Below are the two problems Im working on and my attempt at a solution (understanding the question). My first question is am I interpreting the question from the symbols right, second can you help explain what I need to do for these questions? Ive been reading through chapter 4 of Sipser 3rd ed. Intro to theory of computation and I haven't been able to make much sense the questions. Any help is appreciated.

  1. Show that the following languages are partially decidable:

    (a) ETx = {<M, x> | M(x) ↓ and at the end of computation the tape of M is empty}

    ETx is a universal TM M with input x that terminates and at the end of computation the tape of M is empty. Not sure where to go from (restating the question...)

    (b) ETE = {< M > | (∃x)(M(x) ↓ and at the end of computation the tape of M is empty)}

    ETE is a universal TM M where there exists some input such that input X terminates M and at the end of the computation the tape of M is empty...

  1. Show that the language ETx is not decidable. (Hint: Describe how a decision procedure for ETx could be used to decide the universal language Lu.)

    Not sure where to begin with this one

  • 1
    $\begingroup$ Please ask only one question per post. $\endgroup$
    – D.W.
    Commented Nov 23, 2021 at 6:07

1 Answer 1


Let's start with the first question. One can recursively enumerate triples $\langle M, x, t \rangle$ and print $\langle M,x\rangle $ or $M$ (depending on whether we consider the first or the second language) in case $M$ halts on input $x$ within $t$ steps and the tape is empty when $M$ halts.

For the second question we can reduce halting problem to the language a) by mapping a Turing machine $M$ to a pair $\langle M', M\rangle$, where $M'$ is defined as the Turing machine which runs the Turing machine $M$ (whose encoding it receves as its input) and which clears the computation tape if $M$ halts. Clearly $M$ halts on empty input iff $\langle M', M\rangle$ belongs to the language a).


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