  In a graph G, a dissociation set is a subset of vertices which induces a
subgraph with vertex degree at most 1. Finding a dissociation set of maximum
cardinality in a graph is NP-hard even for bipartite graphs and is called the
maximum dissociation set problem. The complexity of maximum dissociation set
problem in various subclasses of graphs has been extensively studied in the
literature. In this paper, we study the maximum dissociation problem from
different perspectives and characterize the vertices belonging to all maximum
dissociation sets, and to no maximum dissociation set of a tree. We present a
linear time recognition algorithm which can determine whether a given vertex in
a tree is contained in all (or no) maximum dissociation sets of the tree. Thus
for a tree with n vertices, we can find all vertices belonging to all (or no)
maximum dissociation sets of the tree in O(n^2) time.
