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Number theory Difficulty 6.1 National olympiad Prove it Estonia

Find the smallest positive integer nn for which there exist two distinct pairs of positive integers (x,y)(x, y) such that (x21)(y21)=n(x^2 - 1)(y^2 - 1) = n and xyx \le y.

Solution

Notice that for n=360n = 360, the pairs x=2,y=11x = 2, y = 11 and x=4,y=5x = 4, y = 5 satisfy the condition.

We will show that for smaller numbers nn, there do not exist two distinct suitable pairs (x,y)(x, y). If x=1x = 1, then (x21)(y21)(x^2 - 1)(y^2 - 1) is not positive. Thus, we can assume that 1<xy1 < x \le y. Now, as xx or yy increases, (x21)(y21)(x^2 - 1)(y^2 - 1) also increases. Let's examine the cases.

* If x=2x = 2, then y=2,3,4,5,6,7,8,9,10,11,y = 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, \dots give us respectively (x21)(y21)=9,24,45,72,105,144,189,240,297,360,(x^2 - 1)(y^2 - 1) = 9, 24, 45, 72, 105, 144, 189, 240, 297, 360, \dots.
* If x=3x = 3, then y=3,4,5,6,7,y = 3, 4, 5, 6, 7, \dots give similarly (x21)(y21)=64,120,192,280,384,(x^2 - 1)(y^2 - 1) = 64, 120, 192, 280, 384, \dots.
* If x=4x = 4, then y=4,5,y = 4, 5, \dots give (x21)(y21)=225,360,(x^2 - 1)(y^2 - 1) = 225, 360, \dots.

No positive number smaller than 360 appeared repeatedly. If we continue the inspection for x=5,6,x = 5, 6, \dots, even the first case y=xy = x would give (x21)(y21)>360(x^2 - 1)(y^2 - 1) > 360, since x=4x = 4 gives 360 or a larger number for the same yy. In conclusion, 360 is the smallest number with the required property.

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