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The Origin of Complex Numbers

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The Origin of Complex Numbers

So, by using Tartaglia's work and a clever transformation technique, Cardano was able to crack what had seemed to be the impossible task of solving the general cubic equation.Surprisingly, this development played a significant role in helping to establish the legitimacy of imaginary numbers.Roots of negative numbers, of course, had come up earlier in the simplest of quadratic equations, such as[Graphics:Images/ComplexNumberOrigin_gr_47.gif].The solutions we know today as[Graphics:Images/ComplexNumberOrigin_gr_48.gif], however, were easy for mathematicians to ignore.In Cardano's time, negative numbers were still being treated with some suspicion, as it was difficult to conceive of any physical reality corresponding to them. Taking square roots of such quantities was surely all the more ludicrous. Nevertheless, Cardano made some genuine attempts to deal with [Graphics:Images/ComplexNumberOrigin_gr_49.gif]. Unfortunately, his geometric thinking made it hard to make much headway. At one point he commented that the process of arithmetic that deals with quantities such as [Graphics:Images/ComplexNumberOrigin_gr_50.gif] "involves mental tortures and is truly sophisticated." At another point he concluded that the process is "as refined as it is useless." Many mathematicians held this view, but finally there was a breakthrough.

In his 1572 treatise L'Algebra, Rafael Bombelli showed that roots of negative numbers have great utility indeed. Consider the depressed cubic [Graphics:Images/ComplexNumberOrigin_gr_51.gif].Using [Graphics:Images/ComplexNumberOrigin_gr_52.gif] and [Graphics:Images/ComplexNumberOrigin_gr_53.gif] in the "Ferro-Tartaglia" formula for the depressed cubic, we compute [Graphics:Images/ComplexNumberOrigin_gr_54.gif], or in a somewhat different form, [Graphics:Images/ComplexNumberOrigin_gr_55.gif].

Simplifying this expression would have been very difficult if Bombelli had not come up with what he called a "wild thought."He suspected that if the original depressed cubic had real solutions, then the two parts of x in the preceding equation could be written as[Graphics:Images/ComplexNumberOrigin_gr_56.gif]and[Graphics:Images/ComplexNumberOrigin_gr_57.gif]for some real numbers u and v.That is, Bombeli believed[Graphics:Images/ComplexNumberOrigin_gr_58.gif] and[Graphics:Images/ComplexNumberOrigin_gr_59.gif],which would mean[Graphics:Images/ComplexNumberOrigin_gr_60.gif]and[Graphics:Images/ComplexNumberOrigin_gr_61.gif].Then, using the well-known algebraic identity [Graphics:Images/ComplexNumberOrigin_gr_62.gif], and (letting [Graphics:Images/ComplexNumberOrigin_gr_63.gif] and [Graphics:Images/ComplexNumberOrigin_gr_64.gif]),and assuming that roots of negative numbers obey the rules of algebra, he obtained

[Graphics:Images/ComplexNumberOrigin_gr_65.gif] [Graphics:Images/ComplexNumberOrigin_gr_66.gif]
[Graphics:Images/ComplexNumberOrigin_gr_67.gif][Graphics:Images/ComplexNumberOrigin_gr_68.gif]
[Graphics:Images/ComplexNumberOrigin_gr_69.gif]
[Graphics:Images/ComplexNumberOrigin_gr_70.gif]
[Graphics:Images/ComplexNumberOrigin_gr_71.gif].

By equating like parts, Bombelli reasoned that[Graphics:Images/ComplexNumberOrigin_gr_72.gif]and[Graphics:Images/ComplexNumberOrigin_gr_73.gif].Perhaps thinking even more wildly, Bombelli then supposed that u and v were integers.The only integer factors of 2 are 2 and 1, so the equation[Graphics:Images/ComplexNumberOrigin_gr_74.gif]led Bombelli to conclude that[Graphics:Images/ComplexNumberOrigin_gr_75.gif]and[Graphics:Images/ComplexNumberOrigin_gr_76.gif].From this conclusion it follows that[Graphics:Images/ComplexNumberOrigin_gr_77.gif], or[Graphics:Images/ComplexNumberOrigin_gr_78.gif]. Amazingly, [Graphics:Images/ComplexNumberOrigin_gr_79.gif] and [Graphics:Images/ComplexNumberOrigin_gr_80.gif]solve the second equation [Graphics:Images/ComplexNumberOrigin_gr_81.gif], so Bombelli declared the values for u and v to be u=2 and v=1, respectively.

Since [Graphics:Images/ComplexNumberOrigin_gr_82.gif], we clearly have [Graphics:Images/ComplexNumberOrigin_gr_83.gif].Similarly, Bombelli showed that [Graphics:Images/ComplexNumberOrigin_gr_84.gif]. But this means that

[Graphics:Images/ComplexNumberOrigin_gr_85.gif],

which was a proverbial bombshell.Prior to Bombelli, mathematicians could easily scoff at imaginary numbers when they arose as solutions to quadratic equations.With cubic equations, they no longer had this luxury. That [Graphics:Images/ComplexNumberOrigin_gr_86.gif] was a correct solution to the equation[Graphics:Images/ComplexNumberOrigin_gr_87.gif]was indisputable, as it could be checked easily.However, to arrive at this very real solution, mathematicians had to take a detour through the uncharted territory of "imaginary numbers."Thus, whatever else might have been said about these numbers (which, today, we call complex numbers), their utility could no longer be ignored.

Exploration.

[Graphics:../Images/ComplexNumberOrigin_gr_88.gif]


Enter the solution for the cubic equation.



[Graphics:../Images/ComplexNumberOrigin_gr_90.gif]
[Graphics:../Images/ComplexNumberOrigin_gr_91.gif]


Use the Ferro-Tartaglia formula[Graphics:../Images/ComplexNumberOrigin_gr_92.gif]and verify thatzit is a solution.

[Graphics:../Images/ComplexNumberOrigin_gr_93.gif]

[Graphics:../Images/ComplexNumberOrigin_gr_94.gif]



[Graphics:../Images/ComplexNumberOrigin_gr_95.gif]

[Graphics:../Images/ComplexNumberOrigin_gr_96.gif]



Admittedly, Bombelli's technique applies only to a few specialized cases, and lots of work remained to be done even if Bombelli's results could be extended.After all, today we represent real numbers geometrically on the number line.What possible representation could complex numbers have?In 1673 John Wallis made a stab at a geometric picture of complex numbers that comes close to what we use today.He was interested in representing solutions to general quadratic equations, which we can write as [Graphics:Images/ComplexNumberOrigin_gr_97.gif] to make the following discussion easier to follow.When we use the quadratic formula with this equation, we get

[Graphics:Images/ComplexNumberOrigin_gr_98.gif] and [Graphics:Images/ComplexNumberOrigin_gr_99.gif].

Wallis imagined these solutions as displacements to the left and right from the point [Graphics:Images/ComplexNumberOrigin_gr_100.gif]. He saw each displacement, whose value is [Graphics:Images/ComplexNumberOrigin_gr_101.gif], as the length of the sides of the right triangles shown in Figure 1.1. The points P � and P � represent the solutions to our equation, which is clearly correct if[Graphics:Images/ComplexNumberOrigin_gr_102.gif].But how should we picture P � and P � when negative roots arise (i.e., when [Graphics:Images/ComplexNumberOrigin_gr_103.gif])?Wallis reasoned that, with negative roots, b would be less than c, so the lines of length b in Figure 1.1 would no longer be able to reach all the way to the x axis.Instead, they would stop somewhere above it, as Figure 1.2 shows.Wallis argued that P � and P � should represent the geometric locations of the solutions[Graphics:Images/ComplexNumberOrigin_gr_104.gif]and[Graphics:Images/ComplexNumberOrigin_gr_105.gif]when[Graphics:Images/ComplexNumberOrigin_gr_106.gif].He evidently thought that, because b is shorter than c, it could no longer be the hypotenuse of the right triangle as it had been earlier. The side of length c would now have to take that role.

[Graphics:Images/ComplexNumberOrigin_gr_107.gif]

[Graphics:Images/ComplexNumberOrigin_gr_108.gif]


Wallis's method has the undesirable consequence that[Graphics:Images/ComplexNumberOrigin_gr_109.gif]is represented by the same point as is[Graphics:Images/ComplexNumberOrigin_gr_110.gif]. Nevertheless, this interpretation helped set the stage for thinking of complex numbers as "points on the plane." By 1732, the great Swiss mathematicianLeonhard Euler(pronounced "oiler") adopted this view concerning the n solutions to the equation[Graphics:Images/ComplexNumberOrigin_gr_111.gif].You will learn shortly that these solutions can be expressed as[Graphics:Images/ComplexNumberOrigin_gr_112.gif]for various values of[Graphics:Images/ComplexNumberOrigin_gr_113.gif];Euler thought of them as being located at the vertices of a regular polygon in the plane. Euler was also the first to use the symbol [Graphics:Images/ComplexNumberOrigin_gr_114.gif] for [Graphics:Images/ComplexNumberOrigin_gr_115.gif]. Today, this notation is still the most popular, although some electrical engineers prefer the symbol [Graphics:Images/ComplexNumberOrigin_gr_116.gif] instead so that they can use [Graphics:Images/ComplexNumberOrigin_gr_117.gif] to represent current.

Is it possible to modify slightly Wallis's picture of complex numbers so that it is consistent with the representation used today?To help you answer this question, refer to the article by Alec Norton and Benjamin Lotto, "Complex Roots Made Visible," The College Mathematics Journal, 15(3), June 1984, pp. 248--249, Jstor.

Two additional mathematicians deserve mention. The Frenchman Augustin-Louis Cauchy (1789--1857) formulated many of the classic theorems that are now part of the corpus of complex analysis.The German Carl Friedrich Gauss (1777--1855) reinforced the utility of complex numbers by using them in his several proofs of the fundamental theorem of algebra (see Chapter 6).In an 1831 paper, he produced a clear geometric representation of x+iy by identifying it with the point (x, y) in the coordinate plane. He also described how to perform arithmetic operations with these new numbers.

It would be a mistake, however, to conclude that in 1831 complex numbers were transformed into legitimacy. In that same year the prolific logician Augustus De Morgan commented in his book, On the Study and Difficulties of Mathematics, "We have shown the symbol [Graphics:Images/ComplexNumberOrigin_gr_118.gif] to be void of meaning, or rather self-contradictory and absurd. Nevertheless, by means of such symbols, a part of algebra is established which is of great utility."

There are, indeed, genuine logical problems associated with complex numbers. For example, with real numbers[Graphics:Images/ComplexNumberOrigin_gr_119.gif]so long as both sides of the equation are defined. Applying this identity to complex numbers leads to 1=√1=√((-1)(-1))=√(-1)√(-1)=-1.Plausible answers to these problems can be given, however, and you will learn how to resolve this apparent contradiction in Section 2.4. De Morgan's remark illustrates that many factors are needed to persuade mathematicians to adopt new theories. In this case, as always, a firm logical foundation was crucial, but so, too, was a willingness to modify some ideas concerning certain well-established properties of numbers.

As time passed, mathematicians gradually refined their thinking, and by the end of the nineteenth century complex numbers were firmly entrenched. Thus, as it is with many new mathematical or scientific innovations, the theory of complex numbers evolved by way of a very intricate process. But what is the theory that Tartaglia, Ferro, Cardano, Bombelli, Wallis, Euler, Cauchy, Gauss, and so many others helped produce? That is, how do we now think of complex numbers? We explore this question in the remainder of this chapter.

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