2. 0<n
3.
f:(
(n-1)
), m:(n-1+1).
3. sum(f(x) | x < n-1) = sum(f(x) | x < m)+sum(f(x+m) | x < n-1-m)
4. f :
n
5.
m:(n-1+1). sum(f(x) | x < n-1) = sum(f(x) | x < m)+sum(f(x+m) | x < n-1-m)
6. m :
(n+1)
7. m = n-1
8.
n = 0
sum(f(x+n-1) | x < 1) = f(n-1)
