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For regular languages $A,B\subseteq\{0,1\}^*$, is

$$L_2 = \{x \in A \mid \exists y \in B : |x|_1 =|y|_1 \}$$

regular, where $|x|_1$ means the number of appearances of 1 in the word $x$?

i need to demonstrate $L_2$ automata.

Thought about using the multiplication automata, not so sure.

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  • $\begingroup$ Oops, I misread the question, ignore my previous comment. $\endgroup$ – SergGr Dec 2 '18 at 22:09
  • $\begingroup$ Is it enough to just prove $L_2$ is regular? $\endgroup$ – John L. Dec 3 '18 at 4:18
  • $\begingroup$ @Apass.jack yes this is fine $\endgroup$ – Maor Rocky Dec 3 '18 at 5:45
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The idea is as follows:

  • We run two automata in parallel: one verifies that the input is in $A$, and the other verifies the other condition.
  • The second automaton simulates an automaton for $B$, in the following way. At any point in time it may simulate reading a $0$. It ignores all $0$s in the real input, and forwards all $1$s in the real input.

I'll let you construct an NFA given this description.

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