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Doubts on the taylor expansion when deriving 2.53 #3

@xulongwu4

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@xulongwu4

Hi, thanks for the detailed notes on the book. It is truly helpful!

I have a question in the derivation of Eq. 2.53. You introduced a function $T(s) = S(1-s)$ and then used Taylor expansion of T around 0. It seems to me that this implicitly assumes $S'(1)$ exists and takes some non-zero value.

However, if we check the derivative of $S(x)$ using its functional form 2.58, we can show that

a. $S'(1) = -1$ when $m = 1$.
b. $S'(1) = 0$ when $0 < m < 1$.
c. $S'(1)$ does not exist (is infinite) when $m > 1$

This seems to render the Taylor expansion invalid for most of $m$ values. Can you help me figure out if there is anything wrong in my reasoning?

Thanks!

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