2017 USAJMO Problems/Problem 1
Prove that there are infinitely many distinct pairs of relatively prime integers and such that is divisible by .
Let and . We see that . Therefore, we have , as desired.
(Credits to mathmaster2012)
Let be odd where . We have so This means that and since x is odd, or as desired.
Because problems such as this usually are related to expressions along the lines of , it's tempting to try these. After a few cases, we see that is convenient due to the repeated occurrence of when squared and added. We rewrite the given expressions as: After repeatedly factoring the initial equation,we can get: Expanding each of the squares, we can compute each product independently then sum them: Now we place the values back into the expression: Plugging any positive integer value for into yields a valid solution, because there is an infinite number of positive integers, there is an infinite number of distinct pairs .
Let and , where leaves a remainder of when divided by .We seek to show that because that will show that there are infinitely many distinct pairs of relatively prime integers and such that is divisible by .
Claim 1: . We have that the remainder when is divided by is and the remainder when is divided by is always . Therefore, the remainder when is divided by is always going to be .
Claim 2: We know that and , so the remainder when is divided by is always going to be .
Claim 3: Trivial given claim .
I claim (a,b) = (2n-1,2n+1) always satisfies the conditions, n is integer, ,
Proof: We can expand using binomial theorem. However, since 2n-1 + 2n+1 = 4n, all the terms with power 2n larger than 2 modulo 4n evaluate to 0, and thus can be omitted. We are left with the terms: , which is divisible by 4n.
Since there are infinitely many integers larger than 2n, the proof is complete .
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