Hadamard product of GCD matrices.
Let be a square -matrix. Then is a Hall matrix provided it has a nonzero permanent. The Hall exponent of is the smallest positive integer , if such exists, such that is a Hall matrix. The Hall exponent has received considerable attention, and we both review and expand on some of its properties. Viewing as the adjacency matrix of a digraph, we prove several properties of the Hall exponents of line digraphs with some emphasis on line digraphs of tournament (matrices).