Contractible Edges and Contractible Triangles in a 3-Connected Graph

Let G be a 3-connected graph. An edge (a triangle) of G is said to be a 3-contractible edge (a 3-contractible triangle) if the contraction of it results in a 3-connected graph. We denote by $$E_{c}(G)$$ and $$\mathcal {T}_{c}(G)$$ the set of 3-contractible edges of G and the set of 3-contractible triangles of G, respectively. We prove that if $$|V(G)|\ge 7$$ , then $$|E_{c}(G)|+ \frac{15}{14}|\mathcal {T}_{c}(G)|\ge \frac{6}{7}|V(G)|.$$ We also determine the extremal graphs.