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Number theory Difficulty 7.8 National olympiad, round 2 Prove it Bulgaria

Find the least positive integer nn, such that there exist positive integers aa, bb and cc, none of which is a perfect square and
a3+b3+c33abc=2013n. a^3 + b^3 + c^3 - 3abc = 2013^n.

Solution

It is easy to be seen that the left hand side is divisible by 99 implying that the equation has no solution for n=1n = 1.

Let n=2n=2. We have 20132=(31161)22013^2 = (3 \cdot 11 \cdot 61)^2, 32=23+133^2 = 2^3 + 1^3 and 1161=83+43+13+384111 \cdot 61 = 8^3 + 4^3 + 1^3 + 3 \cdot 8 \cdot 4 \cdot 1. We use the identity
(a3+b3+c33abc)(x3+y3+z33xyz)=u3+v3+w33uvw. (a^3 + b^3 + c^3 - 3abc)(x^3 + y^3 + z^3 - 3xyz) = u^3 + v^3 + w^3 - 3uvw.
where u=ax+by+czu = ax + by + cz, v=ay+bz+cxv = ay + bz + cx and w=az+bx+cyw = az + bx + cy. We apply the above identity first for (8,4,1)(8, 4, -1) and (4,8,1)(4, 8, -1), and then for the obtained triples (65,0,56)(65, 0, 56) and (2,1,0)(2, 1, 0) and obtain 653+563=(1161)265^3 + 56^3 = (11 \cdot 61)^2 and 1303+1773+563313017756=20132130^3 + 177^3 + 56^3 - 3 \cdot 130 \cdot 177 \cdot 56 = 2013^2.

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