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L = { < M1,M2 > | M1,M2 are TM's and Ɛ ∈ L(M1) ∪ L(M2) }

Where Ɛ = Epsilon

I know that this language is undecidable, but why it is semidecidable too.

What i have tried is =>

Using Rice's Theorem, part 2

T(no) = (0,1)^+ and T(yes) = (0,1)*

Which gives that T(yes) is not a subset of T(no), and this says that it is semidecidable.

Is my understanding right ?

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  • $\begingroup$ Welcome to Computer Science! Note that you can use LaTeX here to typeset mathematics in a more readable way. See here for a short introduction. $\endgroup$ – Raphael Sep 21 '16 at 14:31
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Rice's second theorem can be used to show for some sets that they are not semi-decidable, but not the opposite.

In order to show that this set is semi-deciable, just follow the definition and construct a semi-decider.

Idea: Simulate both $M_1$ and $M_2$ on $\varepsilon$ simultaneaously using dovetailing. If either one halts and accepts, stop simulating and accept.

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