I am reading Keisler's Elementary Calculus (which can be downloaded here). I am having trouble understanding his proof sketch of Extreme Value Theorem and how he is applying the Transfer Principle.
For reference, he defines the "Transfer Principle" as:
Every real statement that holds for one or more particular functions holds for the hyperreal natural extension of these functions.
On page 164 (using left corner numbering) of the book he provides the following "sketch":
I understand the counter examples and I am able to understand the issues with them using standard tools. I don't understand, however, how one can immediately utilize the Transfer Principle. It is not immediately obvious to me that "there is a partition point $a + K\delta$ at which $f(a + K\delta)$ has the largest value."
To elaborate, the proof seems circular. In trying to to "expand" the sketch to be more precise. I ended up writing instead of:
By the Transfer Principle, there is a partition point $a + K\delta$ at which $f(a + K\delta)$ has the largest value.
Applying the Transfer Principle to the Extreme Value Theorem we see that the Extreme Value holds for hyperreals as well. Hence, there is a partition point $a + K\delta$ at which $f(a + K\delta)$ has the largest value.
But this relies on a proof of the Extreme Value Theorem for reals.
Hopefully what I am saying makes sense, please ask for any clarification.