The crossing number of a projective graph is quadratic in the face–width

We show that for each integer g ⩾ 0 there is a constant c g > 0 such that every graph that embeds in the projective plane with sufficiently large face–width r has crossing number at least c g r 2 in the orientable surface Σ g of genus g. As a corollary, we give a polynomial time constant factor approximation algorithm for the crossing number of projective graphs with bounded degree.