Edge deletion to tree-like graph classes

For a fixed property (graph class) Π, given a graph G and an integer k, the Π-deletion problem consists in deciding if we can turn G into a graph with the property Π by deleting at most k edges. The Π-deletion problem is known to be NP-hard for most of the well-studied graph classes, such as chordal, interval, bipartite, planar, comparability and permutation graphs, among others; even deletion to cacti is known to be NP-hard for general graphs. However, there is a notable exception: the deletion problem to trees is polynomial. Motivated by this fact, we study the deletion problem for some classes similar to trees, addressing in this way a knowledge gap in the literature. We prove that deletion to cacti is hard even when the input is a bipartite graph. On the positive side, we show that the problem becomes tractable when the input is chordal, and for the special case of quasi-threshold graphs we give a simpler and faster algorithm. In addition, we present sufficient structural conditions on the graph class Π that imply the NP-hardness of the Π-deletion problem, and show that deletion from general graphs to some well-known subclasses of forests is NP-hard.