The English term "cross-ratio" was introduced in 1878 by William Kingdon Clifford.
William Kingdon Clifford (1873), "Preliminary Sketch of Biquaternions", Paper XX, Mathematical Papers, p.
His 1902 paper in Mathematische Annalen recounts William Kingdon Clifford's construction of his 2n dimensional algebra with n − 1 anti-commuting square roots of −1.
Madigan's PhD was on the 19th century mathematician and philosopher William Kingdon Clifford, and he wrote a 2009 book about Clifford.
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Mathematical physicists James Cockle, William Kingdon Clifford, and Alexander Macfarlane had all employed in their writings an equivalent mapping of the Cartesian plane by operator (cosh a + r sinh a), where a is the hyperbolic angle and r 2 = +1.