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Romanian Mathematical Olympiad

Number theory Difficulty 5.4 AIME, harder Prove it Romania

Find all positive integers that have exactly 88 positive divisors, among which three are primes of the form aa, bc‾\overline{bc} and cb‾\overline{cb}, given that a+bc‾+cb‾a + \overline{bc} + \overline{cb} is a perfect square and a,b,ca, b, c are digits, with b<cb < c.

Solution

Suppose xx is an integer with the given properties. Then xx is a multiple of y=a⋅bc‾⋅cb‾y = a \cdot \overline{bc} \cdot \overline{cb}. Since yy has eight divisors, it follows that x=yx = y.

The numbers bc‾\overline{bc} and cb‾\overline{cb} are distinct primes, so bc‾∈{13,17,37,79}\overline{bc} \in \{13, 17, 37, 79\}. The condition that a+bc‾+cb‾a + \overline{bc} + \overline{cb} is a square and aa is a prime forces a=5a = 5, bc‾=13\overline{bc} = 13 and cb‾=31\overline{cb} = 31, implying x=2015x = 2015.

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