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.
In essence, a Turing machine is imagined to be a simple computer that reads and writes symbols one at a time on an endless tape by strictly following a set of rules. It determines what action it should perform next according to its internal "state" and what number it currently sees. An example of one of a Turing Machine's rules might thus be: "If you are in state 2 and you see an 'A', change it to a 'B' and move left."
In a deterministic Turing machine, the set of rules prescribes at most one action to be performed for any given situation. A nondeterministic Turing machine (NTM), by contrast, may have a set of rules that prescribes more than one action for a given situation. For example, a nondeterministic Turing machine may have both "If you are in state 2 and you see an 'A', change it to a 'B' and move left" and "If you are in state 2 and you see an 'A', change it to a 'C' and move right" in its rule set.
An ordinary (deterministic) Turing machine (DTM) has a transition function that, for a given state and symbol under the tape head, specifies three things: the symbol to be written to the tape, the direction (left or right) in which the head should move, and the subsequent state of the finite control. For example, an X on the tape in state 3 might make the DTM write a Y on the tape, move the head one position to the right, and switch to state 5.
A nondeterministic Turing machine (NTM) differs in that the state and tape symbol no longer uniquely specify these things; rather, many different actions may apply for the same combination of state and symbol. For example, an X on the tape in state 3 might now allow the NTM to write a Y, move right, and switch to state 5 or to write an X, move left, and stay in state 3.
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Definition
A nondeterministic Turing machine can be formally defined as a 6tuple , where
 Q is a finite set of states
 Σ is a finite set of symbols (the tape alphabet)
 is the initial state
 is the blank symbol
 is the set of accepting states
 is a relation on states and symbols called the transition relation..
The difference with a standard (deterministic) Turing machine is that for those, the transition relation is a function (the transition function).
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