Proof: Assume tc(G) = 1. Then there exists a covering FG of G such that |FG|=1. Let FG = {T}. Since G is connected and nontrivial, G = T. Thus, G is a tree.
Proof: Assume tc(G) = 1. Then there exists a covering FG of G such that|FG|=1. Let FG = {T}. Since G is connected and nontrivial, G = T. Thus,G is a tree.