Complex Analysis : Applications of Harmonic Functions

Study concepts, example questions & explanations for Complex Analysis

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Example Questions

Example Question #53 : Complex Analysis

If \displaystyle f(z) = u(x,y) + iv(x,y) and \displaystyle \overline{f(z)} = u(x,y) - iv(x,y) are both analytic through a domain \displaystyle D, which of the following is true? 

Possible Answers:

\displaystyle f(z) = 0

None of the other answers

\displaystyle f(z) is nowhere differentiable

\displaystyle f(z) is nowhere analytic

\displaystyle f(z) must be constant throughout \displaystyle D

Correct answer:

\displaystyle f(z) must be constant throughout \displaystyle D

Explanation:

Since \displaystyle f(z) = u(x,y) + iv(x,y) is analytic, the Cauchy-Riemann Equations give us \displaystyle u_x(x,y) = v_y(x,y) \text{ and } u_y(x,y) = -v_x(x,y) \forall x,y 

Since \displaystyle \overline{f(z)} = u(x,y) - iv(x,y) is analytic, the Cauchy-Riemann Equations give us \displaystyle u_x(x,y) = -v_y(x,y) \text{ and } u_y(x,y) = v_x(x,y) \forall x,y.

Putting these two results together gives us \displaystyle u_x(x,y) = 0 \text{ and } u_y(x,y) = 0 \forall x,y

which implies that \displaystyle u(x,y) is constant. Similar reasoning gives is that \displaystyle v(x,y) is constant. This implies that \displaystyle f(z)must be constant throughout \displaystyle D.

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