AlgebraDifficulty 6.9National olympiadFind the answer
Let a=2001. Consider the set A of all pairs of integers (m,n) with n=0 such that (i) m<2a; (ii) 2n∣(2am−m2+n2); (iii) n2−m2+2mn≤2a(n−m). For (m,n)∈A, let f(m,n)=n2am−m2−mn. Determine the maximum and minimum values of f.
A number or a short expression. Spacing and $ signs are ignored.
Solution
Let a=2001. Consider the set A of all pairs of integers (m,n) with n=0 such that: 1. m<2a, 2. 2n∣(2am−m2+n2), 3. n2−m2+2mn≤2a(n−m).
For (m,n)∈A, let f(m,n)=n2am−m2−mn. We need to determine the maximum and minimum values of f.
### Minimum Value of f From condition (ii), we have: 2n2am−m2+n2=ℓ∈Z⟹n(2ℓ−n)=m(2a−m). Thus, m≡n(mod2).
Using condition (iii): ℓ=2n(2am−2mn)+(n2−m2+2mn)≤2n(2am−2mn)+2a(n−m)=a−m.
From this, we have: 2ℓ−n<2ℓ≤2a−2m<2a−m. Using this and the previous equation, we conclude n>m. Also, from condition (iii): 2mn≤2a(n−m)−(n2−m2)⟹(n−m)(2a−n−m).
Thus, 2a−n−m>0. Therefore, f(m,n)=nm(2a−m−n)>0.
Hence, f(m,n)=2ℓ−(m+n)≡0(mod2) and f(m,n)>0. Thus, we conclude f(m,n)≥2 and the equality holds for f(2,2000)=2.
### Maximum Value of f Consider: f(n−2,n)=n2a(n−2)−(n−2)2−n(n−2)=2a+6−2(n+n4004).
To maximize this, we need to minimize n+n4004. Choosing n∣4004 such that n and n4004 are as close as possible, we find n=52 and m=50 satisfy the conditions. Therefore, f(50,52)=3750.
Since n>m and m≡n(mod2), it suffices to prove that for n≥m+4, f(m,n)<3750: f(m,n)=n(2a−m)m−m≤m+4(2a−m)m−m=3998−(2(m+4)+m+416024)≤3998−232048<3640.
Thus, the maximum value of f is 3750.
### Conclusion The minimum value of f(m,n) is 2, and the maximum value is 3750.
The answer is: 2 and 3750.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: Omni-MATH,
licensed Apache-2.0.
Statement and solution reproduced as published; topic and difficulty added by this site.