Jane writes down 2024 natural numbers around the perimeter of a circle. She wants the 2024 products of adjacent pairs of numbers to be exactly the set . Can she accomplish this?
Solutions — 2
Solution 1
Given any prime and positive integer , let denote the highest power of dividing . We claim that Jane cannot write 2024 such numbers as that would imply that is the square of the product of the 2024 numbers. Let be a prime and be a natural number such that . Then note that
In particular, let be in . By Bertrand's Postulate, such a prime exists (and must also be odd). Further, the corresponding is either 2 or 3. Either way, is odd from the above formula, and so cannot be a perfect square.
Solution 2
1. Let the numbers on the circle be . Suppose are a permutation of for . If this is possible, then where .
2. Consider the product of all the adjacent pairs:
This simplifies to:
Let . Then:
3. Let be the total number of positive divisors of . We have:
Since is a perfect square, is always odd.
4. Consider the prime number . It divides:
We need to find the exponent of in . Using Legendre's formula for the exponent of a prime in :
Calculating each term:
Thus:
5. The number of divisors of is given by:
Since , the factor contributes to . Therefore, is even.
6. We have a contradiction because must be even, but is odd. Therefore, Jane cannot accomplish her goal.