Famously the intersection of context-free languages need not be context-free. On the other hand the intersection of context-sensitive languages is context-sensitive.

So this leads to the question: what is the closure of context-free languages under finite intersections? Does this class of formal languages have a name? Do we get all context-sensitive languages this way?


2 Answers 2


The intersections of context-free languages will never reach the full family of context-sensitive languages.

The class of languages expressible as the intersection of $k$ context-free languages is shown to be properly contained within the class of languages expressible as the intersection of $k + 1$ context-free languages. Hence an infinite hierarchy of classes of languages is exhibited between the class of context-sensitive languages and the class of context-free languages.

Quoted from: An infinite hierarchy of intersections of context-free languages. Leonard Y. Liu & Peter Weiner Mathematical Systems Theory 7 (1973) 185–192. https://doi.org/10.1007/BF01762237 (paywall)

An explicit example is also given, with a reference to the PhD thesis of the first author.

not every context-sensitive language is expressible as an intersection of a finite number of context-free languages, e.g., $L = \{ a^{2^n} \mid n \in \mathbb N \}$.

Note however, that context-free languages over a one letter alphabet are in fact regular. That means that if a finite number of CF languages intersects to a language in $\{a\}^*$, then we can as well assume each of these languages to be within $\{a\}^*$. This implies we intersect a finite number of regular languages, which is regular again.


Intersections (and more generally, Boolean combinations) of context-free languages are included in the class LOGCFL, which is a quite weak subclass of P (on a low level of the NC hierarchy). Thus, it is very far from the class of all context-sensitive languages. In particular, $\mathrm{LOGCFL\subseteq DSPACE}((\log n)^2)$, while $\mathrm{CSL=NSPACE}(n)$; the space hierarchy theorem ensures that these classes are different, and that there are many intermediate levels of conplexity in between them.


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