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Unknotting Number

Another invariant is the unknotting number. The unknotting number is the least number of crossing changes necessary to turn a knot into the unknot. By "crossing changes" we mean changing the orientation of two strings where they cross. The trefoil knot can be unknotted by changing only one crossing from under to over, so the trefoil knot has an unknotting number of 1.

Supriya Garg 2004-05-02