Paper 1, Section II, A

Complex Analysis or Complex Methods | Part IB, 2011

(i) Let −1<α<0-1<\alpha<0 and let

f(z)=log⁡(z−α)z where −π⩽arg⁡(z−α)<πg(z)=log⁡zz where −π⩽arg⁡(z)<π\begin{aligned} &f(z)=\frac{\log (z-\alpha)}{z} \text { where }-\pi \leqslant \arg (z-\alpha)<\pi \\ &g(z)=\frac{\log z}{z} \quad \text { where }-\pi \leqslant \arg (z)<\pi \end{aligned}

Here the logarithms take their principal values. Give a sketch to indicate the positions of the branch cuts implied by the definitions of f(z)f(z) and g(z)g(z).

(ii) Let h(z)=f(z)−g(z)h(z)=f(z)-g(z). Explain why h(z)h(z) is analytic in the annulus 1⩽∣z∣⩽R1 \leqslant|z| \leqslant R for any R>1R>1. Obtain the first three terms of the Laurent expansion for h(z)h(z) around z=0z=0 in this annulus and hence evaluate

∮∣z∣=2h(z)dz\oint_{|z|=2} h(z) d z

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