A halting configuration of a DFA is a configuration C h = ( q, ) ∈ C( M) with the property that δ ( q, σ ) is undefined. This can occur whenever the state transition function is undefined. We say the DFA halts when there is no next state or when the machine moves off the end of the tape. The tape alphabet Σ T is the set of all possible symbols that appear on the tape, and so it is Σ plus ( by transition 3 ) ⊢ M ( q 2, ) ( by transition 2 ) ⊢ M ( q 2, ) ( by transition 6 ) End markers are not allowed as data symbols for obvious reasons. These are the symbols that can occur on the input tape between 〈 and 〉. Since q 0 ∈ Q, it follows that Q is nonempty however, we prefer to write this condition explicitly in the definition. Typically, we represent individual states by the symbols q 0, q 1, and so on, but keep in mind that other names would work as well. Let's examine each component of this definition in turn. The M stands for “machine.” We will usually use the symbols M, M', M 1, and so on to denote a machine. Q 0- the initial state or start state, q 0 ∈ Q 5.
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