Question #235599

expand f(z)=z/(z-1)(2-z) in a laurent series valid 1<|z|<2


1
Expert's answer
2021-09-14T11:19:28-0400

z(z1)(2z)=Az1+B2z=2AAz+BzB(z1)(2z)\dfrac{z}{(z-1)(2-z)}=\dfrac{A}{z-1}+\dfrac{B}{2-z}=\dfrac{2A-Az+Bz-B}{(z-1)(2-z)}

z0:0=2ABz^0: 0=2A-B


z1:1=A+Bz^1: 1=-A+B


A=1,B=2A=1, B=2

f(z)=z(z1)(2z)=1z1+22zf(z)=\dfrac{z}{(z-1)(2-z)}=\dfrac{1}{z-1}+\dfrac{2}{2-z}

Geometric series


1z1=1z(111z)=i=1n1zn,z>1\dfrac{1}{z-1}=\dfrac{1}{z}(\dfrac{1}{1-\dfrac{1}{z}})=\displaystyle\sum_{i=1}^n\dfrac{1}{z^n}, |z|>1

22z=11z2=i=0nzn2n,z<2\dfrac{2}{2-z}=\dfrac{1}{1-\dfrac{z}{2}}=\displaystyle\sum_{i=0}^n\dfrac{z^n}{2^n}, |z|<2

z(z1)(2z)=1+i=1n(1zn+zn2n),1<z<2\dfrac{z}{(z-1)(2-z)}=1+\displaystyle\sum_{i=1}^n\bigg(\dfrac{1}{z^n}+\dfrac{z^n}{2^n}\bigg),1<|z|<2


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