Explain the two equivalent definitions of the class NP and how they relate to polynomial time verifiers and non-deterministic Turing machines.
In the field of computational complexity theory, the class NP (Non-deterministic Polynomial time) is a fundamental concept that plays a important role in understanding the complexity of computational problems. There are two equivalent definitions of NP that are commonly used: the polynomial time verifier definition and the non-deterministic Turing machine definition. These definitions provide different perspectives on the class NP and help us understand its properties and relationships with other complexity classes.
The polynomial time verifier definition of NP is based on the concept of problem verification. A language L is said to be in NP if there exists a polynomial time verifier V such that for every string x in L, there exists a certificate y of polynomial length such that V accepts the pair (x, y) in polynomial time. In other words, given an instance x of the problem, there exists a short proof y that can be efficiently checked by the verifier V. The verifier V acts as a witness to the fact that x belongs to the language L.
To illustrate this definition, let's consider the problem of determining whether a given graph has a Hamiltonian cycle, which is a cycle that visits every vertex exactly once. The language associated with this problem is the set of all graphs that have a Hamiltonian cycle. A polynomial time verifier for this problem could be a deterministic Turing machine that takes as input a graph G and a permutation of its vertices, and checks whether the permutation forms a valid Hamiltonian cycle in G. If the graph has a Hamiltonian cycle, there exists a permutation that serves as a certificate, which can be efficiently verified by the verifier.
The non-deterministic Turing machine definition of NP provides a different perspective on the class. A language L is said to be in NP if there exists a non-deterministic Turing machine M that decides L in polynomial time. Non-deterministic Turing machines are theoretical models of computation that can make non-deterministic choices at each step. If there exists a non-deterministic Turing machine that can decide a language L in polynomial time, then L is said to be in NP.
Continuing with the example of the Hamiltonian cycle problem, a non-deterministic Turing machine for this problem would guess a permutation of the vertices of the graph and then verify whether it forms a Hamiltonian cycle. The machine can make non-deterministic choices at each step, exploring different possibilities in parallel. If there exists at least one accepting computation path for a given input, the machine accepts the input, indicating that the graph has a Hamiltonian cycle.
The two definitions of NP are equivalent, meaning that a language L is in NP according to one definition if and only if it is in NP according to the other definition. This equivalence can be proven by showing that any non-deterministic Turing machine can be simulated by a polynomial time verifier, and vice versa.
The relationship between NP and other complexity classes is also important to understand. NP is known to be a subset of the class PSPACE, which consists of problems that can be solved using polynomial space on a deterministic Turing machine. It is an open question whether NP is equal to PSPACE or if they are distinct classes.
The class NP can be defined in two equivalent ways: the polynomial time verifier definition and the non-deterministic Turing machine definition. The polynomial time verifier definition emphasizes problem verification, while the non-deterministic Turing machine definition focuses on non-deterministic computation. Both definitions provide valuable insights into the complexity of computational problems and their relationships with other complexity classes.
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