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Number theory Difficulty 6.2 National olympiad Prove it Ukraine

Find all triples pairwise distinct positive integer numbers (a,b,c)(a, b, c) that satisfy the condition: number 2a12a-1 is divisible by bb, number 2b12b-1 is divisible by cc and number 2c12c-1 is divisible by aa.

Solution

Let's rewrite the conditions in form of a system: there exist natural numbers k,m,nk, m, n, for which equalities are true:
2a1=kb,2b1=nc and 2c1=ma. 2a-1=kb, \quad 2b-1=nc \text{ and } 2c-1=ma.

It is obvious, that all the numbers k,m,nk, m, n and a,b,ca, b, c are odd.
Then we have, that
b=2a1k2b1=4a2k1=ncc=4a2kkn2c1=8a42kkn1=ma8a42kkn=knmaa=4+2k+kn8knm. b = \frac{2a-1}{k} \Rightarrow 2b-1 = \frac{4a-2}{k}-1 = nc \Rightarrow c = \frac{4a-2-k}{kn} \Rightarrow 2c-1 = \frac{8a-4-2k}{kn}-1 = ma \Rightarrow \\ 8a-4-2k-kn = knma \Rightarrow a = \frac{4+2k+kn}{8-knm}.
Now we see, that an equality must be true: knm<8knm<8. Then from symmetry of conditions and oddness of numbers k,m,nk, m, n it follows, that the following variants are possible (with accuracy to cycle).

Variant 1: k=m=n=1k=m=n=1. Then we have c=2b1a=2c1=4b3b=2a1=8b7a=b=c=1c=2b-1 \Rightarrow a=2c-1=4b-3 \Rightarrow b=2a-1=8b-7 \Rightarrow a=b=c=1, but numbers a,b,ca, b, c must be different.

Variant 2: k=3,m=n=1k=3, m=n=1. Then we have c=2b1a=2c1=4b33b=2a1=8b75b=7c=2b-1 \Rightarrow a=2c-1=4b-3 \Rightarrow 3b=2a-1=8b-7 \Rightarrow 5b=7 – contradiction, because bb is not integer.

Variant 3: k=5,m=n=1k=5, m=n=1. Then we have c=2b1a=2c1=4b35b=2a1=8b73b=7c=2b-1 \Rightarrow a=2c-1=4b-3 \Rightarrow 5b=2a-1=8b-7 \Rightarrow 3b=7 – contradiction, because bb is not integer.

Variant 4: k=7,m=n=1k=7, m=n=1. Then we have c=2b1a=2c1=4b37b=2a1=8b7b=7c=13,a=25c=2b-1 \Rightarrow a=2c-1=4b-3 \Rightarrow 7b=2a-1=8b-7 \Rightarrow b=7 \Rightarrow c=13, a=25 and this set satisfies the condition.

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