P (complexity)

# P (complexity)

Discussion

Encyclopedia
In computational complexity theory
Computational complexity theory
Computational complexity theory is a branch of the theory of computation in theoretical computer science and mathematics that focuses on classifying computational problems according to their inherent difficulty, and relating those classes to each other...

, P, also known as PTIME or DTIME
DTIME
In computational complexity theory, DTIME is the computational resource of computation time for a deterministic Turing machine. It represents the amount of time that a "normal" physical computer would take to solve a certain computational problem using a certain algorithm...

(nO(1)), is one of the most fundamental complexity class
Complexity class
In computational complexity theory, a complexity class is a set of problems of related resource-based complexity. A typical complexity class has a definition of the form:...

es. It contains all decision problem
Decision problem
In computability theory and computational complexity theory, a decision problem is a question in some formal system with a yes-or-no answer, depending on the values of some input parameters. For example, the problem "given two numbers x and y, does x evenly divide y?" is a decision problem...

s which can be solved by a deterministic Turing machine using a polynomial
Polynomial
In mathematics, a polynomial is an expression of finite length constructed from variables and constants, using only the operations of addition, subtraction, multiplication, and non-negative integer exponents...

amount of computation time, or polynomial time.

Cobham's thesis
Cobham's Thesis
Cobham's thesis, also known as Cobham–Edmonds thesis , asserts that computational problems can be feasibly computed on some computational device only if they can be computed in polynomial time; that is, if they lie in the complexity class P.Formally, to say that a problem can be solved in...

holds that P is the class of computational problems which are "efficiently solvable" or "tractable"; in practice, some problems not known to be in P have practical solutions, and some that are in P do not, but this is a useful rule of thumb.

## Definition

A language L is in P if and only if there exists a deterministic Turing machine M, such that
• M runs for polynomial time on all inputs
• For all x in L, M outputs 1
• For all x not in L, M outputs 0

P can also be viewed as a uniform family of boolean circuit
Boolean circuit
A Boolean circuit is a mathematical model of computation used in studying computational complexity theory. Boolean circuits are the main object of study in circuit complexity and are a special kind of circuits; a formal language can be decided by a family of Boolean circuits, one circuit for each...

s. A language L is in P if and only if there exists a polynomial-time uniform family of boolean circuits , such that
• For all , takes n bits as input and outputs 1 bit
• For all x in L,
• For all x not in L,

The circuit definition can be weakened to use only a logspace uniform family without changing the complexity class P.

## Notable problems in P

P is known to contain many natural problems, including the decision versions of linear programming
Linear programming
Linear programming is a mathematical method for determining a way to achieve the best outcome in a given mathematical model for some list of requirements represented as linear relationships...

, calculating the greatest common divisor
Greatest common divisor
In mathematics, the greatest common divisor , also known as the greatest common factor , or highest common factor , of two or more non-zero integers, is the largest positive integer that divides the numbers without a remainder.For example, the GCD of 8 and 12 is 4.This notion can be extended to...

, and finding a maximum matching. In 2002, it was shown that the problem of determining if a number is prime
Prime number
A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. A natural number greater than 1 that is not a prime number is called a composite number. For example 5 is prime, as only 1 and 5 divide it, whereas 6 is composite, since it has the divisors 2...

is in P. The related class of function problem
Function problem
In computational complexity theory, a function problem is a computational problem where a single output is expected for every input, but the output is more complex than that of a decision problem, that is, it isn't just YES or NO...

s is FP
FP (complexity)
In computational complexity theory, the complexity class FP is the set of function problems which can be solved by a deterministic Turing machine in polynomial time; it is the function problem version of the decision problem class P...

.

Several natural problems are complete for P, including st-connectivity
ST-connectivity
In computer science and computational complexity theory, st-connectivity or STCON is a decision problem asking, for vertices s and t in a directed graph, if t is reachable from s.Formally, the decision problem is given by- Complexity :...

(or reachability
Reachability
In graph theory, reachability is the notion of being able to get from one vertex in a directed graph to some other vertex. Note that reachability in undirected graphs is trivial — it is sufficient to find the connected components in the graph, which can be done in linear time.- Definition :For a...

) on alternating graphs. The article on P-complete problems
P-complete
In complexity theory, the notion of P-complete decision problems is useful in the analysis of both:# which problems are difficult to parallelize effectively, and;# which problems are difficult to solve in limited space....

lists further relevant problems in P.

## Relationships to other classes

A generalization of P is NP
NP (complexity)
In computational complexity theory, NP is one of the most fundamental complexity classes.The abbreviation NP refers to "nondeterministic polynomial time."...

, which is the class of languages decidable in polynomial
Polynomial
In mathematics, a polynomial is an expression of finite length constructed from variables and constants, using only the operations of addition, subtraction, multiplication, and non-negative integer exponents...

time on a non-deterministic Turing machine
Non-deterministic Turing machine
In theoretical computer science, a Turing machine is a theoretical machine that is used in thought experiments to examine the abilities and limitations of computers....

. We then trivially have P is a subset of NP. Though unproven, most experts believe this is a strict subset.

P is also known to be at least as large as L
L (complexity)
In computational complexity theory, L is the complexity class containing decision problems which can be solved by a deterministic Turing machine using a logarithmic amount of memory space...

, the class of problems decidable in a logarithm
Logarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...

ic amount of memory space. A decider using space cannot use more than time, because this is the total number of possible configurations; thus, L is a subset of P. Another important problem is whether L = P. We do know that P = AL, the set of problems solvable in logarithmic memory by alternating Turing machine
Alternating Turing machine
In computational complexity theory, an alternating Turing machine is a non-deterministic Turing machine with a rule for accepting computations that generalizes the rules used in the definition of the complexity classes NP and co-NP...

s. P is also known to be no larger than PSPACE
PSPACE
In computational complexity theory, PSPACE is the set of all decision problems which can be solved by a Turing machine using a polynomial amount of space.- Formal definition :...

, the class of problems decidable in polynomial space. Again, whether P = PSPACE is an open problem. To summarize:

Here, EXPTIME
EXPTIME
In computational complexity theory, the complexity class EXPTIME is the set of all decision problems solvable by a deterministic Turing machine in O time, where p is a polynomial function of n....

is the class of problems solvable in exponential time. Of all the classes shown above, only two strict containments are known:
• P is strictly contained in EXPTIME. Consequently, all EXPTIME-hard problems lie outside P, and at least one of the containments to the right of P above is strict (in fact, it is widely believed that all three are strict).
• L is strictly contained in PSPACE.

The most difficult problems in P are P-complete
P-complete
In complexity theory, the notion of P-complete decision problems is useful in the analysis of both:# which problems are difficult to parallelize effectively, and;# which problems are difficult to solve in limited space....

problems.

Another generalization of P is P/poly, or Nonuniform Polynomial-Time. If a problem is in P/poly, then it can be solved in deterministic polynomial time provided that an advice string
In computational complexity theory, an advice string is an extra input to a Turing machine which is allowed to depend on the length n of the input, but not on input itself...

is given that depends only on the length of the input. Unlike for NP, however, the polynomial-time machine doesn't need to detect fraudulent advice strings; it is not a verifier. P/poly is a large class containing nearly all practical problems, including all of BPP. If it contains NP, then the polynomial hierarchy
Polynomial hierarchy
In computational complexity theory, the polynomial hierarchy is a hierarchy of complexity classes that generalize the classes P, NP and co-NP to oracle machines...

collapses to the second level. On the other hand, it also contains some impractical problems, including some undecidable problem
Undecidable problem
In computability theory and computational complexity theory, an undecidable problem is a decision problem for which it is impossible to construct a single algorithm that always leads to a correct yes-or-no answer....

s such as the unary version of any undecidable problem.

In 1999, Jin-Yi Cai and D. Sivakumar, building on work by Mitsunori Ogihara, showed that if there exists a sparse language
Sparse language
In computational complexity theory, a sparse language is a formal language such that the number of strings of length n in the language is bounded by a polynomial function of n. They are used primarily in the study of the relationship of the complexity class NP with other classes...

which is P-complete, then L = P.

## Properties

Polynomial-time algorithms are closed under composition. Intuitively, this says that if one writes a function which is polynomial-time assuming that function calls are constant-time, and if those called functions themselves require polynomial time, then the entire algorithm takes polynomial time. One consequence of this is that P is low
Low (complexity)
In computational complexity theory, a complexity class B is said to be low for a complexity class A if AB = A; that is, A with an oracle for B is equal to A. Such a statement implies that an abstract machine which solves problems in A achieves no additional power if it is given the ability to solve...

for itself. This is also one of the main reasons that P is considered to be a machine-independent class; any machine "feature", such as random access
Random access
In computer science, random access is the ability to access an element at an arbitrary position in a sequence in equal time, independent of sequence size. The position is arbitrary in the sense that it is unpredictable, thus the use of the term "random" in "random access"...

, which can be simulated in polynomial time can simply be composed with the main polynomial-time algorithm to reduce it to a polynomial-time algorithm on a more basic machine.

## Pure existence proofs of polynomial-time algorithms

Some problems are known to be solvable in polynomial-time, but no concrete algorithm is known for solving them. For example, the Robertson-Seymour theorem guarantees that there is a finite list of forbidden minors that characterizes (for example) the set of graphs that can be embedded on a torus; moreover, Robertson and Seymour showed that there is an O(n3) algorithm for determining whether a graph has a given graph as a minor. This yields a nonconstructive proof that there is a polynomial-time algorithm for determining if a given graph can be embedded on a torus, despite the fact that no concrete algorithm is known for this problem.

## Alternative characterizations

In descriptive complexity
Descriptive complexity
Descriptive complexity is a branch of computational complexity theory and of finite model theory that characterizes complexity classes by the type of logic needed to express the languages in them. For example, PH, the union of all complexity classes in the polynomial hierarchy, is precisely the...

, P can be described as the problems expressible in FO (LFP), the class of first-order logic
First-order logic
First-order logic is a formal logical system used in mathematics, philosophy, linguistics, and computer science. It goes by many names, including: first-order predicate calculus, the lower predicate calculus, quantification theory, and predicate logic...

with a least fixed point
Least fixed point
In order theory, a branch of mathematics, the least fixed point of a function is the fixed point which is less than or equal to all other fixed points, according to some partial order....

operator added to it. In Immerman's 1999 textbook on descriptive complexity, Immerman ascribes this result to Vardi and to Immerman.

## History

Kozen
Dexter Kozen
Dexter Campbell Kozen is an American theoretical computer scientist. He is currently Joseph Newton Pew, Jr. Professor in Engineering at Cornell University. He received his B.A...

states that Cobham and Edmonds
Jack Edmonds
Jack R. Edmonds is a mathematician, regarded as one of the most important contributors to the field of combinatorial optimization...

are "generally credited with the invention of the notion of polynomial time." Cobham invented the class as a robust way of characterizing efficient algorithms, leading to Cobham's thesis
Cobham's Thesis
Cobham's thesis, also known as Cobham–Edmonds thesis , asserts that computational problems can be feasibly computed on some computational device only if they can be computed in polynomial time; that is, if they lie in the complexity class P.Formally, to say that a problem can be solved in...

. However, H. C. Pocklington
Henry Cabourn Pocklington
Henry Cabourn Pocklington was an English physicist and mathematician. His primary profession was as a schoolmaster, but he has made important contributions to number theory with the discovery of Pocklington's primality test in 1914.-References:...

, in a 1910 paper, analyzed two algorithms for solving quadratic congruences, and observed that one took time "proportional to a power of the logarithm of the modulus" and contrasted this with one that took time proportional "to the modulus itself or its square root", thus explicitly drawing a distinction between an algorithm that ran in polynomial time versus one that did not.