Definition of vectors for a polygonal curve. Definition of vectors used in the computation of the writhe number of two segments of a polygonal curve. Points 1 and 2 define the first segment, and 3 and 4 the second. The vectors r13, r14, r23, and r24, are translated so that they originate at the center of a unit sphere. The area A of the quadrangle they span is given by A = α+β+γ+δ-2π, where α is the size of the angle measured in radians.
Chang et al. BMC Bioinformatics 2006 7:346 doi:10.1186/1471-2105-7-346