The regular expression equivalence problem is decidable. The C++ program equivalence problem is decidable. Every language gencrated by an unambiguous context-free grammar is accepted by some DPDA. The language {a"a"d" | n > 0} is recursive Let L be the language over fa, b,c consisting of all strings which have more a's than b's and more b's than c's. There is some PDA that accepts L The context-sensitive membership problem is decidable. The language sa"b" | n 201 is context-free. The language (a"b"e" | n20 is context-free. -The language {ai bek I j = 1 + k} is context-free. The intersection of any three regular languages is context-frec