Cluster Edge Deletion on 2-trees Definitions:
Cluster Edge Deletion problem is a graph modification problem, in which we want to remove the minimum number of edges such that the resulting graph does not contain a $P_3$ as an induced subgraph (that is, the resulting graph is a disjoin union of cliques).

The class of $k$-trees is defined as follows: a complete graph with $k$ vertices is a $k$-tree; a $k$-trees with $n + 1$ vertices $(n &gt; k)$ can be constructed from a $k$-tree&nbsp;$T$ with $n$ vertices by adding a vertex adjacent to all vertices of a $k$-clique of $T$, and only to these vertices.

Question:
Does exist a linear-time algorithm to solve Cluster Edge Deletion on 2-trees?
The edge sets whose deletion leaves a cluster graph can easily be described as a formula with one free variable (the edge set) in monadic second-order graph logic. Therefore, by an optimization version of Courcelle's theorem, the minimum cardinality set can be found in linear time for graphs of bounded treewidth, which obviously include the 2-trees.

See e.g. Arnborg, Lagergren, and Seese, "Easy problems for tree-decomposable graphs", J. Algorithms 1991.