Is CFL $\cap$ DCFL = CFL, always true?

CFL - Any Context Free Language

DCFL - Any Deterministic Context Free Language

  • $\begingroup$ Your notation is off. $\mathrm{CFL} \cap \mathrm{DCFL}$ would denote the class of languages which are both context-free and deterministic context free. $\endgroup$
    – Arno
    Commented Nov 19, 2022 at 12:03

1 Answer 1


Even the intersection of two deterministic context-free languages may not be context-free.

Consider $L_1 = \{a^nb^nc^m\mid n,m\geqslant 0\}$ and $L_2 = \{a^nb^mc^m\mid n,m\geqslant 0\}$. Both languages are DCFL, but their intersection is $\{a^nb^nc^n\mid n\geqslant 0\}$ which is not context-free.


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