The chromatic number u(M) of a map M is the smallest number n for which M is n- colourable. By using the Heawood theorem, and the Peterson graph, show that u(projective plane) = 6 The chromatic number u(M) of a map M is the smallest number n for which M is n- colourable. By using the Heawood theorem, and the Peterson graph, show that u(projective plane) = 6