In this paper, we consider the non-existence of some bi-level orthogonal arrays (O-arrays) of strength six, with \( m \) constraints (\( 6 \leq m \leq 32 \)), and with index set \( \mu \) (\( 1 \leq \mu \leq 512 \)). The results presented here tend to improve upon the results available in the literature.

We present constructions and results about GDDs with two groups and block size five in which each block has configuration \((s, t)\), that is, in which each block has exactly \(s\) points from one of the two groups and \(t\) points from the other. After some results for a general \(k\), \(s\), and \(t\), we consider the \((2,3)\) case for block size \(5\). We give new necessary conditions for this family of GDDs and give minimal or near-minimal index examples for all group sizes \(n \geq 4\) except for \(n = 24s + 17\).

The covering number for a subset of leaves in a finite rooted tree is defined as the number of subtrees that remain after deleting all the paths connecting the root to the other leaves. We find the formula for the total sum (hence the average) of the covering numbers for a given subset of labeled leaves over all unordered binary trees with \( n \) leaves.