Finding Radius of Convergence of the Power Series

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SUMMARY

The radius of convergence for the power series of the function 1 / (z - 1), expanded about the point z = i, is determined to be sqrt(2). The discussion emphasizes the importance of correctly identifying the center of the series expansion, which in this case is z = i, rather than z = 0. The Ratio Test is applied, confirming that the only singularity affecting convergence is at z = 1, which is sqrt(2) units away from the center.

PREREQUISITES
  • Understanding of power series expansions
  • Familiarity with the Ratio Test for convergence
  • Knowledge of complex analysis, specifically functions of a complex variable
  • Ability to manipulate algebraic expressions involving complex numbers
NEXT STEPS
  • Study the application of the Ratio Test in more complex series
  • Explore the concept of singularities in complex functions
  • Learn about Taylor and Laurent series expansions
  • Investigate convergence criteria for power series in complex analysis
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Students and educators in mathematics, particularly those focused on complex analysis and series convergence, as well as anyone preparing for advanced calculus or analysis courses.

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Homework Statement


"Find the radius of convergence of the power series for the following functions, expanded about the indicated point.

1 / (z - 1), about z = i.



Homework Equations



1 / (1 - z) = 1 + z + z^2 + z^3 + z^4 + ... +

Ratio Test: limsup sqrt(an)^k)^1/k



The Attempt at a Solution



1 / (z - 1) = -1 / (1 - z) = -1(1 + z + z^2 + z^3 + z^4 + ... +)

Ratio test gives 1?

Answer but I'm not sure how sqrt(2)
 
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Your relevant equation is irrelevant. Your series is not about z = 0 - it's about z = i, so your power series will be in powers of z - i, not powers of z. So your power series should be defined for all z near i. The only value of z where you'll run into problems is z = 1, which happens to be sqrt(2) away from i. Hope that helps.
 
Question: A clock's minute hand has length 4 and its hour hand has length 3. What is the distance between the tips at the moment when it is increasing most rapidly?(Putnam Exam Question) Answer: Making assumption that both the hands moves at constant angular velocities, the answer is ## \sqrt{7} .## But don't you think this assumption is somewhat doubtful and wrong?

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