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Number theory Difficulty 5.8 AIME, harder Prove it Belarus

Prove that there are at most a finite number of primes pp such that the equation a3+b3=2016paba^3 + b^3 = 2016pab has a solution in positive integers aa and bb which are not divisible by pp.

Solution

Let dd denote the greatest common divisor of aa and bb, i.e., a=da1a = d a_1, b=db1b = d b_1, where gcd(a1,b1)=1\gcd(a_1, b_1) = 1. Then the given equality can be presented in the form
d(a13+b13)=2016pa1b1. d(a_1^3 + b_1^3) = 2016p a_1 b_1.
It follows that da13+b13d \mid a_1^3 + b_1^3, and, since a1a_1 and b1b_1 are coprime, db13d \mid b_1^3 and da13d \mid a_1^3. Similarly, da13d \mid a_1^3, so da1b1d \mid a_1 b_1 because a1a_1 and b1b_1 are coprime. Let d=d1a1b1d = d_1 a_1 b_1, then the equation can be presented in the form
d1(a13+b13)=2016p.(1) d_1(a_1^3 + b_1^3) = 2016p. \qquad (1)
There are exactly two possible cases:

1) a1+b1pa_1 + b_1 \neq p.

2) a1+b1=pa_1 + b_1 = p.

1) If a1+b1pa_1 + b_1 \neq p, then
pa1+b1    p3(a1+b1)34(a13+b13)42016p    p242016    p22016245=90. \begin{aligned} p \le a_1 + b_1 &\implies p^3 \le (a_1 + b_1)^3 \le 4(a_1^3 + b_1^3) \le 4 \cdot 2016p \\ &\implies p^2 \le 4 \cdot 2016 \\ &\implies p \le 2 \cdot \sqrt{2016} \le 2 \cdot 45 = 90. \end{aligned}
Therefore there are only a finite number of pp satisfying the problem condition.

2) By condition, d1pd_1 \nmid p (otherwise aa and bb are divisible by pp), then from (1) it follows that a13+b13pa_1^3 + b_1^3 \neq p. Since
a13+b13=(a1+b1)(a12a1b1+b12)=(a1+b1)((a1+b1)23a1b1)(2) a_1^3 + b_1^3 = (a_1 + b_1)(a_1^2 - a_1 b_1 + b_1^2) = (a_1 + b_1)((a_1 + b_1)^2 - 3 a_1 b_1) \quad (2)
and a1+b1pa_1 + b_1 \nmid p, we have (a1+b1)23a1b1p(a_1 + b_1)^2 - 3 a_1 b_1 \neq p. Hence
p(a1+b1)23a1b1.(3) p \le (a_1 + b_1)^2 - 3 a_1 b_1. \qquad (3)
Since a1+b1pa_1 + b_1 \nmid p, from (1) and (2) it follows that a1+b1a_1 + b_1 is a divisor of 20162016, i.e., a1+b12016a_1 + b_1 \le 2016. Then from (3) we obtain
p(a1+b1)23a1b1<(a1+b1)220162. p \le (a_1 + b_1)^2 - 3 a_1 b_1 < (a_1 + b_1)^2 \le 2016^2.
Thus, in this case there are also only a finite number of pp satisfying the problem condition.

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