Convergence of Power Series with Coefficients Multiplied by n

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SUMMARY

The discussion centers on the convergence of the power series sum(n*a_n*x^n) given that the series sum(a_n*x^n) has a radius of convergence (-R, R). The user applies the ratio test, concluding that lim[b_(n+1)/b_n] = lim [(n+1)/n] * [lim x*a_(n+1)/a_n/a_n] < 1, which supports the claim of convergence under the stated conditions. The logic appears sound, as the limit of x*a_(n+1)/a_n is less than 1 based on the initial hypothesis.

PREREQUISITES
  • Understanding of power series and their convergence properties
  • Familiarity with the ratio test for series convergence
  • Knowledge of limits and their application in calculus
  • Basic concepts of sequences and series in mathematical analysis
NEXT STEPS
  • Study the implications of the ratio test in greater detail
  • Explore the concept of radius of convergence in power series
  • Investigate counterexamples to convergence in series
  • Learn about other convergence tests such as the root test and comparison test
USEFUL FOR

Mathematics students, educators, and anyone studying series convergence in calculus or analysis will benefit from this discussion.

ohreally1234
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Homework Statement


Suppose sum(a_n*x^n) represents a power series with radius of convergence (-R, R). Is it true that the series sum(n*a_n*x^n) is convergent? Prove or give a counter example.


Homework Equations





The Attempt at a Solution



Let b_n = n*a_n*x^n
Using ratio test:
lim[b_(n+1)/b_n] = lim [(n+1)/n] * [lim x*a_(n+1)/a_n] < 1 because lim x*a_(n+1)/a_n <1 from hypothesis.

Any gaps in logic?
 
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I think that's correct.
 

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