Gurari (1989). An Introduction to the Theory of Computation. Computer Science Press. ISBN 0716781824. http://www.cse.ohiostate.edu/~gurari/theorybk/theorybk.html.
 Hein, James L. (1996) Theory of Computation. Sudbury, MA: Jones & Bartlett. ISBN 9780867204971 A gentle introduction to the field, appropriate for secondyear undergraduate computer science students.
 Taylor, R. Gregory (1998). Models of Computation and Formal Languages. New York: Oxford University Press. ISBN 9780195109832 An unusually readable textbook, appropriate for upperlevel undergraduates or beginning graduate students.
 Lewis, F. D. (2007). Essentials of theoretical computer science A textbook covering the topics of formal languages, automata and grammars. The emphasis appears to be on presenting an overview of the results and their applications rather than providing proofs of the results.
 Martin Davis, Ron Sigal, Elaine J. Weyuker, Computability, complexity, and languages: fundamentals of theoretical computer science, 2nd ed., Academic Press, 1994, ISBN 0122063821. Covers a wider range of topics than most other introductory books, including program semantics and quantification theory. Aimed at graduate students.
 Books on computability theory from the (wider) mathematical perspective
 Hartley Rogers, Jr (1987). Theory of Recursive Functions and Effective Computability, MIT Press. ISBN 0262680521
 S. Barry Cooper (2004). Computability Theory. Chapman and Hall/CRC. ISBN 1584882379..
 Carl H. Smith, A recursive introduction to the theory of computation, Springer, 1994, ISBN 0387943323. A shorter textbook suitable for graduate students in Computer Science.
 Historical perspective
 Richard L. Epstein and Walter A. Carnielli (2000). Computability: Computable Functions, Logic, and the Foundations of Mathematics, with Computability: A Timeline (2nd ed.). Wadsworth/Thomson Learning. ISBN 0534546447..
External links
 Computability Logic  A theory of interactive computation. The main web source on this subject.
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