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Non-deterministic Turing Machine - Automata
2023-03-10 18:17:15

Non-deterministic Turing Machine - Automata

A non-deterministic Turing machine (NTM) is a theoretical machine used in computability theory to study the extent to which a machine can "guess" at a solution and yet still be considered as a computer. It is an extension of the standard Turing machine which is a model of a general-purpose computer.

NTMs are more powerful than deterministic Turing machines (DTM), which are the theoretical machines used to model standard computers. DTMs follow a predetermined set of rules to perform a computation, whereas NTMs can make choices during the computation, allowing them to potentially solve problems faster. However, this added power comes at the cost of being harder to analyze and understand.

NTMs have been used to study the concept of NP-completeness in computer science, which refers to a class of problems that are believed to be difficult to solve on a DTM. These problems can be solved quickly on an NTM, but it is not known if they can be solved efficiently on a DTM. This has important implications for the study of algorithms and complexity theory.

Formal definition

A non-deterministic Turing machine (NTM) is a theoretical machine used in computability theory to study the computational power of computers. It can be formally defined as a 7-tuple (Q, Σ, Γ, δ, q0, B, F), where:

  • Q is a finite set of states
  • Σ is a finite set of input symbols
  • Γ is a finite set of tape symbols, which includes Σ as a subset
  • δ is a transition function that takes as input a state and a tape symbol and produces a set of possible next states and tape symbols to write
  • q0 is the initial state
  • B is the blank symbol, which is an element of Γ
  • F is a set of final or accepting states, which is a subset of Q.

The NTM operates on an infinite tape, divided into cells, which can be either blank or contain a symbol from the set Γ. The NTM reads and writes symbols on the tape and moves the tape left or right, depending on the transition function δ. The NTM begins in the initial state q0 and reads the symbol on the tape at its current position. It then uses the transition function to determine the next state and the symbol to be written on the tape. If the NTM reaches a state in the set F, it is said to accept the input string. If the NTM goes into a non-final state and the tape is exhausted, it is said to reject the input string.

Key properties of NTMs

  • Non-determinism: As the name suggests NTMs are non-deterministic, meaning they are allowed to make choices during the computation. This allows them to potentially solve problems faster than DTMs but it also makes them harder to analyze and understand.
  • Multiple computations: NTMs can perform multiple computations simultaneously by branching into multiple computational paths. This allows them to "guess" at a solution and check all possible paths at once.
  • Acceptance: NTMs accept a string if there exists at least one computational path that leads to an accepting state. This is in contrast to DTMs, which must follow a predetermined set of rules to reach an accepting state.
  • Formal language: NTMs can recognize the same set of formal languages as DTMs. However, the class of languages that can be recognized by an NTM in a certain amount of time is a superset of the class of languages that can be recognized by a DTM in the same amount of time. This means that NTMs can recognize more languages than DTMs but it is not known if they can do so efficiently.

Complexity of non-deterministic Turing Machines

NTMs are more powerful than deterministic Turing machines (DTMs) in terms of their computational complexity, but it is not known if they can solve certain problems efficiently. This is because NTMs are allowed to make choices during the computation, which allows them to potentially solve problems faster than DTMs. However, this added power comes at the cost of being harder to analyze and understand.

The complexity of a non-deterministic Turing machine (NTM) refers to how long it takes the machine to solve a problem. In general, the time complexity of an NTM is the number of steps it takes to solve a problem, where each step corresponds to a single operation such as moving the read/write head or changing the symbol on the tape.

One important concept in complexity theory is NP-completeness, which refers to a class of problems that are believed to be difficult to solve on a DTM. These problems can be solved quickly on an NTM, but it is not known if they can be solved efficiently on a DTM. This has important implications for the study of algorithms and the design of efficient computational systems.

Conclusion

In conclusion, non-deterministic Turing machines (NTMs) are theoretical machines used to study the computational power of computers and the complexity of algorithms. They are an extension of the standard Turing machine which is a model of a general-purpose computer. NTMs are more powerful than deterministic Turing machines (DTMs) in terms of their computational complexity but they are harder to analyze and understand. NTMs have been used to study the concept of NP-completeness in computer science which refers to a class of problems that are believed to be difficult to solve on a DTM. The complexity of an NTM refers to how long it takes the machine to solve a problem.