Product of Even Numbers

Number Theory Level pending

What is the largest integer N 2017 N\leq2017 such that there is exactly one way to express N N as a product of even positive integers p p and q q with

  • None of p p and q q is divisible by 4 4 and

  • p q p\leq q .

Hint :

  • For N = 12 N=12 , there is exactly one way; namely, 12 = 2 × 6 12=2\times 6 .


The answer is 2012.

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1 solution

Oliver Papillo
Dec 31, 2016

For there to be only one way to express N N as a product of p p and q q through the conditions, N N must be the product of 4 4 and a prime, k k .

This means that N = 2 2 k N = 2^2 * k .

There is only one pair of integers p p and q q that will suffice here, and they are 2 2 and 2 k 2k .

k k must be the largest prime 2017 4 = 504 ≤\left \lfloor{\frac{2017}{4}}\right \rfloor =504 .

Thus k k is 503, and N N is 4 k = 4 503 = 2012 4k = 4*503 = 2012 .

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