2 1. The Complex Numbers
where a, b and c are real numbers, formally has two solutions given by the quadratic
formula
(1.1.2) x =
−b ±

b2 4ac
2a
,
but these will not be real numbers if
b2
4ac is negative. If we could take square
roots of negative numbers, then the quadratic formula would give us solutions to
(1.1.1) for all choices of real coefficients a, b, c. To make this possible, we expand
the real number system in the following way, thus creating the complex number
system C.
Constructing C. We begin by adjoining a single new number to our old number
system R. We will denote it by i and declare it to be a square root of −1. Thus,
i2
= −1.
Our new number system is to contain both R and the new number i and it should be
closed under addition and multiplication. If it is to be closed under multiplication,
we need a number iy for every real number y. Likewise, if it is to be closed under
addition, there should be a number x + iy in our new number system for each pair
of real numbers (x, y). It turns out that this is enough. If we define the set of
complex numbers C to be the set of all symbols of the form x + iy where (x, y) is
a pair of real numbers, and if we define addition and multiplication appropriately,
then the resulting number system is a field in which every polynomial equation has
a root. We will be a long time proving the latter half of this statement, but it is
not hard to prove the first part.
To define the operations of addition and multiplication in C, we begin by noting
that, as a set, C may be identified with
R2
the set of all pairs (x, y) of real
numbers. Obviously, each pair (x, y) determines a symbol x + iy and vice versa.
This identification makes C into a vector space over R and gives us operations of
addition and scalar multiplication by reals which satisfy the usual associative and
distributive rules. The resulting operation of addition is
(x1
+ iy1) + (x2 + iy2) = (x1 + x2) + i(y1 + y2).
It remains to define a product on C.
We have already declared that i2 = −1. If we also require that the associative
and distributive laws of multiplication should hold and that the multiplication of
real numbers should remain as before, then the product of two complex numbers
x1 + iy1 and x2 + iy2 must be
(x1 +iy1)(x2 +iy2) = x1x2 +ix1y2 +iy1x2
+i2y1y2
= (x1x2 −y1y2)+i(x1y2 +y1x2).
We formalize this conclusion in the following definition.
Definition 1.1.1. We define the system C of complex numbers to be the set of
all symbols of the form x + iy with (x, y)
R2,
with addition and multiplication
defined by
(x1 + iy1) + (x2 + iy2) = (x1 + x2) + i(y1 + y2)
and
(x1 + iy1)(x2 + iy2) = (x1x2 y1y2) + i(x1y2 + y1x2).
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