5. Alternative definitions of C 17
z = x + iy
w = a + ib
z + w
Figure 1.5. Addition of complex numbers.
to be the set of expressions of the form a + ib for real numbers a,b. We add and
multiply as expected, using the distributive law; then we set
i2
equal to -1. Thus
(15) (x + iy) + (a + ib) = (x + a) + i(y + b),
(16) (x + iy) * (a + ib) = xa + i(ya + xb) +
i2(yb)
= (xa - yb) + i(ya + xb).
Equations (15) and (16) give the same results as (12) and (13). While this new
approach is more elegant, it makes some readers feel uneasy. After all, we are
assuming the existence of an object, namely 0 + i1, whose square is -1. In the
first approach we never assume the existence of such a thing, but such a thing does
exist: the square of (0, 1) is (-1, 0), which is the additive inverse of (1, 0).
The reader will be on safe logical ground if he or she regards the above para-
graph as an abbreviation for the previous discussion. In the next section we will
give two additional equivalent ways of defining C.
I
Exercise 1.23. Prove the distributive law for addition and multiplication, as
defined in (12) and (13). Do the same using (15) and (16). Compare.
The next lemma reveals a crucial difference between R and C.
Lemma 4.1. The complex numbers do not form an ordered field.
Proof. Assume that a positive subset P exists. By Lemma 3.1, each nonzero
square is in P. Since 12 = 1 and i2 = -1, both 1 and -1 are squares and hence
must be positive, contradicting 2) of Definition 3.1.
5. Alternative definitions of C
In this section we discuss alternative approaches to defining C. We use some
basic ideas from linear and abstract algebra that might be new to many students.
The primary purpose of this section is to assuage readers who find the rules (12)
and (13) unappealing but who find the rules (15) and (16) dubious, because we
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