anonymous
  • anonymous
expand the following f(z) in a Laurent series expansion valid for |z-1|>1
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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chestercat
  • chestercat
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anonymous
  • anonymous
\[f(z)=\frac{ z }{ (z-1)(2-z) }\]
anonymous
  • anonymous
oh kay i think i know where i went wrong.
anonymous
  • anonymous
let u=z-1 so that z=u+1 \[\frac{ z }{ (z-1)(2-z) }=\frac{ u+1 }{ u(1-u) }=\frac{ 1 }{ u }+\frac{ 2 }{ 1-u }\]\[=\frac{ 1 }{ u }+2\sum_{n=0}^{\infty}u ^{n}\]\[=\frac{ 1 }{ z-1 }+2\sum_{n=0}^{\infty}(z-1) ^{n}\] did i make any error or its not simplified yet? the answer provided is\[=\frac{ 1 }{ z-1 }-\sum_{n=0}^{\infty}2(\frac{ 1 }{ z-1 }) ^{n+1}\]

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anonymous
  • anonymous
start with partial fractions i think
anonymous
  • anonymous
you get \[\frac{1}{z-1}-\frac{2}{z-2}\]
anonymous
  • anonymous
yep
anonymous
  • anonymous
oops now i got stuck, i am not getting the right thing
anonymous
  • anonymous
ouh substitution after the partial fraction works.
anonymous
  • anonymous
second part is \(\frac{2}{1+(1-z)}\) right?
anonymous
  • anonymous
yea
anonymous
  • anonymous
so i am confused as to why this doesn't alternate, but if you got it, then i guess i have something wrong probably in my mind
anonymous
  • anonymous
for the region of validity, like in this case |z-1|>1, after we let u=z-1, any series we obtain using that substitution will always be in that region? my english aint good enough lol
anonymous
  • anonymous
yes, i believe so
anonymous
  • anonymous
oh kay thx alot =)
anonymous
  • anonymous
oh wait i am confusing myself. it is \(|z-1|>1\) not less than that is why you need to flip it
anonymous
  • anonymous
so if i turn the partial fraction into \[\frac{ 1 }{ u }-\frac{ 2 }{ u }(\frac{ 1 }{ 1-\frac{ 1 }{ u } })\]should be ok right?

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