On minimally 3-connected graphs on a surface

It is well known that the maximal size of minimally 3-connected graphs of order n ≥ 7 is 3n-9. In this paper, we shall prove that if G is a minimally 3-connected graph of order n, and is embedded in a closed surface with Euler characteristic χ, then G contains at most 2n- min {2, 2χ} edges. This bound is best possible for every closed surface.