I am trying to solve a problem in Sipser's Introduction to the Theory of Computation book, which reads:
4.22 A useless state in a pushdown automaton is never entered on any input string. Consider the problem of determining whether a pushdown automaton has any useless states. Formulate this problem as a language and show that it is decidable.
I feel that I have a solution, but at the same time the solution makes no reference to the fact that the machine in question is a PDA, which makes me suspicious. Here is my proposed solution:
Formulate the problem as the language $$L = \{\langle M, q, w \rangle \mid M \text{ is a PDA }, q \text{ is a state of M}, w \text{ is any word in } \Sigma(M)^*\}.$$ Then to decide this language, simply run $w$ in $M$, and see if we ever encounter $q$. Since PDAs are non-deterministic by definition, there may be branching choices, and so we just go through all possible branches. We accept if we encounter $q$ at some point, and reject if we don't. Since the PDA can't loop, then this process won't loop either, and so $L$ is decidable.
This seems to work to me, but as mentioned, it makes no real use of PDAs. It seems like this argument would also work if $M$ was more generally a Turing-decider. So basically, my question is, is this argument truly valid, or am I missing something? My only thought is that perhaps this language is not a true formulation of the problem. If so, how does one understand whether the language they have written is a correct formulation or not?
Thanks for reading, and for any insight.
