There are several types of unknotting algorithms. Other numerical knot invariants include the bridge number, linking number, stick number, and unknotting number. And type IA topoisomerase can catalyze catenation, decatenation, knotting and unknotting of the DNA. In general, it is relatively difficult to determine the unknotting number of a given knot. The unknotting number of a knot is always less than half of its crossing number.