Question 416822: Find the greatest four-digit number that has exactly three positive factors.
Answer by richard1234(7193)   (Show Source):
You can put this solution on YOUR website! If it has an odd number of positive factors the number must be a perfect square, as one of its factors will be duplicated, reducing the number of factors by 1. Hence we are looking for the perfect square less than 10000, or 100^2. Therefore, 99^2 = 9801 is the greatest such four-digit number.
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