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Writhe

A knot property, also called the twist number, defined as the sum of crossings of a link ,

(1)

where defined to be if the overpass slants from top left to bottom right or bottom left to top right and is the set of crossings of an oriented link.
The writhe of a minimal knot diagram is not a knot invariant, as exemplified by the Perko pair, which have differing writhes (Hoste et al. 1998). This is because while the writhe is invariant under Reidemeister moves II and III, it may increase or decrease by one for a Reidemeister move of type I (Adams 1994, p. 153).
Thistlethwaite (1988) proved that if the writhe of a reduced alternating projection of a knot is not 0, then the knot is not amphichiral (Adams 1994).
A formula for the writhe is given by

(2)

where is parameterized by for along the length of the knot by parameter , and the frame associated with is