Find out where this power series converges

In summary, a power series is an infinite series that can be used to represent functions. The convergence of a power series depends on the value of x and the coefficients c<sub>n</sub>, and can be determined using methods such as the ratio test, root test, and integral test. A power series can converge at multiple points, and the radius of convergence, denoted by R, is the range of values for x in which the series converges. However, it is also possible for a power series to diverge for all values of x if the limit of |c<sub>n+1</sub>|/|c<sub>n</sub>| is equal to 1 or does not exist, resulting in a radius of
  • #1
tamtam402
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Homework Statement


Find out where this power series converges.

Ʃ(xn2n) / (3n + n3)


Homework Equations





The Attempt at a Solution



I'm trying to use the ratio test to solve it. I end up with the following equation, which I am unable to reduce further:

pn = 2x (3n + n3)/[(3)(3)n+n3(1+1/n)3]
 
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  • #2
My guess is, since [itex]3^n[/itex] goes to infinity faster than [itex]n^3[/itex] (exponentials are faster than polynomials), is that your ratios go to [itex]\frac{2}{3}x[/itex]. Tnen you want [itex]|x|<\frac{3}{2}[/itex]. Not sure what happens at the boundaries. To check the limit I guessed at, maybe use l'Hopital's rule 3 times?
 

FAQ: Find out where this power series converges

What is a power series?

A power series is an infinite series of the form ∑n=0^∞cn(x-a)n, where cn are constants and x is a variable. It is a type of mathematical series that can be used to represent functions.

How do you determine where a power series converges?

The convergence of a power series depends on the value of x and the coefficients cn. One method to determine convergence is to use the ratio test, where the limit of |cn+1|/|cn| as n approaches infinity is calculated. If this limit is less than 1, the series converges. Other methods, such as the root test and the integral test, can also be used.

Can a power series converge at more than one point?

Yes, it is possible for a power series to converge at multiple points. This depends on the values of x and the coefficients cn. For example, a power series may converge at a specific value of x but diverge at another value.

What is the radius of convergence for a power series?

The radius of convergence is the range of values for x in which a power series converges. It is typically denoted by R and can be found using the ratio test, where R = 1/limn→∞|cn|^(1/n). The series will converge for all values of x within this radius and diverge for values outside of it.

Can a power series diverge for all values of x?

Yes, it is possible for a power series to diverge for all values of x. This can happen if the limit of |cn+1|/|cn| as n approaches infinity is equal to 1 or if the limit does not exist. In this case, the series is said to have a radius of convergence of 0 and does not converge for any value of x.

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