Hybrid trees and hybrid tree graphs

A subgraph "a hybrid tree" in a linear graph
G
which has properties of both a tree and a cotree is defined. In the definition, the set
E
of edges of
G
is partitioned into two subsets. The relations between the number of edges of a hybrid tree and the partition of
E
are discussed. Furthermore, a hybrid tree graph which is a generalized concept of a tree graph and a cotree graph is defined and its some properties are presented.