Question on Arithmetic Series

[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!

[rwinston]

Got it, d'oh!


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