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Let $L$ be the language of all the words with the shape $<M_1,M_2,w>$ such that $M_1,M_2$ are turing machines that accepts $w$, show that $L$ isn't decidable.

I thought to use reduction from $L_{TM}=\{<M,w>\mid M \text{ accepts } w\}$

how can I defined the function $f$ ?

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If $L$ were decidable, then you could make a decider for $A_{TM}=\{\langle M, w\rangle\mid M\text{ accepts }w\}$ by deciding $\langle M, M, w\rangle$. We know, however that that's impossible, since $A_{TM}$ is undecidable.

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