Maths Olympiad Prep

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Number theory Difficulty 5.5 AIME, harder Prove it JBMO

Problem:
If aa, bb, cc, dd are integers and A=2(a2b+c)4+2(b2c+a)4+2(c2a+b)4A = 2(a-2b+c)^{4} + 2(b-2c+a)^{4} + 2(c-2a+b)^{4}, B=d(d+1)(d+2)(d+3)+1B = d(d+1)(d+2)(d+3) + 1, prove that (A+1)2+B(\sqrt{A} + 1)^{2} + B cannot be a perfect square.

Solution

Solution:
First we prove the following Lemma
Lemma: If xx, yy, zz are real numbers such that x+y+z=0x + y + z = 0, then 2(x4+y4+z4)=(x2+y2+z2)22\left(x^{4} + y^{4} + z^{4}\right) = \left(x^{2} + y^{2} + z^{2}\right)^{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)2x4y4z4 \begin{aligned} x^{4} + y^{4} + z^{4} & = x^{2} x^{2} + y^{2} y^{2} + z^{2} z^{2} = x^{2}(y+z)^{2} + y^{2}(z+x)^{2} + z^{2}(x+y)^{2} \\ & = 2\left(x^{2} y^{2} + y^{2} z^{2} + z^{2} x^{2}\right) + 2 x y z(x + y + z) \\ & = \left(x^{2} + y^{2} + z^{2}\right)^{2} - x^{4} - y^{4} - z^{4} \end{aligned}
and the claim follows.

Now back to our problem notice that (a2b+c)+(b2c+a)+(c2a+b)=0(a-2b+c) + (b-2c+a) + (c-2a+b) = 0, thus according to the lemma it holds
A=2(a2b+c)4+2(b2c+a)4+2(c2a+b)4=[(a2b+c)2+(b2c+a)2+(c2a+b)2]=[6(a2+b2+c2abbcca)]2 \begin{aligned} A & = 2(a-2b+c)^{4} + 2(b-2c+a)^{4} + 2(c-2a+b)^{4} \\ & = \left[(a-2b+c)^{2} + (b-2c+a)^{2} + (c-2a+b)^{2}\right] = \left[6\left(a^{2} + b^{2} + c^{2} - ab - bc - ca\right)\right]^{2} \end{aligned}
Since a2+b2+c2ab+bc+caa^{2} + b^{2} + c^{2} \geq ab + bc + ca we have that
A+1=6(a2+b2+c2abbcca)+1 \sqrt{A} + 1 = 6\left(a^{2} + b^{2} + c^{2} - ab - bc - ca\right) + 1
In addition, it is easy to check that
B=d(d+1)(d+2)(d+3)=(d2+3d+1)2 B = d(d+1)(d+2)(d+3) = \left(d^{2} + 3d + 1\right)^{2}
Let us set 6(a2+b2+c2abbcca)+1=m6\left(a^{2} + b^{2} + c^{2} - ab - bc - ca\right) + 1 = m, d2+3d+1=nd^{2} + 3d + 1 = n. We need to prove that the number (A+1)2+B=m2+n2(\sqrt{A} + 1)^{2} + B = m^{2} + n^{2} is not a perfect square.
Since both mm, nn are odd integers, both m2m^{2}, n2n^{2} are integers of the form 4k+14k + 1, so the number m2+n2m^{2} + n^{2} is an integer of the form 4k+24k + 2. But it is well known that all perfect squares are of the form 4k4k or 4k+14k + 1, and we are done.

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