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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0

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?
 

Related to Find out where this power series converges

1. 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.

2. 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.

3. 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.

4. 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.

5. 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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