Complex Analysis: Theory and Applications · Problem 7.2.6

Taylor and Laurent Series — Howell & Matthews

Mathematics — Complex Analysis · taylor-series-binomial

Problem

Let f(z)=(1+z)β=exp⁡[β Log(1+z)]f(z) = (1+z)^{\beta} = \exp[\beta\,\mathrm{Log}(1+z)] be the principal branch of (1+z)β(1+z)^{\beta}, where β\beta is a fixed complex number. Establish the validity for z∈D1(0)z \in D_1(0) of the binomial expansion (1+z)β=1+βz+β(β−1)2!z2+β(β−1)(β−2)3!z3+⋯=1+∑n=1∞β(β−1)(β−2)⋯(β−n+1)n!zn.\begin{aligned} (1+z)^{\beta} &= 1 + \beta z + \frac{\beta(\beta-1)}{2!}z^2 + \frac{\beta(\beta-1)(\beta-2)}{3!}z^3 + \cdots \\ &= 1 + \sum_{n=1}^{\infty}\frac{\beta(\beta-1)(\beta-2)\cdots(\beta-n+1)}{n!}z^n. \end{aligned}

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