Number theoryDifficulty 5.6AIME, harderProve itJBMO
Problem: Find all the integers pairs (x,y) which satisfy the equation x5−y5=16xy
Solution
Solution: If one of x,y is 0, the other has to be 0 too, and (x,y)=(0,0) is one solution.
If xy=0, let d=gcd(x,y) and we write x=da, y=db, a,b∈Z with (a,b)=1. Then, the given equation is transformed into d3a5−d3b5=16ab So, by the above equation, we conclude that a∣d3b5 and thus a∣d3. Similarly b∣d3. Since (a,b)=1, we get that ab∣d3, so we can write d3=abr with r∈Z. Then, equation (1) becomes abra5−abrb5r(a5−b5)=16ab⇒=16 Therefore, the difference a5−b5 must divide 16. Therefore, the difference a5−b5 must divide 16. This means that a5−b5=±1,±2,±4,±8,±16 The smaller values of a5−b5 are 1 or 2. Indeed, if a5−b5=1 then a=±1 and b=0 or a=0 and b=±1, a contradiction. If a5−b5=2, then a=1 and b=−1 or a=−1 and b=1. Then r=−8, and d3=−8 or d=−2. Therefore, (x,y)=(−2,2).
If a5−b5>2 then, without loss of generality, let a>b and a≥2. Putting a=x+1 with x≥1, we have a5−b5=(x+1)5−b5≥(x+1)5−x5=5x4+10x3+10x2+5x+1≥31 which is impossible. Thus, the only solutions are (x,y)=(0,0) or (−2,2).
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Source: MathNet,
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