The history of the discovery of imaginary unity as the square root of negative unity passed unavoidably through the solution of a depressed cubic polynomial equation. A bold assertion was made in 1494 by the Franciscan friar Luca Pacioli (1445-1514) that a solution of the cubic equation is as impossible as squaring the circle (constructing by straightedge and compass alone a square equal in area to a circle). Although squaring the circle was shown to be truly impossible by the year 1882, in 1504 Scipione del Ferro (1465-1526), a mathematician at the University of Bologna in Italy discovered a special case of the general cubic x+ax+bx+c=0 with the term ax=0. Therefore, the depressed cubic equation solved by del Ferro is x+ax=b where both a and b are non-negative.
The Polish-American mathematician Mark Kac (1914-84) answered the FAQ “How did del Ferro know to do this?” It was his famous distinction between the ordinary genius and the magician genius: “An ordinary genius is someone you and I would be just as good as, if we were only many times better….It is different with the magicians…the working of their minds is for all intends and purposes incomprehensible. Even after we understand what they have done….” Del Ferro’s idea was a typical example of the magician class of genius.
Reference: Paul J. Nahim, An Imaginary Tale: The Story of √-1, Princeton University Press, Princeton, NJ, 1998.


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