# Finding the exact value of a summation.

1. Feb 8, 2015

### NATURE.M

1. The problem statement, all variables and given/known data
The sum we are given is Σ(from x=0->∞) [(x^2)(2^x)]/x!. We are asked to find the exact value of this sum using concepts discussed in class which include poisson random variables, and their expected values.

3. The attempt at a solution

So i know the solution to the infinite sum is divergent. I know this since:

e^2 * Σ(from x=0->∞) (x^2) * Σ(from x=0->∞) (2^x)(e^-2)/x! = e^-2 * Σ(from x=0->∞) (x^2) since Σ(from x=0->∞) (2^x)(e^-2)/x! = 1 from poisson distribution. And we know Σ(from x=0->∞) (x^2) is divergent from simple infinite series. So the sum itself is ∞/or diverges.

But I don't know if theres a better way to solve this using only properties of the poisson distributions and their expected values/variances instead of invoking results from infinite series.

2. Feb 8, 2015

### Dick

The sum is not divergent. A ratio test will tell you that. I don't know how to sum it exactly, but it is convergent.

3. Feb 8, 2015

### NATURE.M

oh I think I noticed my error of breaking the sum up since its a property of finite sums only.

4. Feb 8, 2015

### LCKurtz

Hint: If that sum had an $e^{-2}$ factor, what expected value of what distribution would it be calculating?

5. Feb 9, 2015

### NATURE.M

Yeah I kinda understand that what I'm uncertain about is how does Σ(from x=0->∞) (x^2)(2^x)(e^-2)/x! = 6. The x^2 confuses me ?

6. Feb 9, 2015

### LCKurtz

7. Feb 15, 2015

### lurflurf

use homogeneous differentiation for these types of exercises

$$\sum_{x=0}^\infty \dfrac{x^22^x}{x!}=\left. \left(r\dfrac{\mathrm{d}\phantom{r}}{\mathrm{d}r}\right)^2\sum_{x=0}^\infty \dfrac{r^x}{x!}\right|_{r=2}$$

or use
$$\sum_{x=0}^\infty \dfrac{r^x}{x!} =\sum_{x=0}^\infty x\dfrac{r^{x-1}}{x!} =\sum_{x=0}^\infty x(x-1)\dfrac{r^{x-2}}{x!}=e^x$$

Last edited: Feb 15, 2015