1-
Morera’s Theorem
The following theorem can be considered a converse of
Cauchy’s Theorem and is often useful in proving that a
function is analytic:
Theorem (Morera).
Suppose
that f(z) is continuous in a domain D and
for every triangle T which together with its
interior lie in D. Then f(z) is analytic in
D.
To prove Morera’s Theorem we use the following lemma whose
proof is a slight modification of the proof of Cauchy’s
Theorem:
Lemma.
Suppose
that f(z) is continuous in a star domain D
and
for every triangle T which together with its
interior lie in D. Then there exists a function
F(z) analytic in D such that F’(z)=f(z).
Proof of Morera’s Theorem. Let
be any point in D.
Choose a positive number
r small enough so that the disk
Thus the star
domain
satisfies the hypothesis of the lemma and so there
is an analytic function F(z) such that F’(z)=f(z)
in
. Since
F(z) is analytic in
it has
analytic derivatives of all orders in
and, in
particular, F’(z)=f(z) is analytic in
. But
was an arbitrary point in D.
Consequently, f(z)
is analytic in D.
Corollary. Suppose
that f(z) is continuous in a domain D and analytic
at all points of D except possibly at the points of
some line
. Then f(z) is analytic on
D.
Proof. Rotating if necessary, we may assume that
is parallel to
the x-axis. Now the result follows from Morera’s
Theorem.
2- Analytic Continuation Principle
We state without proof the following useful:
Theorem (Analytic Continuation Principle). If two analytic functions f and g agree in a
neighborhood of a point of a domain D, then they
agree on D.
3- Reflection Principle
Theorem. Let
D be a domain of
C
symmetric with respect to the real axis. Let
,
,
and
.
Let g(z) be continuous on
, holomorphic on
G and
R.
Then there is a unique function
holomorphic on D
such that its restriction to
is g(z).
Proof. Consider the function h(z) defined on
by
. Since the function
is continuous, h(z)
is continuous on
. We now prove that
h(z)
is holomorphic on L by proving that it is
holomorphic at each
Now
. Since g(z) is
holomorphic on G, it is analytic there and so there
exists r>0 such that
and
.
Now for all
we
have
.
So by a limiting process we get
and therefore h(z) is analytic on L
and hence holomorphic there. Consider the function
on
D defined by
= g(z) if
and h(z) if
. By
hypothesis, g(z) is real on
. Thus
. So
is continuous on D and holomorphic on D
except possibly on
R.
Thus,
is holomorphic on D by the previous
corollary. Clearly,
extends g(z) and
is
unique by the Analytic Continuation Principle.
Example. Suppose that f(z) is an entire function that is
real-valued on an interval (a,b) of the real axis.
Put
if
and
if
. By the reflection principle F(z)
is analytic on the union D of the upper and lower
semi-planes and (a,b). Moreover, f(z) is
analytic on D and agrees with F(z)
for. Thus,
on D.
In particular,. Furthermore,
f(z) is continuous on
C
. Consequently, for any
R,
and therefore f(z) is real on the real axis. |