Let be a natural number whose all positive divisors are denoted as in such a way that (thus and ). Determine all the values of for which both equalities and hold. (Matúš Harminc)
Solutions — 2
Solution 1
We distinguish whether is odd or even.
i) The case of odd. Since all the 's are odd too, it follows from that as well as , hence . In view of , the relations imply that . Substituting this into , we obtain or . Thus the four numbers 1, 3, 11 and 33 are divisors of , more exactly all its divisors smaller than 50, since the fifth divisor satisfies . Consequently, it holds that , , , , , and thus . The number 2013 is satisfactory indeed, because its first small divisors as indicated in the last sentence and moreover, the subsequent divisors are and ; hence as required.
ii) The case of even. Now the equality implies that and hence as well. Since , and , we conclude that either , or , with some integer . But the last is impossible (otherwise is a divisor of with , a contradiction), Therefore, we have , and hence 3 is a divisor of between and , a contradiction. In this way, the nonexistence of any satisfactory even is proven.
Answer. The problem has the only solution .
Solution 2
The divisors and of , with , can be represented as and , where and () are some positive divisors of again. Substituting this into , we obtain (after cancelling ) an equation which can be solved in a standard way, for example by a simple factorization:
The first equation implies that and hence as well. Note that yields . Taking into account the prime factorization , we conclude that the ordered pair of factors must belong to the following set
However, congruences modulo 3 imply the only two pairs (88, 1) and (22, 4) are admissible. The corresponding pairs are (33, 3) and (11, 4), respectively.
If , then (and ), thus 1, 3, 11 and 33 are divisors of which leads (as in the above solution) to the solution .
If , then and , thus 1, 2, 4, 11, 22 and 44 are divisors of , which contradicts .