“…Virtual knots were introduced by Kauffman [11] as a generalization of classical knot theory, and since then many invariants have been developed to help distinguish virtual knots, and to determine when a virtual knot is equivalent to a classical knot. In the past few years, several authors have developed invariants that generalize the classical writhe of a knot [3,4,6,9,12,13,17]. These invariants have been used to define Vassiliev invariants of virtual knots [9,13], give bounds on the unknotting number (when it exists) and forbidden number of virtual knots [17,5], and distinguish mutant virtual knots [6], among other applications.…”