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Odious Suspect
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Mod note: Fixed the broken LaTeX in the following, and edited the difference quotient definition of f'(z).
The following is from a mathematical introduction to a physics book:
"The real part u and the imaginary part v of w=u+i v are functions of the two variables x and y. Nevertheless, two arbitrary functions u(x,y) and v(x,y) cannot, in general, be considered to be the real and imaginary parts of a function of a complex variable. If the functions u and v originate in a complex function, they satisfy certain special conditions. If we denote by f'(z) the derivative of the function f with respect to its argument, then, since u+i v=f(x+i y)=f(z),
[itex] \frac{\partial u}{\partial x}+ i \frac{\partial v}{\partial x}=f'(z)\frac{\partial z}{\partial x}=f'(z) [/itex] ;
[itex] \frac{\partial u}{\partial y}+\imath\frac{\partial v}{\partial y}=f'(z)\frac{\partial z}{\partial y}=\imath f'(z) [/itex].
Comparison gives the Cauchy-Riemann Differential Equations
[itex]\frac{\partial u}{\partial x}=\frac{\partial v}{\partial y}[/itex]
[itex]\frac{\partial u}{\partial y}=-\frac{\partial v}{\partial x}[/itex]."
All of this makes sense to me except what [itex]f'(z)[/itex] means.
Simply doing what I do with functions of real numbers, I write:
[tex]z=x+\imath y[/tex]
[tex]f(z)=u+\imath v[/tex]
[tex]f'(z)=\lim_{\Delta z\to 0} \frac{f(z + \Delta z)-f (z)}{\Delta z}[/tex]
But [itex]\Delta z[/itex] seems to be an infinite set of small displacements of the form [itex]\Delta x+\imath \Delta y[/itex]. What the book seems to be saying is that a continuously differentiable complex function of a complex argument is not simply a continuously differentiable mapping of a region of [itex]R^2[/itex] onto another region of [itex]R^2[/itex]. There are some other conditions implied.
So my question is, what is meant by f'(z)?
The following is from a mathematical introduction to a physics book:
"The real part u and the imaginary part v of w=u+i v are functions of the two variables x and y. Nevertheless, two arbitrary functions u(x,y) and v(x,y) cannot, in general, be considered to be the real and imaginary parts of a function of a complex variable. If the functions u and v originate in a complex function, they satisfy certain special conditions. If we denote by f'(z) the derivative of the function f with respect to its argument, then, since u+i v=f(x+i y)=f(z),
[itex] \frac{\partial u}{\partial x}+ i \frac{\partial v}{\partial x}=f'(z)\frac{\partial z}{\partial x}=f'(z) [/itex] ;
[itex] \frac{\partial u}{\partial y}+\imath\frac{\partial v}{\partial y}=f'(z)\frac{\partial z}{\partial y}=\imath f'(z) [/itex].
Comparison gives the Cauchy-Riemann Differential Equations
[itex]\frac{\partial u}{\partial x}=\frac{\partial v}{\partial y}[/itex]
[itex]\frac{\partial u}{\partial y}=-\frac{\partial v}{\partial x}[/itex]."
All of this makes sense to me except what [itex]f'(z)[/itex] means.
Simply doing what I do with functions of real numbers, I write:
[tex]z=x+\imath y[/tex]
[tex]f(z)=u+\imath v[/tex]
[tex]f'(z)=\lim_{\Delta z\to 0} \frac{f(z + \Delta z)-f (z)}{\Delta z}[/tex]
But [itex]\Delta z[/itex] seems to be an infinite set of small displacements of the form [itex]\Delta x+\imath \Delta y[/itex]. What the book seems to be saying is that a continuously differentiable complex function of a complex argument is not simply a continuously differentiable mapping of a region of [itex]R^2[/itex] onto another region of [itex]R^2[/itex]. There are some other conditions implied.
So my question is, what is meant by f'(z)?
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