Discrete Morse Theory on Join of Digraphs

For given two digraphs, we can construct a larger digraph through join. The two digraphs that make up the join are called the factors of the join. In this paper, we give a necessary and sufficient condition that the function on the join determined by the discrete Morse functions on factors is a discrete Morse function. Moreover, we further prove the discrete Morse theory on join when the factors satisfy certain conditions.