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Number theory Difficulty 5.0 AIME Prove it Estonia

Does there exist a positive integer nn which has exactly 9 positive divisors and whose all divisors can be placed in a 3-by-3 table such that the products of the 3 numbers in each row, each column and on each diagonal are all the same?

Solutions — 2

Solution 1

The number 3636 has 99 positive divisors 11, 22, 33, 44, 66, 99, 1212, 1818, 3636. Let the first row be 1818, 11, 1212, second row 44, 66, 99, and third row 33, 3636, 22. Then the product of each row, column and diagonal is 216216.

Solution 2

For each prime pp the number p8p^8 has exactly 99 divisors p0,p1,,p8p^0, p^1, \dots, p^8. It is known that the numbers 11 to 99 can be placed as a 3×33 \times 3 magic square, with an equal sum of the numbers in each row, column and diagonal. By subtracting 11 from each number, we reduce the sum of each row, column and diagonal by 33. By replacing in the magic square each number ii by the respective power pip^i we obtain a placement of the divisors of p8p^8 in which each row, column and diagonal has an equal product.

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