On the positive definiteness and eigenvalues of meet and join matrices

In this paper we study the positive definiteness of meet and join matrices using a novel approach. When the set SnSn is meet closed, we give a necessary and sufficient condition for the positive definiteness of the matrix (f(Sn))(f(Sn)). From this condition we obtain some sufficient conditions for positive definiteness as corollaries. We also use graph theory and show that by making some graph theoretic assumptions on the set SnSn we are able to reduce the assumptions on the function ff while still preserving the positive definiteness of the matrix (f(Sn))(f(Sn)). Dual theorems of these results for join matrices are also presented. As examples we consider the so-called power GCD and reciprocal power LCM matrices as well as MIN and MAX matrices. Finally we give bounds for the eigenvalues of meet and join matrices in cases when the function ff possesses certain monotonic behaviour.