Math - Applied combinatorics

Similar to the proof of m<=3n-6. In this case, we have kf<=2m
By Euler Polyhedron Formula, n - m + f = 2, , so, kn - km + kf = 2k. Since kf =< 2m
kn - km + 2m<= 2k
(2-k)m<=2k-kn
m<=(2k-kn)/(2-k)
or m<=k/(k-2)(n-2)
where m is the no. of edge and n is the no. of vertices

what is f represents?