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Number theory Difficulty 6.5 National Olympiad Prove it Romania

Let M1M \ge 1 be a real number. Determine all natural numbers nn for which there exist pairwise distinct natural numbers a,b,c>Ma, b, c > M, such that
n=(a,b)(b,c)+(b,c)(c,a)+(c,a)(a,b) n = (a, b) \cdot (b, c) + (b, c) \cdot (c, a) + (c, a) \cdot (a, b)
(where (x,y)(x, y) denotes the greatest common divisor of natural numbers xx and yy).

Solution

Call good a number nNn \in \mathbb{N} for which there exist a,b,cN1a, b, c \in \mathbb{N}_{\ge 1} such that
n=(a,b)(b,c)+(b,c)(c,a)+(c,a)(a,b).n = (a, b) \cdot (b, c) + (b, c) \cdot (c, a) + (c, a) \cdot (a, b).
We will prove that, regardless of the value of MM, the good numbers are those of the form n=22t(2k+1)n = 2^{2t}(2k+1), where t,kNt, k \in \mathbb{N}, k1k \ge 1.
Let k1k \ge 1 and p,q,r>Mp, q, r > M three distinct primes such that r>max{kp,kq}r > \max\{kp, kq\}. Considering a=kp,b=kqa = kp, b = kq and c=rc = r, we have (a,b)=k(a, b) = k and (b,c)=(c,a)=1(b, c) = (c, a) = 1, which leads to (a,b)(b,c)+(b,c)(c,a)+(c,a)(a,b)=2k+1(a, b) \cdot (b, c) + (b, c) \cdot (c, a) + (c, a) \cdot (a, b) = 2k+1, so 2k+12k+1 is good.
Let tNt \in \mathbb{N} and nn be a natural number for which there exist a,b,c>Ma, b, c > M, pairwise distinct, such that n=(a,b)(b,c)+(b,c)(c,a)+(c,a)(a,b)n = (a, b) \cdot (b, c) + (b, c) \cdot (c, a) + (c, a) \cdot (a, b). For every tNt \in \mathbb{N}, we have 2ta,2tb,2tc>M2^t a, 2^t b, 2^t c > M and 22tn=(2ta,2tb)(2tb,2tc)+(2tb,2tc)(2tc,2ta)+(2tc,2ta)(2ta,2tb)2^{2t} \cdot n = (2^t a, 2^t b) \cdot (2^t b, 2^t c) + (2^t b, 2^t c) \cdot (2^t c, 2^t a) + (2^t c, 2^t a) \cdot (2^t a, 2^t b), so all the numbers of the form n=22t(2k+1)n = 2^{2t}(2k+1) are good.

Further, we prove that the numbers of the form n=2tn = 2^t or n=22t+1(2k+1)n = 2^{2t+1}(2k+1), where t,kNt, k \in \mathbb{N}, k1k \ge 1, are not good, so they don't satisfy the conditions of the problem either.
First, we prove that any good number that is even is actually divisible by 4.
Consider n0n_0 an even number. If n0=(a,b)(b,c)+(b,c)(c,a)+(c,a)(a,b)n_0 = (a, b) \cdot (b, c) + (b, c) \cdot (c, a) + (c, a) \cdot (a, b), then (a,b)(a, b), (b,c)(b, c) and (c,a)(c, a) can't be simultaneously odd. For example, if 2(a,b)2 \mid (a, b), then 2(b,c)(c,a)2 \mid (b, c) \cdot (c, a), so a,ba, b and cc are even (the other cases are completely similar). It follows that 4n04 \mid n_0. Also, the number n04\frac{n_0}{4} is good too, because n04=(a,b)(b,c)+(b,c)(c,a)+(c,a)(a,b)\frac{n_0}{4} = (a', b') \cdot (b', c') + (b', c') \cdot (c', a') + (c', a') \cdot (a', b'), where a=a2,b=b2,c=c2a' = \frac{a}{2}, b' = \frac{b}{2}, c' = \frac{c}{2}.
Suppose, for the sake of contradiction, that there exists t2t \ge 2 such that n=2tn = 2^t is good. Consequently, 2t2,2t4,2t6,2^{t-2}, 2^{t-4}, 2^{t-6}, \dots etc. are also good, so either 1 or 2 should be also good. This is a contradiction, since if a,b,c1a, b, c \ge 1, then (a,b)(b,c)+(b,c)(c,a)+(c,a)(a,b)3(a, b) \cdot (b, c) + (b, c) \cdot (c, a) + (c, a) \cdot (a, b) \ge 3.
Similarly, if n=22t+1(2k+1)n = 2^{2t+1}(2k+1), with t,kNt, k \in \mathbb{N}, k1k \ge 1 is good, then 22t1(2k+1)2^{2t-1}(2k+1), 22t3(2k+1),,2(2k+1)2^{2t-3}(2k+1), \dots, 2(2k+1) are also good, impossible, since 2(2k+1)2(2k+1) is not divisible by 4.

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