Bourbaki

Exercise 3, § 2

Functions of a Real Variable, III, 125

For every real x>0x > 0 and every complex m=μ+iνm = \mu + i\nu one puts xm=emlog⁡xx^m = e^{m \log x}; show that the formula (19) of III, p. 108 remains valid for mm complex and x>−1x > -1 and that the remainder rn(x)r_n(x) in this formula satisfies the inequalities

∣rn(x)∣≤∣(m−1n)mμxn[(1+x)μ−1]∣if μ≠0∣rn(x)∣≤∣(m−1n)mxnlog⁡(1+x)∣if μ=0. |r_n(x)| \leq \left| \binom{m-1}{n} \frac{m}{\mu} x^n [(1+x)^{\mu} - 1] \right| \quad \text{if } \mu \neq 0 \\ |r_n(x)| \leq \left| \binom{m-1}{n} m x^n \log(1+x) \right| \quad \text{if } \mu = 0.
Generalize the study of the convergence of the binomial series to the case where mm is complex.

Solution

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Transcribed from the printed volume. Not translated.