“…Determining the feedback number is quite difficult even for some elementary graphs. However, the problem has been studied for some special graphs and digraphs, such as hypercubes, meshes, toroids, butterflies, cubeconnected cycles, directed split-stars (see [3,4,5,6,7,8,9,10,11,12,13]). In fact, the minimum feedback set problem is known to be NP-hard for general graphs [14] and the best known approximation algorithm is one with an approximation ratio two [5].…”