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White’s formula A closed DNA molecule modeled as a ribbon has a twist (Tw) with respect to its central line (the line of centroids) and a writhe (Wr) which represents the contortion of the central line in space. It was shown by J.H. White, that Tw + Wr = linking number, a topological invariant. White’s formula relates DNA structure to knot theory (see also DNA supercoil; cf. J.H. White, Am. J. Math., 91, 693 (1969)).