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Let $\Sigma = \{a, b\}$. For every language $L \subseteq \Sigma^*$ we denote $\widetilde{L} := \{xy \mid xxy\in L\}$. Prove that if $L$ is regular, then so is $\widetilde{L}$. I tried playing around with the automaton for $L$ and extending it in ways, but I always keep adding unwanted words to the new language and therefore get a language actually bigger than $\widetilde{L}$. I also thought about somehow employing the Myhill-Nerode theorem and the finite equivalence classes of $\approx_L$, but I couldn't rerally figure out an useful way of doing it... Any help is welcome!

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    $\begingroup$ cs.stackexchange.com/q/41281/755 $\endgroup$
    – D.W.
    Commented Jun 22, 2021 at 7:41
  • $\begingroup$ Wow, okay, that was much heavier than expected... $\endgroup$
    – D. Petrov
    Commented Jun 22, 2021 at 8:04

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