## Motivation

Consider the following languages, are they context-free?

- $ \{x \# y: x \neq y\}$
- $ \{x y: |x|=|y|, x \neq y\}$
- $ \{x \# y: |x|=|y|, x \neq y\}$
- $ \{x y: |x|=|y|,d(x,y)>1\}$

The first three are explained here, the last one is this question.

I’m wondering whether there is an algorithm to solve this kind of problem in general.

## Question

Given two strings $ x,y$ , let $ \operatorname{zip}(x,y)$ denote the string $ (x_1,y_1)(x_2,y_2)\dots(x_n,y_n)$ . Note that letters in $ \operatorname{zip}(x,y)$ are pairs. If one string is shorter, we pad it with an extra blank symbol.

Is the following problem decidable?

Given a regular language $ L$ , is $ \{x y: \operatorname{zip}(x,y) \in L\}$ context-free?

If the answer is positive, we can solve the four problems from the motivation section by picking a suitable $ L$ .