Proving Non-Existence of f'(z) for f(z) = e^x * e^-iy | Cauchy-Riemann Question

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In summary, The homework task is to show that the derivative of f(z) does not exist. The equation for f(z) is given as e^x * e^-iy. The approach suggested is to write f(z) as u+iv, where u and v are real functions. The question is how to write the given function in this form. The solution involves using deMoivre and the equation e^(it)=cos(t)+i*sin(t).
  • #1
tylerc1991
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Homework Statement


Show that f'(z) DNE

f(z) = e^x * e^-iy

Homework Equations



so I have to show that u_x =/= v_y or v_x =/= -u_y

The Attempt at a Solution



my question is this: what is u(x,y) and v(x,y)? is it e^x and e^-iy respectively? Thank you for your help!
 
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  • #2
You have to write f=u+iv where u and v are real functions. e^(-iy) isn't a real function. And there's a '*' between them, not a '+'.
 
  • #3
That is the basis of my question, how would I write that function in f(z) = u + iv? e^(x-iy)?
 
  • #4
tylerc1991 said:
That is the basis of my question, how would I write that function in f(z) = u + iv? e^(x-iy)?

Use deMoivre. e^(it)=cos(t)+i*sin(t).
 
  • #5
ahh I see, thank you!
 

1. What is the Cauchy-Riemann equation and why is it important?

The Cauchy-Riemann equation is a set of necessary and sufficient conditions for a complex-valued function to be differentiable at a point. It is important because it allows us to determine if a function is analytic (and therefore differentiable) at a given point on the complex plane.

2. Can you prove the non-existence of the derivative of a function using the Cauchy-Riemann equation?

Yes, the Cauchy-Riemann equation can be used to prove that the derivative of a function does not exist at a given point. If the Cauchy-Riemann equations are not satisfied at a point, then the function is not differentiable at that point and therefore does not have a derivative.

3. How do you apply the Cauchy-Riemann equation to prove the non-existence of the derivative of f(z) = e^x * e^-iy?

To apply the Cauchy-Riemann equation to this function, we can rewrite it in terms of its real and imaginary parts: f(z) = e^x * cos(y) - i * e^x * sin(y). Then, we can use the Cauchy-Riemann equations to show that the partial derivatives of the real and imaginary parts do not satisfy the equations, thus proving that the derivative of f(z) does not exist.

4. What are the implications of proving the non-existence of the derivative of a function?

Proving that the derivative of a function does not exist at a given point means that the function is not differentiable at that point. This can have implications for the continuity and smoothness of the function, as well as its use in other mathematical calculations.

5. Are there any other methods for proving the non-existence of a derivative?

Yes, there are other methods for proving that a derivative does not exist, such as using the definition of the derivative and showing that the limit does not exist, or using other necessary conditions for differentiability such as the existence of partial derivatives.

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