Number theoryDifficulty 5.8AIME, harderProve itBelarus
Prove that there are at most a finite number of primes p such that the equation a3+b3=2016pab has a solution in positive integers a and b which are not divisible by p.
Solution
Let d denote the greatest common divisor of a and b, i.e., a=da1, b=db1, where gcd(a1,b1)=1. Then the given equality can be presented in the form d(a13+b13)=2016pa1b1. It follows that d∣a13+b13, and, since a1 and b1 are coprime, d∣b13 and d∣a13. Similarly, d∣a13, so d∣a1b1 because a1 and b1 are coprime. Let d=d1a1b1, then the equation can be presented in the form d1(a13+b13)=2016p.(1) There are exactly two possible cases:
1) a1+b1=p.
2) a1+b1=p.
1) If a1+b1=p, then p≤a1+b1⟹p3≤(a1+b1)3≤4(a13+b13)≤4⋅2016p⟹p2≤4⋅2016⟹p≤2⋅2016≤2⋅45=90. Therefore there are only a finite number of p satisfying the problem condition.
2) By condition, d1∤p (otherwise a and b are divisible by p), then from (1) it follows that a13+b13=p. Since a13+b13=(a1+b1)(a12−a1b1+b12)=(a1+b1)((a1+b1)2−3a1b1)(2) and a1+b1∤p, we have (a1+b1)2−3a1b1=p. Hence p≤(a1+b1)2−3a1b1.(3) Since a1+b1∤p, from (1) and (2) it follows that a1+b1 is a divisor of 2016, i.e., a1+b1≤2016. Then from (3) we obtain p≤(a1+b1)2−3a1b1<(a1+b1)2≤20162. Thus, in this case there are also only a finite number of p satisfying the problem condition.
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Source: MathNet,
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