# In graph theory, what does $o(G)$ usually mean?

I'm completing a graph theory assignment, and one of the problems states,

Prove that a tree $T$ has a perfect matching if and only if $o(T-v) = 1$ for every $v \in V (T)$.

I'm not asking for help answering this question, but rather for help understanding the notation. What does $o(G)$ usually mean in graph theory? If it has multiple conventional meanings, which are the most common?

One thought I had was that it could be the number of odd disconnected components left after removing some vertices, but I believe that following that definition's implications has lead me to a disproof of the statement in the question, so I don't think that's correct.

• Please do not answer the homework question for me. I would like to figure it out myself once I am sure I understand precisely what it is asking. – Kevin Dec 5 '14 at 3:22
• The $o(G)$ is referring to the number of components of odd order in $G$. – ml0105 Dec 5 '14 at 3:26
• Positive. I had this homework question last semester. I just double checked as well. Theorem 3.3.3 of West may offer some clarification as well. – ml0105 Dec 5 '14 at 3:34
• Thank you for answering my question, @ml0105! – Kevin Dec 5 '14 at 3:44

I think that $o(G)$ refers to the number of components of G which have an odd number of vertices (also denoted by odd($G$)).
In this case, the graph induced by $T-v$ has at most $1$ connected component with an odd number of vertices.