I know the theorem:
for an undirected graph on $n$ nodes, any of the following two imply the third:
- $G$ is connected
- $G$ does not contain a cycle
- $G$ has $n-1$ edges
(source)
Would it be correct to add "graph is a tree" and have any two conditions imply all four?
for an undirected graph, any of the following two imply the third:
- $G$ is connected
- $G$ does not contain a cycle
- $G$ has $n-1$ edges
- $G$ is a tree
I think this is true because, by definition, if you know it is a tree then the first two conditions are implied.
The definition of tree I am using is
An undirected graph is a tree if it is connected and does not contain a cycle
With that definition of a tree, your formulation is correct, it's just not very useful; the definition includes the first two statements. The power of the initial theorem is that from any two of the three statements, we can conclude the third.