n:
, f:(n
), i:(n-1). sum(f((i, i+1)(x)) | x < n) = sum(f(x) | x < n)k:, i,j:k. (i, j) 
kkn:, f:(n), m:(n+1).
Thm* sum(f(x) | x < n) = sum(f(x) | x < m)+sum(f(x+m) | x < n-m)
n:, f,g:(n).
Thm* (
i:n. f(i) = g(i)) 
sum(f(x) | x < n) = sum(g(x) | x < n)