On the extremal Merrifield–Simmons index and Hosoya index of quasi-tree graphs Original Research Article

It is well known that the two graph invariants, “the Merrifield–Simmons index” and “the Hosoya index” are important in structural chemistry. A graph image is called a quasi-tree graph, if there exists image in image such that image is a tree. In this paper, at first we characterize the image-vertex quasi-tree graphs with the largest, the second-largest, the smallest and the second-smallest Merrifield–Simmons indices. Then we characterize the image-vertex quasi-tree graphs with the largest, the second-largest, the smallest and the second-smallest Hosoya indices, as well as those image-vertex quasi-tree graphs with image pendent vertices having the smallest Hosoya index.