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Chapter 22: Problem 1
Man bestimme die Taylor-Reihe der Funktion \(x \longmapsto x^{a}\) mitEntwicklungspunkt \(a \in \mathbb{R}_{+}^{*}\)
Short Answer
Expert verified
The Taylor series for the function \(f(x)=x^{a}\) around the point \(a\) is \[f(x) = a^{a} + \sum_{n=1}^{\infty} \frac{(a)_n (x-a)^n}{n!}\] where (a)_n denotes the falling factorial of \(a\).
Step by step solution
01
Root Selection
We first need to decide which root we will use to expand the power series around. Since the problem specifically mentions \(a \in \mathbb{R}_{+}^{*}\), that means the root will be the point \(a\).
02
Taylor Series Expansion
The general definition of the Taylor series for a function \(f(x)\) about the point \(a\) is: \[f(x) = f(a) + f'(a)(x-a) + \frac{f''(a)(x-a)^{2}}{2} + \frac{f'''(a)(x-a)^3}{3!} + \ldots\] We now need to apply this formula to our function \(f(x) = x^{a}\) and use our development point \(a\).
03
Calculating Derivatives
The derivative of \(f(x) = x^{a}\) is \(f'(x) = a * x^{a-1}\), the second derivative is \(f''(x) = a * (a-1) * x^{a-2}\), and, generally, the nth derivative is given by \(f^{(n)}(x) = a * (a-1) * \ldots * (a-n+1) * x^{a-n}\).
04
Substituting the Derivatives
Substituting these derivatives into the expansion, we find: \[f(x) = a^{a} + a * a^{a-1} * (x-a) + \frac{a * (a-1) * a^{a-2} * (x-a)^{2}}{2} + \frac{a * (a-1) * (a-2) * a^{a-3} * (x-a)^3}{3!} + \ldots\]
05
Simplifying the Expansion
Each term can be simplified by recognising that the numerator of the fraction in each term forms a falling factorial, while the denominator is a rising factorial. Thus, the series simplifies to: \[f(x) = a^{a} + \sum_{n=1}^{\infty} \frac{(a)_n (x-a)^n}{n!}\] where \((a)_n\) denotes the falling factorial of \(a\).
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