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Do all states in a DFA must be connected?

Submitted by: @import:stackexchange-cs··
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dfamustallstatesconnected

Problem

Could I construct (for some wired reason) a DFA that has a state that is not connected to anything, and it would still be legal?

I'm studying for a test, and I found a question that asks if an infinite DFA could represent a regular language, and I want to use a regular DFA and add all the infinite states not connected to the original. Can I do that?

Solution

I suspect a trick question in the material you are studying - there are no DFAs with an infinite number of states in the first place.

in fact, the whole point of D/NFAs is their finiteness. if you could have an inifinite number of (reachable) states in your NFA you'd actually be able to recognize any language; just take for every word in your language the trivial deterministic recognizer and connect a global start state with $\lambda$-transitions to the start state of each of these automata.

Context

StackExchange Computer Science Q#23127, answer score: 4

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