Minimize W(n) w.r.t k.
Find 
& solve by setting to 0.
Set 
and solve to get
(can not be 1)
Further check that 
at k=2 is 
for a minimum value.
Thus, k=2 is the best. So, dividing in 3 partition is not
better than that in 2.
W(n)=k-1+w(n/k)
=2-1+w(n/2)
