Does Using Maximum Coefficients Determine the Smallest Radius of Convergence?

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SUMMARY

The discussion centers on determining the radius of convergence for the series Ʃdnx^n, where dn is defined as max(|an|, |bn|) from two power series Ʃanx^n and Ʃbnx^n with convergence radii A and B. It is established that the radius of convergence D for the series Ʃdnx^n is D = min(A, B). The approach suggested involves using the inequality |an| + |bn| ≥ dn to support the conclusion that the radius of convergence is at least min(A, B), while also emphasizing the need to prove it is not greater than that value.

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Homework Statement


Let Ʃanx^n and Ʃbnx^n be two power series and let A and B be their converging radii. define dn=max(lanl,lcnl) and consider the series Ʃdnx^n. Show that the convergence radius of this series D, is D=min(A,B)


Homework Equations


My idea is to use that the series Ʃ(lanl+lbnl)x^n has convergence radius min(A,B) and use that lanl+lbnl≥dn. Do you agree that this is a good idea from a rigorous perspective? Last assigment I really got punished for not being rigorous enough, so I want to make sure this time, that I do it properly.


The Attempt at a Solution


 
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aaaa202 said:

Homework Statement


Let Ʃanx^n and Ʃbnx^n be two power series and let A and B be their converging radii. define dn=max(lanl,lcnl) and consider the series Ʃdnx^n. Show that the convergence radius of this series D, is D=min(A,B)


Homework Equations


My idea is to use that the series Ʃ(lanl+lbnl)x^n has convergence radius min(A,B) and use that lanl+lbnl≥dn. Do you agree that this is a good idea from a rigorous perspective? Last assigment I really got punished for not being rigorous enough, so I want to make sure this time, that I do it properly.

That will show the radius of convergence of ##\sum d_n## is at least min(A,B). You would still have to show it isn't greater than that.
 

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