An Imaginary Tale: The Story of ?-1 by Paul J. Nahin

By Paul J. Nahin

Today complicated numbers have such frequent functional use--from electric engineering to aeronautics--that few humans could anticipate the tale at the back of their derivation to be jam-packed with event and enigma. In An Imaginary Tale, Paul Nahin tells the 2000-year-old historical past of 1 of mathematics' such a lot elusive numbers, the sq. root of minus one, sometimes called i. He recreates the baffling mathematical difficulties that conjured it up, and the colourful characters who attempted to resolve them.

In 1878, while brothers stole a mathematical papyrus from the traditional Egyptian burial website within the Valley of Kings, they led students to the earliest recognized incidence of the sq. root of a unfavourable quantity. The papyrus provided a selected numerical instance of ways to calculate the amount of a truncated sq. pyramid, which implied the necessity for i. within the first century, the mathematician-engineer Heron of Alexandria encountered I in a separate undertaking, yet fudged the mathematics; medieval mathematicians stumbled upon the concept that whereas grappling with the which means of destructive numbers, yet disregarded their sq. roots as nonsense. by the point of Descartes, a theoretical use for those elusive sq. roots--now known as "imaginary numbers"--was suspected, yet efforts to resolve them ended in excessive, sour debates. The infamous i eventually gained reputation and was once positioned to take advantage of in complicated research and theoretical physics in Napoleonic times.

Addressing readers with either a normal and scholarly curiosity in arithmetic, Nahin weaves into this narrative interesting old proof and mathematical discussions, together with the appliance of complicated numbers and capabilities to special difficulties, equivalent to Kepler's legislation of planetary movement and ac electric circuits. This ebook might be learn as an interesting historical past, virtually a biography, of 1 of the main evasive and pervasive "numbers" in all of mathematics.

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Extra resources for An Imaginary Tale: The Story of ?-1

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Vi`ete’s next step was to suppose that one can always find a such that x ϭ 2a cos( ). I’ll now show you that this supposition is in fact true by actually calculating the required value of . From the supposition we have cos( ) ϭ x/2a, and if this is substituted into the above trigonometric identity then you can quickly show that x3 ϭ 3a2x ϩ 2a3cos(3 ). But this is just the cubic we are trying to solve if we write 2a3cos(3 ) ϭ a2b. That is, 22 THE PUZZLES OF IMAGINARY NUMBERS θ= 1 b cos −1   .  2a  3 into x ϭ 2a cos ( ) immediately gives us the Inserting this result for solution b  1 x = 2 a cos cos −1     3 2 a   or, in terms of p and q, x=2  3 3q   p  1 cos cos −1   .

Now, if you look at the original cubic long enough, perhaps you’ll have the lucky thought that x ϭ 2 works (8 ϩ 12 ϭ 20). So could that complicatedlooking thing with all the radical signs that I just wrote actually be 2? Well, yes, it is. 7320508 = 2. Then, to find the other two roots to f (x) ϭ 0 ϭ x3 ϩ 6x Ϫ 20, we use the fact that one factor of f (x) is (x Ϫ 2) to find, with some long division, that (x Ϫ 2)(x2 ϩ 2x ϩ 10) ϭ x3 ϩ 6x Ϫ 20. Applying the quadratic formula to the quadratic factor quickly gives the two complex roots (solutions to the original cubic) of r2 ϭ Ϫ1 ϩ 3͙Ϫ1 and r3 ϭ Ϫ1 Ϫ 3͙Ϫ1.

The first of these two statements says that 2 ϩ ͙Ϫ121 ϭ (a ϩ b͙Ϫ1)3. 2 The Irreducible Case Means There Are Three Real Roots To study the nature of the roots to x3 ϭ px ϩ q, where p and q are both non-negative, consider the function f (x) ϭ x3 Ϫ px Ϫ q. , the local extrema of the depressed cubic that can lead to the irreducible case are symmetrically located about the vertical axis. The values of f (x) at these two local extrema are, if we denote them by M1 and M2, M1 = 2 p p p p −p −q=− p − q, at x = + , 3 3 3 3 3 p 3 M2 = − p 3 2 p p p p +p −q= p − q, at x = − .

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