Maths Olympiad Prep

Library / /5 of 22

Number theory Difficulty 7.9 National Olympiad, round 2 Prove it United States

Find all triples (x,y,z)(x, y, z) of positive integers such that xyzx \le y \le z and
x3(y3+z3)=2012(xyz+2). x^3(y^3 + z^3) = 2012(xyz + 2).

Solution

The solution is (x,y,z)=(2,251,252)(x, y, z) = (2, 251, 252).
By the given, xx divides 22012=235032 \cdot 2012 = 2^3 \cdot 503. Moreover, xx cannot be a multiple of 503503, by considering the exponent of 503503 in both terms of the equation. Similarly, by considering the powers of 22 on both sides of the equation, we conclude that xx cannot be a multiple of 44, thus x=1x = 1 or x=2x = 2. Thus, we have either
y3+z3=2012(yz+2) or y3+z3=503(yz+1). y^3 + z^3 = 2012(yz + 2) \text{ or } y^3 + z^3 = 503(yz + 1).
In either case, 503503 divides y3+z3y^3 + z^3, so y3z3(mod503)y^3 \equiv -z^3 \pmod{503}. Consequently, by Fermat's little theorem, we have y2y504y3168z3168z2(mod503)y^2 \equiv y^{504} \equiv y^{3 \cdot 168} \equiv z^{3 \cdot 168} \equiv z^2 \pmod{503}. From y3z3(mod503)y^3 \equiv -z^3 \pmod{503} and z2y2(mod503)z^2 \equiv y^2 \pmod{503}, we conclude that either both yy and zz are divisible by 503503 or yz(mod503)y \equiv -z \pmod{503}. Hence in all cases y+zy+z is a multiple of 503503. Write y+z=503ky+z=503k with kk a positive integer.
Next, we cannot have y=zy = z, as it would then follow that either y2+1y^2 + 1 or y2+2y^2 + 2 divides 2y32y^3, which forces y1y \le 1. For (yz)2=1(y-z)^2 = 1, say y=z+1y = z + 1 the first equation has no solution because y+zy+z must be even, while the second one has only the solution (y,z)=(251,252)(y, z) = (251, 252), as in this case the equation simply becomes x+y=503x + y = 503. We now prove that we cannot have (yz)24(y-z)^2 \ge 4. Indeed, if this happened, write y+z=503ky+z = 503k and note that y2yz+z2>yz+2y^2 - yz + z^2 > yz + 2. Thus for the first equation we would get k<4k < 4, while for the second one k<1k < 1. Consider the first equation with k<4k < 4. As y+zy+z is even, kk is even and so k=3k = 3. But then 33 divides both y+zy+z and yz+2yz+2, so 33 divides y2+1y^2+1, which is impossible.
Thus, the only solution is (2,251,252)(2, 251, 252).

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: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.