Operations on Automata with All States Final
  We study the complexity of basic regular operations on languages represented by incomplete deterministic or nondeterministic automata, in which all states are final. Such languages are known to be prefix-closed. We get tight bounds on both incomplete and nondeterministic state complexity of complement, intersection, union, concatenation, star, and reversal on prefix-closed languages. 
Krist\'ina \v{C}evorov\'a, Galina Jir\'askov\'a, Peter   Mlyn\'ar\v{c}ik, Mat\'u\v{s} Palmovsk\'y, Juraj \v{S}ebej
cs.FL