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Number theory Difficulty 5.6 AIME, harder Prove it Croatia

In how many ways can the number 20112010\frac{2011}{2010} be represented as a product of two fractions of the form n+1n\frac{n+1}{n}, where nn is a positive integer? (Order of the factors is not important.)

Solution

Let pp and qq be positive integers such that 20112010=p+1pq+1q\frac{2011}{2010} = \frac{p+1}{p} \cdot \frac{q+1}{q}.
Then 2011pq=2010(pq+p+q+1)2011pq = 2010(pq + p + q + 1) i.e. pq=2010(p+q+1)pq = 2010(p + q + 1).

From the last equation we find
p=2010(q+1)q2010=2010(q2010)+20102011q2010=2010+20102011q2010. p = \frac{2010(q+1)}{q-2010} = \frac{2010(q-2010)+2010 \cdot 2011}{q-2010} = 2010 + \frac{2010 \cdot 2011}{q-2010}.
Since pp and qq are positive integers, it follows that q2010q - 2010 is a positive divisor of 201020112010 \cdot 2011. Each divisor of 201020112010 \cdot 2011 corresponds to exactly one pair (p,q)(p, q). Since 20102011=2356720112010 \cdot 2011 = 2 \cdot 3 \cdot 5 \cdot 67 \cdot 2011, the number of its divisors is 25=322^5 = 32. Finally, since the pairs (p,q)(p, q) and (q,p)(q, p) determine the same representation, the number of required representations is 16.

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