# Find the limit of sequence

1. Jun 14, 2012

### aaaa202

1. The problem statement, all variables and given/known data
I want to find the limit of ƩK(n+m,n)zn
K(a,b) being the binomial coefficient.

2. Relevant equations
Cauchy root test?

3. The attempt at a solution

Trying the cauchy root test I get:

1/R = limn->∞[(K(n+m,n))½]

But what do I do from here?

2. Jun 14, 2012

### HallsofIvy

Staff Emeritus
The "Cauchy root test" tells you whether or not a series converges. It says nothing about what it converges to. If I read this correctly, you have
$$\sum \begin{pmatrix}n+m \\ n\end{pmatrix}z^n$$

The sum is over n with m fixed? And it is a finite sum? n goes from 0 to what?

3. Jun 14, 2012

### aaaa202

well maybe I named it wrong, but I meant the formula stated above, which gives an explicit expression for the radius of convergence, R.
And the sum is from zero to infinity. Sorry for the lack of information :)

4. Jun 14, 2012

### Ray Vickson

If you set z = -t, the coefficient of t^n is the "negative binomial" coefficient:
$$(-1)^n {n+m \choose n} = {-m \choose n}.$$ That should allow you to evaluate the sum explicitly.

RGV

Last edited: Jun 14, 2012