“…In this paper, problems are investigated involving three given languages R, L 1 , L 2 , and the goal is to determine decidability and complexity of testing if R ⊆ L 1 L 2 , L 1 L 2 ⊆ R, and L 1 L 2 = R, depending on the language families of L 1 , L 2 and R. In Section 3, it is demonstrated that the following three problems are NP-complete: to determine, given an NFA M and two words u, v whether u v ⊆ L(M ) is true, L(M ) ⊆ u v is true, and u v = L(M ) is true. Then, the DFA algorithm from [16] that can output a "candidate solution" is extended to an algorithm on NFAs that operates in polynomial time, and outputs two words u, v such that if the NFA is decomposable into the shuffle of words, then u v is the unique solution. And in Section 4, decidability and the complexity of testing if L 1 L 2 ⊆ R is investigated involving more general language families.…”