Completable nilpotent Lie algebra
THE theory of nilpotent Lie algebra is very important in the theory of finite-dimensional Lie al-gebras. Because of its extraordinary complexity, one usually studies various classes of specialnilpotent Lie algebras. In the study of complete Lie algebras, a class of special nilpotent Lie al-gebras (called completable nilpotent Lie algebras) was discovered. In this letter, we will study
We continue the study of bottom-up unranked tree automata with equality and ::: disequality constraints between direct subtrees. In particular, we show that ::: the emptiness problem for the nondeterministic automata is decidable. In ::: addition, we show that the universality problem, in contrast, is undecidable.