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Saturday, May 17, 2014

Behavior of a Complex-Valued Series (3.6(d))

Walter Rudin Principles of Mathematical Analysis, chapter 3, exercise 6(d):

MathJax TeX Test Page Investigate the behavior of an when an=11+zn for complex values of z.

Proof: Assume |z|>1. Then ||an+1an|1|z||=||11+zn+111+zn||1z||=||1+zn1+zn+1||1z|||1+zn1+zn+11z|=|z1z+zn+2|=|z1z|·1|1+zn||z1z|·1|z|n10 as n, so lim |an+1an|=1|z|<1 and an converges by the ratio test. Assume |z|1. Then |11+zn|11+|z|n12 and an diverges seeing as an doesn't converge to 0. 

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