  The entanglement quantified by negativity of pure bipartite superposed states
is studied. We show that if the entanglement is quantified by the concurrence
two pure states of high fidelity to one another still have nearly the same
entanglement. Furthermore this conclusion can be guaranteed by our obtained
inequality, and the concurrence is shown to be a continuous function even in
infinite dimensions. The bounds on the negativity of superposed states in terms
of those of the states being superposed are obtained. These bounds can find
useful applications in estimating the amount of the entanglement of a given
pure state.
