# Question on Arithmetic Series

1. Oct 11, 2007

### rwinston

Hi

I am currently working through the following issue: I am trying to read an list of values which contains the data points for a binomial lattice. If I have a list of N values that describes a binary tree, and I want to find out how many levels deep L the tree is, I can easily do it via the following method, since at each level, the number of nodes in the tree is 2^N-1:

$$N=2^L-1$$

$$N+1=2^L$$

$$log_2{N}=L$$

So the number of nodes increases like: 1, 3, 7, 15, 31....But a binary lattice is different - the number of nodes increases like 1,3,6,10,15....i.e. it is an arithmetic sum:

$$N = \sum_{i=1}^L i$$

My issue is: given N, how can I solve for L?

Thanks!

Last edited: Oct 11, 2007
2. Oct 11, 2007

### rwinston

Got it, d'oh!

$$L = \frac{\sqrt{8N+1}-1}{2}$$