On the sizes of generalized cactus graphs

Abstract: A cactus is a connected graph in which each edge is contained in at most one cycle. We generalize the concept of cactus graphs, i.e., a $k$-cactus is a connected graph in which each edge is contained in at most $k$ cycles where $k\ge 1$. It is well known that every cactus with $n$ vertices has at most $\lfloor\frac{3}{2}(n-1) \rfloor$ edges. Inspired by it, we attempt to establish analogous upper bounds for general $k$-cactus graphs. In this paper, we first characterize $k$-cactus graphs for $2\le k\le 4$ based on the block decompositions. Subsequently, we give tight upper bounds on their sizes. Moreover, the corresponding extremal graphs are also characterized. However, the case of $k\ge 5$ remains open. For the case of 2-connectedness, the range of $k$ is expanded to all positive integers in our research. We prove that every $2$-connected $k ~(\ge 1)$-cactus graphs with $n$ vertices has at most $n+k-1$ edges, and the bound is tight if $n \ge k + 2$. But, for $n < k+1$, determining best bounds remains a mystery except for some small values of $k$.