Reduction from $L$ to $L_{TM}$ Let $L$ be the language of all the words with the shape $&lt;M_1,M_2,w&gt;$ such that $M_1,M_2$ are turing machines that accepts $w$, show that $L$ isn't decidable. 


I thought to use reduction from $L_{TM}=\{&lt;M,w&gt;\mid M \text{ accepts } w\}$

how can I defined the function $f$ ?
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.