I'm having trouble with proving that "Intersection of a recognizable language anda decidable language is decidable.

I assume this is true although I have no idea how to proof it. Can somebody point me in the right direction?


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    $\begingroup$ Looks like homework. Voting to close. $\endgroup$ – Aryeh Jun 2 at 9:53
  • $\begingroup$ It's not homework, just extra questions in our course notes to think about to understand the subject matter better. $\endgroup$ – Bobby Jun 2 at 9:57

It's false. Let $L_1=\Sigma^*$ be a decidable language and $L_2=L_{HALT}$ be the (recognizable) language of all halting TM-string pairs. Then $L_1\cap L_2=L_2$, which is not decidable.


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