文摘
The triangle graph of a graph G, denoted by T(G), is the graph whose vertices represent the triangles (K3 subgraphs) of G, and two vertices of T(G) are adjacent if and only if the corresponding triangles share an edge. In this paper, we characterize graphs whose triangle graph is a cycle and then extend the result to obtain a characterization of Cn-free triangle graphs. As a consequence, we give a forbidden subgraph characterization of graph G for which T(G) is a tree, a chordal graph, or a perfect graph. For the class of graphs whose triangle graph is perfect, we verify a conjecture of the third author concerning packing and covering of triangles.