Complex Analysis - Maths KU

184 Chapter 4 Elementary Functions
That is,
w = log e 2+(2n +1)πi, n =0, ±1, ±2,... .
Each value of w satisfies the equation e w = −2.
Logarithmic Identities Definition 4.2 can be used to prove that
the complex logarithm satisfies the following identities, which are analogous
to identitiesfor the real logarithm.
Theorem 4.3 Algebraic Properties of ln z
If z1 and z2 are nonzero complex numbersand n isan integer, then
(i) ln(z1z2) =lnz1 +lnz2
� �
z1
(ii) ln =lnz1− ln z2
z2
(iii) lnzn 1 = n ln z1.
Proof of (i) By Definition 4.2,
ln z1 +lnz2 = loge |z1| + i arg (z1) + loge |z2| + i arg (z2)
= loge |z1| + loge |z2| + i (arg (z1) + arg (z2)) . (13)
Because the real logarithm has the property log e a + log e b = log e (ab) for
a>0 and b>0, we can write log e |z1z2| = log e |z1| + log e |z2|. Moreover,
from (8) of Section 1.3, we have arg (z1) + arg (z2) = arg (z1z2). Therefore,
(13) can be rewritten as:
ln z1 +lnz2 = log e |z1z2| + i arg (z1z2) = ln (z1z2) . ✎
Proofsof Theorems4.3(ii) and 4.3(iii) are similar. See Problems 53 and
54 in Exercises 4.1.
Principal Value of a Complex Logarithm It isinteresting
to note that the complex logarithm of a positive real number has infinitely
many values. For example, the complex logarithm ln 5 isthe set of values
1.6094 + 2nπi, where n isany integer, whereasthe real logarithm log e 5 has
a single value: log e 5 ≈ 1.6094. The unique value of ln 5 corresponding to
n = 0 isthe same asthe value of the real logarithm log e 5. In general, this
value of the complex logarithm iscalled the principal value of the complex
logarithm since it is found by using the principal argument Arg(z) in place of
the argument arg(z) in (11). We denote the principal value of the logarithm