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Number theory Difficulty 4.9 AIME Prove it Estonia

How many positive integers nn are there for which 2014n2014 \cdot n is divisible by 2014+n2014 + n?

Solutions — 2

Solution 1

If a=1,d=2014a = 1, d = 2014, then a+x2014a+x \mid 2014. As a+x>1a+x > 1, there are 7 possibilities.
If a=2,d=1007a = 2, d = 1007, then a+x1007a+x \mid 1007. As a+x>2a+x > 2, there are 3 possibilities.
If a=19,d=106a = 19, d = 106, then a+x106a+x \mid 106. As a+x>19a+x > 19, there are 2 possibilities.
If a=38,d=53a = 38, d = 53, then a+x53a+x \mid 53. As a+x>38a+x > 38, there is 1 possibility.
Those add up to 13 possibilities.

Solution 2

As the number 2014+n2014 + n is always a divisor of 2014(2014+n)=20142+2014n2014 \cdot (2014 + n) = 2014^2 + 2014n, the number 2014+n2014 + n is a divisor of 2014n2014n if and only if 2014+n2014 + n is a divisor of 201422014^2. The canonical representation 20142=221925322014^2 = 2^2 \cdot 19^2 \cdot 53^2 shows that the number of divisors of 201422014^2 is (2+1)(2+1)(2+1)=27(2+1) \cdot (2+1) \cdot (2+1) = 27. The middle one of them is 2014 and 13 divisors are expressible in a form 2014+n2014 + n for some positive integer nn. Thus there are 13 possibilities for nn.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.