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A regular language (also called a rational language) is a formal language that can be expressed using a regular expression.

Assume that $L=L_1.L_2$ is a regular language. Also assume that $L_1$ is regular. Is it true that $L_2$ is regular too ?

Note: Please prove it using a DFA. I know how to do that with regular expressions. Thank you in advance.

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No. The empty language $\emptyset$ is regular and for every language $L$, $\emptyset L = \emptyset$ is also regular. This does not imply that $L$ is regular.

Edit. To answer your comment, here is another counterexample. Let $A$ be the alphabet. Then, for every language $L$ containing the empty word, $A^*L = A^*$. This does not imply that $L$ is regular.

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  • $\begingroup$ Assume that $L_1$ is not empty. then what ? $\endgroup$ – Arman Malekzadeh Apr 20 '16 at 17:45
  • $\begingroup$ Also, i don't get it ... why is $\lambda.L=\lambda $?? i thought it should be equal to $L$!!! $\endgroup$ – Arman Malekzadeh Apr 20 '16 at 17:48
  • $\begingroup$ What does denote $\lambda$ for you? You seem to confuse the empty language and the language reduced to the empty word. $\endgroup$ – J.-E. Pin Apr 20 '16 at 18:43

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