MHB Understanding Theorem 3.4: Mean Value Theorem & Cauchy Riemann Equations

Math Amateur
Gold Member
MHB
Messages
3,920
Reaction score
48
I am reading "Complex Analysis for Mathematics and Engineering" by John H. Mathews and Russel W. Howell (M&H) [Fifth Edition] ... ...

I am focused on Section 3.2 The Cauchy Riemann Equations ...

I need help in fully understanding the Proof of Theorem 3.4 ...The start of Theorem 3.4 and its proof reads as follows:View attachment 9350In the above proof by Mathews and Howell we read the following:

" ... ... The partial derivatives $$u_x$$ and $$u_y$$ exist, so the mean value theorem for real functions of two variables implies that a value $$x*$$ exists between $$x_0$$ and $$x_0 + \Delta x$$ such that we can write the first term in brackets on the right side of equation (3-17) as

$$ u(x_0 + \Delta x, y_0 + \Delta y) - u(x_0, y_0 + \Delta y) = u_x(x*, y_0 + \Delta y) \Delta x $$ ... ... "

Can someone please explain how exactly the mean value theorem for real functions of two variables implies that a value $$x*$$ exists between $$x_0$$ and $$x_0 + \Delta x$$ such that we can write the first term in brackets on the right side of equation (3-17) as

$$u(x_0 + \Delta x, y_0 + \Delta y) - u(x_0, y_0 + \Delta y) = u_x(x*, y_0 + \Delta y) \Delta x$$ ... ...
Peter[ NOTE ... ... In Wendell Fleming's book: "Functions of Several Variables" (Second Edition) the Mean Value Theorem reads as follows:View attachment 9351... ... ]
 

Attachments

  • M&H - 1 - Theorem 3.1 ... .PART 1 ... .png
    M&H - 1 - Theorem 3.1 ... .PART 1 ... .png
    49.7 KB · Views: 155
  • Fleming - Mean Value Theorem ... .png
    Fleming - Mean Value Theorem ... .png
    13.5 KB · Views: 144
Last edited:
Physics news on Phys.org
The Mean Value Theorem for real functions of two variables states that if a function f(x,y) is continuous on some rectangle R and differentiable on the interior of R then there exists some point (x*,y*) in the interior of R such that f(x_2,y_2)-f(x_1,y_2) =f_x(x*,y_2)(x_2-x_1)andf(x_2,y_2)-f(x_2,y_1)=f_y(x_2,y*)(y_2-y_1). In our case, we are dealing with a function u(x,y) which is assumed to be differentiable on some region. Applying the Mean Value Theorem to this function, we see that there exists some point (x*,y_0+\Delta y) in the interior of the region such that u(x_0 + \Delta x, y_0 + \Delta y) - u(x_0, y_0 + \Delta y) = u_x(x*, y_0 + \Delta y) \Delta x. We can see from this that x* must lie between x_0 and x_0+\Delta x, since otherwise the right side of the equation would not be equal to the left side.
 


Hi Peter,

The mean value theorem for real functions of two variables states that if a function f(x,y) is continuous on a closed and bounded region R and has continuous partial derivatives u_x and u_y at every point in R, then there exists a point (x*, y*) in R such that:

f(x_2,y_2) - f(x_1,y_1) = u_x(x*, y*) (x_2 - x_1) + u_y(x*, y*) (y_2 - y_1)

In this case, the function u(x,y) is continuous on the closed and bounded region R = [x_0, x_0 + \Delta x] x [y_0, y_0 + \Delta y] and has continuous partial derivatives u_x and u_y at every point in R. Therefore, the mean value theorem can be applied to the first term in brackets on the right side of equation (3-17) to obtain:

u(x_0 + \Delta x, y_0 + \Delta y) - u(x_0, y_0 + \Delta y) = u_x(x*, y_0 + \Delta y) \Delta x

where (x*, y_0 + \Delta y) is a point in the region R = [x_0, x_0 + \Delta x] x [y_0, y_0 + \Delta y]. This is because the mean value theorem guarantees the existence of such a point x* between x_0 and x_0 + \Delta x, and since u_x is continuous, it can be evaluated at this point.

I hope this helps clarify the proof for you. Let me know if you have any other questions. Happy studying!
 
A sphere as topological manifold can be defined by gluing together the boundary of two disk. Basically one starts assigning each disk the subspace topology from ##\mathbb R^2## and then taking the quotient topology obtained by gluing their boundaries. Starting from the above definition of 2-sphere as topological manifold, shows that it is homeomorphic to the "embedded" sphere understood as subset of ##\mathbb R^3## in the subspace topology.
Back
Top