How to show that the following language is undecidable using reduction on the halting problem?

$L: = \{w \in \{0,1\}^* |$ TM $M$ with $w = \langle M \rangle$ does not accept any input $\}$

When TM doesn't accept any input, does it mean that the Turing machine halts or it just rejects any words?

And how to show that this language is not semi-decidable? I think to do it with a complement but I don't know how to start. Should I use the Rice's theorem in this proof?


1 Answer 1


I'd assume that a TM does not accept any input if for every input $w$ it either loops or halts and rejects.

The general approach to show some language is undecidable goes as follows:

  1. Assume that there exists a machine $M$ that recognizes the language you are given
  2. Take some language that you know is undecidable
  3. Create a machine $N$ that using $M$ solves the undecidable language
  4. Since the language is undecidable, you have reached a contradiction

Do you see how being able to answer the question "is it true that machine $T$ doesn't accept any input?" would help you answering the question "does machine $T'$ accept $\epsilon$?"?

Hint: Sometimes it is useful to create a machine that ignores its input!

About semi-decidability: There's a theorem that says that a language is decidable if and only if it is semi-decidable and its complement is semi-decidable. Once you prove that your language is undecidable you can easily check that its component is semi-decidable, which immediately gives you that the language itself cannot be semi-decidable.

Regarding Rice's Theorem: Generally I'd say it is a very useful theorem, but I would recommend to solve some undecidability exercises using just pure contradictory reasoning, since it allows you to understand the concept of reducing one problem to another better, in my opinion :)

  • $\begingroup$ Thank you for explanations, I proved at last that this language is undecidable. But I got stuck in proving that it's not semi-decidable. I know that one of the properties of semi-decidability is when TM accepts the language, which is not this case. How to check that the complement is semi-decidable? Should I use characteristic function? $\endgroup$ May 24, 2021 at 12:45
  • $\begingroup$ There's a trick to this - assume $M \in \bar{L}$ that is machine $M$ accepts some word. The problem is we don't know what word that is. If we just ran $M$ on the empty input and then $0$, then $1$ then $00$ and so on, we could never find the word $M$ accepts because for example $M$ loops on $\epsilon$. What you do is the following - enumerate all binary words $w_0, w_1, w_2, \ldots$ (you don't actually write them anywhere) and in the $i$-th step run $M$ on words $w_0, \ldots, w_i$ for $i$ steps each. This way if $M$ accepts $w_k$ you will eventually find out that $M$ accepts $w_k$. $\endgroup$ May 25, 2021 at 13:11
  • $\begingroup$ If this is not clear to You I'd suggest reading about Universal Turing Machines $\endgroup$ May 25, 2021 at 13:13

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