Maths Olympiad Prep

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Number theory Difficulty 5.8 AIME, harder Prove it Ukraine

Find all solutions of the equation x3+y3=z3\sqrt[3]{x} + \sqrt[3]{y} = \sqrt[3]{z}, where xx, yy, zz are integer numbers.

Solution

Any group of three (da3,db3,dc3)(da^3, db^3, dc^3) if a+b=ca + b = c satisfies the condition of the problem.

Let's find solutions of the equation x3+y3+z3=0\sqrt[3]{x} + \sqrt[3]{y} + \sqrt[3]{z} = 0. From the known equation
a3+b3+c33abc=(a+b+c)(a2+b2+c2abbcca) a^3 + b^3 + c^3 - 3abc = (a + b + c)(a^2 + b^2 + c^2 - ab - bc - ca)
it follows that x+y+z=3xyz3x + y + z = 3\sqrt[3]{xyz}, thus (x+y+z)3=27xyz(x + y + z)^3 = 27xyz. We can assume that any two from xx, yy, zz are coprime. Really, if the prime pp divides xx, yy, then it can divide the equation (x+y+z)3=27xyz(x + y + z)^3 = 27xyz and therefore z0(modp)z \equiv 0 \pmod{p}.

Therefore, we can think that x=dx1x = d x_1, y=dy1y = d y_1, z=dz1z = d z_1, where x1x_1, y1y_1, z1z_1 are coprime. Then x1x_1, y1y_1, z1z_1 are the third powers of integers aa, bb, cc. On the other side, any group of three (da3,db3,dc3)(da^3, db^3, dc^3), if a+b=ca + b = c, satisfy the condition of the problem.

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