Quotients of Context-Free Languages
Seymour Ginsburg, Edwin H. Spanier · Journal of the ACM · 1963
The following results on the quotient of context-free languages (CFL) are shown: (1) It is reeursively unsolvable to determine for arbitrary CFL whether the quotient of one by another is a CFL.(2) If either set is regular and the other is a CFL, then the quotient is a CFL.