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Number theory Difficulty 7.7 National olympiad, round 2 Prove it Argentina

A positive integer n>1n > 1, whose positive divisors are
1=d1<d2<<dk=n, 1 = d_1 < d_2 < \dots < d_k = n,
is called sureño if all of the numbers d2d1,d3d2,,dkdk1d_2 - d_1, d_3 - d_2, \dots, d_k - d_{k-1} are divisors of nn.

a. Find a positive integer that is not sureño and has exactly 2022 positive divisors that are sureño.

b. Prove that there are infinitely many positive integers that are not sureño and have exactly 2022 positive divisors that are sureño.

Solution

We will prove that the number 220227k2^{2022}7^k is not sureño and has exactly 2022 sureño divisors (for every positive integer kk).

Every power of 2 is sureño. Indeed, the divisors of 2k2^k are 2j2^j for j=0,,kj = 0, \dots, k and djdj1=2j2j1=2j1d_j - d_{j-1} = 2^j - 2^{j-1} = 2^{j-1} is a divisor of 2k2^k. Therefore we have that the powers of 2 that divide 220227k2^{2022} \cdot 7^k are sureño, that is, 2,22,23,,220222, 2^2, 2^3, \dots, 2^{2022}. We have showed that the number has 2022 sureño divisors.

It remains for us to prove that the other divisors as well as the number itself are not sureño. That is, we have to show that the numbers 2j7k2^j \cdot 7^k are not sureño for j0j \ge 0 and k1k \ge 1.

If j=0j = 0 then the first divisors of the number are 1 and 7. Hence the number is not sureño, since 71=67 - 1 = 6 is not a divisor of 2j7k2^j \cdot 7^k.

If j=1j = 1 then the first divisors of the number are 1, 2 and 7. Again, the number is not sureño, since 72=57 - 2 = 5 is not a divisor of 2j7k2^j \cdot 7^k.

Finally, if j2j \ge 2 then the first divisors of the number are 1, 2, 4 and 7. Hence the number is not sureño, since 74=37 - 4 = 3 is not a divisor of 2j7k2^j \cdot 7^k.

We have completed the proof.

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