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Number theory Difficulty 6.3 National olympiad Prove it

A special number is a positive integer nn for which there exist positive integers a,b,ca, b, c and dd with

n=a3+2b3c3+2d3 n=\frac{a^{3}+2 b^{3}}{c^{3}+2 d^{3}}

Prove that:
(a) there are infinitely many special numbers;
(b) 2014 is not a special number.

Solution

(a) Every perfect cube k3k^{3} of a positive integer is special because we can write

k3=k3a3+2b3a3+2b3=(ka)3+2(kb)3a3+2b3 k^{3}=k^{3} \frac{a^{3}+2 b^{3}}{a^{3}+2 b^{3}}=\frac{(k a)^{3}+2(k b)^{3}}{a^{3}+2 b^{3}}

for some positive integers a,ba, b.
(b) Observe that 2014=219532014=2 \cdot 19 \cdot 53. If 2014 is special, then we have,

x3+2y3=2014(u3+2v3) x^{3}+2 y^{3}=2014\left(u^{3}+2 v^{3}\right)

for some positive integers x,y,u,vx, y, u, v. We may assume that x3+2y3x^{3}+2 y^{3} is minimal with this property. Now, we will use the fact that if 19 divides x3+2y3x^{3}+2 y^{3}, then it divides both xx and yy. Indeed, if 19 does not divide xx, then it does not divide yy too. The relation x32y3(mod19)x^{3} \equiv -2 y^{3} \pmod{19} implies (x3)6(2y3)6(mod19)\left(x^{3}\right)^{6} \equiv \left(-2 y^{3}\right)^{6} \pmod{19}. The latter congruence is equivalent to x1826y18(mod19)x^{18} \equiv 2^{6} y^{18} \pmod{19}. Now, according to Fermat's Little Theorem, we obtain 126(mod19)1 \equiv 2^{6} \pmod{19}, that is 19 divides 63, not possible.
It follows x=19x1,y=19y1x=19 x_{1}, y=19 y_{1}, for some positive integers x1x_{1} and y1y_{1}. Replacing in (1) we get

192(x13+2y13)=253(u3+2v3) 19^{2}\left(x_{1}^{3}+2 y_{1}^{3}\right)=2 \cdot 53\left(u^{3}+2 v^{3}\right)

i.e. 19u3+2v319 \mid u^{3}+2 v^{3}. It follows u=19u1u=19 u_{1} and v=19v1v=19 v_{1}, and replacing in (2) we get

x13+2y13=2014(u13+2v13). x_{1}^{3}+2 y_{1}^{3}=2014\left(u_{1}^{3}+2 v_{1}^{3}\right) .

Clearly, x13+2y13<x3+2y3x_{1}^{3}+2 y_{1}^{3}<x^{3}+2 y^{3}, contradicting the minimality of x3+2y3x^{3}+2 y^{3}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.