Maths Olympiad Prep

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Number theory Difficulty 5.7 AIME, harder Prove it China

Find all positive integers nn such that equation 1x+1y=1n\frac{1}{x} + \frac{1}{y} = \frac{1}{n} has exactly 2011 positive integer solutions (x,y)(x, y) with xyx \le y.

Solution

From the given equation, we have xynxny=0(xn)(yn)=n2xy - nx - ny = 0 \Rightarrow (x-n)(y-n) = n^2. Then, besides x=y=2nx = y = 2n, for any xnx-n equal to a proper divisor of n2n^2, we will get a positive integer solution (x,y)(x, y) satisfying the required condition. Therefore, n2n^2 should have exactly 2010 proper divisors that are less than nn.

Suppose n=p1α1pkαkn = p_1^{\alpha_1} \cdots p_k^{\alpha_k}, where p1,,pkp_1, \dots, p_k are prime numbers different from each other. Then the number of proper divisors of n2n^2 less than nn is
(2α1+1)(2αk+1)12 \frac{(2\alpha_1 + 1) \cdots (2\alpha_k + 1) - 1}{2}
So (2α1+1)(2αk+1)=4021(2\alpha_1 + 1) \cdots (2\alpha_k + 1) = 4021. Since 4021 is prime, we get k=1,2α1+1=4021k = 1, 2\alpha_1 + 1 = 4021, and α1=2010\alpha_1 = 2010.

Therefore, n=p2010n = p^{2010}, where pp is any prime number.

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