f(xy)=f(x)+f(y),(x,y>0),f(2)=1,
f(2*1)=f(2)=f(2)+f(1)=1,
f(1)=0.
f(2*2)=f(2)+f(2)=2f(2)=2,
f(4)=2.
f(x)+f(x-3)0,x>3,
x(x-3)