This is a follow-up to this question, which asks for an example of a non-regular language $L$ which satisfies the following condition, intersection resilience:

If $L'$ is context-free then so is $L \cap L'$.

We know that all regular languages are intersection-resilient, and clearly every intersection-resilient language is context-free. Ginsburg and Spanier showed in 1964, in their paper Bounded ALGOL-like languages, that every context-free subset of $x^*y^*$ is intersection-resilient (Theorem 6.1, Corollary 2).

Which languages are intersection-resilient?

  • 1
    $\begingroup$ A related question is whether it is decidable whether a given context-free language is intersection-resilient. Another related question is whether the intersection of an intersection-resilient language and an context-free language is intersection-resilient. Note that the intersection of two intersection-resilient languages is intersection-resilient. $\endgroup$
    – John L.
    Commented May 29, 2019 at 15:44

1 Answer 1


Just a few notes (I'll try to add the proofs later).

Intersection-resilience seems closed under homomorphism.

A necessary condition for intersection-resilience is that for all homomorphisms $\varphi : \Sigma \to \Sigma \cup\ \{\epsilon \}$; $\varphi(L) \cap x^*y^*x^*$ must be a regular language (where $x$ and $y$ are any two distinct symbols of $\Sigma$).

So the result of Ginsburg and Spanier seems "tight" (if we use the same homomorphism trick).


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