2. m :
3. f :
n
m
4. x1 :
n
5. x2 :
n
6. y1 :
m
7. y2 :
m
8.
x1 = x2 
y1 = y2
9.
x:n, y:m. (x = x1 & y = y1) 
(x = x2 & y = y2) f(x,y) = 0
10.
x1 = x2
11. x :
n
12.
x = x1
13.
x = x2
sum(f(x,y) | y < m) = 0
nm
n
n
m
m
x1 = x2 y1 = y2
x:n, y:m. (x = x1 & y = y1) (x = x2 & y = y2) f(x,y) = 0
x1 = x2
n
x = x1
x = x2
sum(f(x,y) | y < m) = 0| By: |
n:, f:(n). (x:n. f(x) = 0) sum(f(x) | x < n) = 0 |
| 1 |
m
f(x,y) = 0 | 1 step |
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