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.The
solutions we know today as,
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
.
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
"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
.Using
and
in the "Ferro-Tartaglia" formula for the depressed cubic, we compute
,
or in a somewhat different form,
.
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 asandfor
some real numbers u and v.That is, Bombeli believed
and,which
would meanand.Then,
using the well-known algebraic identity
,
and (letting
and
),and
assuming that roots of negative numbers obey the rules of algebra, he obtained
.
By equating like parts, Bombelli reasoned thatand.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 equationled
Bombelli to conclude thatand.From
this conclusion it follows that,
or.
Amazingly,
and
solve
the second equation
,
so Bombelli declared the values for u and v to be u=2 and v=1, respectively.
Since
,
we clearly have
.Similarly,
Bombelli showed that
.
But this means that
,
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
was a correct solution to the equationwas
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.
Enter the solution for the cubic equation.
Use the Ferro-Tartaglia formulaand
verify thatzit is a solution.
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
to make the following discussion easier to follow.When we use the quadratic
formula with this equation, we get
and
.
Wallis imagined these solutions as displacements to the left and right
from the point
.
He saw each displacement, whose value is
,
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.But
how should we picture P � and P � when negative roots arise (i.e., when
)?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
solutionsandwhen.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.
Wallis's method has the undesirable consequence thatis
represented by the same point as is.
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.You
will learn shortly that these solutions can be expressed asfor
various values of;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
for
.
Today, this notation is still the most popular, although some electrical
engineers prefer the symbol
instead so that they can use
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
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 numbersso
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.
|