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Hrvatska 2011

Croatia 2011 number theory

Problem

In how many ways can the number be represented as a product of two fractions of the form , where is a positive integer? (Order of the factors is not important.)
Solution
Let and be positive integers such that . Then , i.e. .

From the last equation we find Since and are positive integers, it follows that is a positive divisor of . Each divisor of corresponds to exactly one pair . Since , the number of its divisors is . Finally, since the pairs and determine the same representation, the number of required representations is 16.
Final answer
16

Techniques

Factorization techniquesτ (number of divisors)Techniques: modulo, size analysis, order analysis, inequalities