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If there is a Turing Machine $M_e$ (computing some partially computable function $f$), is there an algorithm to decide if $f$ diverges for all possible inputs?

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migrated from stackoverflow.com Apr 10 '13 at 17:08

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1 Answer 1

No. It is very easy to reduce $A_{TM}=\{(M,w): M$ accepts $w\}$ to the complement of the problem you describe: given input $(M,w)$, output a machine $M'$ that simulates $M(w)$. If $M$ accepts $w$ then $M'$ halts and accepts. Otherwise, $M'$ goes into an infinite loop (either if $M$ loops on $w$, or manually if $M$ rejects $w$).

Now, if $M$ accepts $w$, then $M'$ does not diverge on every input, and otherwise it does.

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@YuvalFilmus: I added the definition of A_TM to the answer, thanks. –  Shaull Apr 11 '13 at 4:45

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