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Algebra Difficulty 6.9 National olympiad Find the answer

Let a=2001a=2001. Consider the set AA of all pairs of integers (m,n)(m,n) with n0n\neq0 such that
(i) m<2am<2a;
(ii) 2n(2amm2+n2)2n|(2am-m^2+n^2);
(iii) n2m2+2mn2a(nm)n^2-m^2+2mn\leq2a(n-m).
For (m,n)A(m, n)\in A, let f(m,n)=2amm2mnn.f(m,n)=\frac{2am-m^2-mn}{n}.
Determine the maximum and minimum values of ff.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let a=2001 a = 2001 . Consider the set A A of all pairs of integers (m,n)(m, n) with n0 n \neq 0 such that:
1. m<2a m < 2a ,
2. 2n(2amm2+n2) 2n \mid (2am - m^2 + n^2) ,
3. n2m2+2mn2a(nm) n^2 - m^2 + 2mn \leq 2a(n - m) .

For (m,n)A(m, n) \in A, let
f(m,n)=2amm2mnn. f(m, n) = \frac{2am - m^2 - mn}{n}.
We need to determine the maximum and minimum values of f f .

### Minimum Value of f f
From condition (ii), we have:
2amm2+n22n=Z    n(2n)=m(2am). \frac{2am - m^2 + n^2}{2n} = \ell \in \mathbb{Z} \implies n(2\ell - n) = m(2a - m).
Thus, mn(mod2) m \equiv n \pmod{2} .

Using condition (iii):
=(2am2mn)+(n2m2+2mn)2n(2am2mn)+2a(nm)2n=am. \ell = \frac{(2am - 2mn) + (n^2 - m^2 + 2mn)}{2n} \leq \frac{(2am - 2mn) + 2a(n - m)}{2n} = a - m.

From this, we have:
2n<22a2m<2am. 2\ell - n < 2\ell \leq 2a - 2m < 2a - m.
Using this and the previous equation, we conclude n>m n > m . Also, from condition (iii):
2mn2a(nm)(n2m2)    (nm)(2anm). 2mn \leq 2a(n - m) - (n^2 - m^2) \implies (n - m)(2a - n - m).

Thus, 2anm>0 2a - n - m > 0 . Therefore, f(m,n)=m(2amn)n>0 f(m, n) = \frac{m(2a - m - n)}{n} > 0 .

Hence, f(m,n)=2(m+n)0(mod2) f(m, n) = 2\ell - (m + n) \equiv 0 \pmod{2} and f(m,n)>0 f(m, n) > 0 . Thus, we conclude f(m,n)2 f(m, n) \geq 2 and the equality holds for f(2,2000)=2 f(2, 2000) = 2 .

### Maximum Value of f f
Consider:
f(n2,n)=2a(n2)(n2)2n(n2)n=2a+62(n+4004n). f(n - 2, n) = \frac{2a(n - 2) - (n - 2)^2 - n(n - 2)}{n} = 2a + 6 - 2\left(n + \frac{4004}{n}\right).

To maximize this, we need to minimize n+4004n n + \frac{4004}{n} . Choosing n4004 n \mid 4004 such that n n and 4004n \frac{4004}{n} are as close as possible, we find n=52 n = 52 and m=50 m = 50 satisfy the conditions. Therefore, f(50,52)=3750 f(50, 52) = 3750 .

Since n>m n > m and mn(mod2) m \equiv n \pmod{2} , it suffices to prove that for nm+4 n \geq m + 4 , f(m,n)<3750 f(m, n) < 3750 :
f(m,n)=(2am)mnm(2am)mm+4m=3998(2(m+4)+16024m+4)3998232048<3640. f(m, n) = \frac{(2a - m)m}{n} - m \leq \frac{(2a - m)m}{m + 4} - m = 3998 - \left(2(m + 4) + \frac{16024}{m + 4}\right) \leq 3998 - 2\sqrt{32048} < 3640.

Thus, the maximum value of f f is 3750 3750 .

### Conclusion
The minimum value of f(m,n) f(m, n) is 2 2 , and the maximum value is 3750 3750 .

The answer is: 2 and 3750\boxed{2 \text{ and } 3750}.

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