Square of an infinite sum


by Char. Limit
Tags: infinite, square
Char. Limit
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Feb29-12, 11:25 PM
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Given a general, convergent infinite sum, say, one like this:

[tex]\sum_{n=0}^\infty a_n x^n[/tex]

Is there a formula for the square of such a sum? I looked at the Cauchy product, but I'd rather not get a double sum as an answer...
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LCKurtz
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Feb29-12, 11:33 PM
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Quote Quote by Char. Limit View Post
Given a general, convergent infinite sum, say, one like this:

[tex]\sum_{n=0}^\infty a_n x^n[/tex]

Is there a formula for the square of such a sum? I looked at the Cauchy product, but I'd rather not get a double sum as an answer...
YMMV but I wouldn't call the Cauchy product a double sum. And if you wish to write the result as a power series, you're stuck with it. Those finite inner sums give the coefficients of ##x^n##. It is what it is.
Char. Limit
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Feb29-12, 11:42 PM
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Quote Quote by LCKurtz View Post
YMMV but I wouldn't call the Cauchy product a double sum. And if you wish to write the result as a power series, you're stuck with it. Those finite inner sums give the coefficients of ##x^n##. It is what it is.
Aww, well I guess if I need to use the Cauchy product I'll have to figure out how to use it... thanks for the help though!


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