State Complexity of Finite Tree Width NFAs

We consider nondeterministic finite automata having finite tree width (ftw-NFA) where the computation on any input string has a constant number of branches. We give effective characterizations of ftw-NFAs and a tight worst-case state size bound for determinizing an ftw-NFA $A$ as a function of the tree width and the number of states of $A$. We also introduce a lower bound technique for ftw-NFAs.