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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... ... ]
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... ... ]
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