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Number theory Difficulty 7.7 National Olympiad, round 2 Prove it United States

Determine, with proof, whether or not there exist integers a,b,c>2010a, b, c > 2010 satisfying the equation
a3+2b3+4c3=6abc+1. a^3 + 2b^3 + 4c^3 = 6abc + 1.

Solution

We claim there do exist such integers a,b,ca, b, c.

Note that (a1,b1,c1)=(1,1,1)(a_1, b_1, c_1) = (1, 1, 1) satisfy the given equation. For n>1n > 1, define (an+1,bn+1,cn+1)(a_{n+1}, b_{n+1}, c_{n+1}) by
(an+1,bn+1,cn+1)=(an+2cn+2bn,bn+an+2cn,cn+bn+an). (a_{n+1}, b_{n+1}, c_{n+1}) = (a_n + 2c_n + 2b_n, b_n + a_n + 2c_n, c_n + b_n + a_n).
It is not hard to verify algebraically that
an+13+2bn+13+4cn+136an+1bn+1cn+1=an3+2bn3+4cn36anbncn, a_{n+1}^3 + 2b_{n+1}^3 + 4c_{n+1}^3 - 6a_{n+1}b_{n+1}c_{n+1} = a_n^3 + 2b_n^3 + 4c_n^3 - 6a_n b_n c_n,
which shows that (an,bn,cn)(a_n, b_n, c_n) satisfy the given equation for all n1n \ge 1. Moreover, notice that an+1>ana_{n+1} > a_n, bn+1>bnb_{n+1} > b_n, and cn+1>cnc_{n+1} > c_n for all n1n \ge 1; this implies that for sufficiently large nn we will have an,bn,cn>2010a_n, b_n, c_n > 2010, yielding the desired result.

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