Number theoryDifficulty 5.5AIME, harderProve itJBMO
Problem: If a, b, c, d are integers and A=2(a−2b+c)4+2(b−2c+a)4+2(c−2a+b)4, B=d(d+1)(d+2)(d+3)+1, prove that (A+1)2+B cannot be a perfect square.
Solution
Solution: First we prove the following Lemma Lemma: If x, y, z are real numbers such that x+y+z=0, then 2(x4+y4+z4)=(x2+y2+z2)2. Proof of the Lemma: x4+y4+z4=x2x2+y2y2+z2z2=x2(y+z)2+y2(z+x)2+z2(x+y)2=2(x2y2+y2z2+z2x2)+2xyz(x+y+z)=(x2+y2+z2)2−x4−y4−z4 and the claim follows.
Now back to our problem notice that (a−2b+c)+(b−2c+a)+(c−2a+b)=0, thus according to the lemma it holds A=2(a−2b+c)4+2(b−2c+a)4+2(c−2a+b)4=[(a−2b+c)2+(b−2c+a)2+(c−2a+b)2]=[6(a2+b2+c2−ab−bc−ca)]2 Since a2+b2+c2≥ab+bc+ca we have that A+1=6(a2+b2+c2−ab−bc−ca)+1 In addition, it is easy to check that B=d(d+1)(d+2)(d+3)=(d2+3d+1)2 Let us set 6(a2+b2+c2−ab−bc−ca)+1=m, d2+3d+1=n. We need to prove that the number (A+1)2+B=m2+n2 is not a perfect square. Since both m, n are odd integers, both m2, n2 are integers of the form 4k+1, so the number m2+n2 is an integer of the form 4k+2. But it is well known that all perfect squares are of the form 4k or 4k+1, and we are done.
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