Sentence:  Kotel'nikov and Study showed, the dual distance between two points of this sphere representing two lines, forming an angle \varphi and having shortest distance d, is equal to the <WSD>dual number</WSD> \varphi + \epsilon d, while the motions of Euclidean space are represented by rotations of the dual sphere, and consequently depend on three dual parameters. 
Meaning of the word marked with <WSD> tags:  An element of an algebra (the "algebra of dual numbers") which includes the real numbers and an element &epsilon; which satisfies &epsilon; ≠ 0 and &epsilon;&sup2; = 0.