Proof: Assume |z|>1. Then ||an+1an|−1|z||=||11+zn+111+zn|−|1z||=||1+zn1+zn+1|−|1z||≤|1+zn1+zn+1−1z|=|z−1z+zn+2|=|z−1z|·1|1+zn|≤|z−1z|·1|z|n−1→0 as n→∞, so lim |an+1an|=1|z|<1 and ∑an converges by the ratio test. Assume |z|≤1. Then |11+zn|≥11+|z|n≥12 and ∑an diverges seeing as an doesn't converge to 0. ◻
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