Let's say that L1 is regular and that L1∩L2 is regular as well.
What L2 can be?Is the only option regular? I have ruled out CF due to a theorem that says CF intersection Regular = Regular.
Furthermore,is it even worth trying to find a solution via the Venn diagram?Because I find odd that the union of 2 CF can be regular.
If L1 is defined on even length strings, and L2 is defined to include all even-length strings, and an arbitrary subset of the odd-length strings, then if L1 is regular, L1 $\cap$ L2 $=$ L1 has to be regular, but L2 can be arbitrary.