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Number theory Difficulty 5.6 AIME, harder Prove it Estonia

Determine all triples (a,b,c)(a,b,c) of integers which satisfy the equation
(ab)3(a+b)2=c2+2(ab)+1. (a-b)^3(a+b)^2 = c^2 + 2(a-b) + 1.

Solution

Answer: (0,1,0),(1,0,0)(0,1,0), (-1,0,0).

Substituting x=abx = a - b, y=a+by = a + b we obtain the equation
x3y2=c2+2x+1, x^3 y^2 = c^2 + 2x + 1,
which suffices to be solved in integers such that xx and yy have equal parity.
We shall consider the equivalent equation
x(x2y22)=c2+1. x(x^2 y^2 - 2) = c^2 + 1.
If both xx and yy are even then x2y22x^2 y^2 - 2 is even, whence the l.h.s. is divisible by 4. Thus c21(mod4)c^2 \equiv -1 \pmod 4, but this is impossible since 1-1 is not a quadratic residue modulo 4.
Let now both xx and yy be odd; then x2y2x^2 y^2 is a positive odd number. Consider
two cases:
* If x2y2=1x^2 y^2 = 1 then x2=1x^2 = 1 and y2=1y^2 = 1. Since x2y22=1<0x^2 y^2 - 2 = -1 < 0 whereas the product x(x2y22)x(x^2 y^2 - 2) equals the positive number c2+1c^2 + 1, we must have x=1x = -1. The possibilities x=1,y=1x = -1, y = 1 and x=1,y=1x = -1, y = -1 give a=0,b=1a = 0, b = 1 and a=1,b=0a = -1, b = 0, respectively. In both cases c=0c = 0.
* If x2y2>1x^2 y^2 > 1 then x2y22>0x^2 y^2 - 2 > 0. Squares of odd numbers give remainder 1 upon division by 4, whence x2y22112=1(mod4)x^2 y^2 - 2 \equiv 1 \cdot 1 - 2 = -1 \pmod 4. Consequently there exists a prime divisor pp of x2y22x^2 y^2 - 2 such that p1(mod4)p \equiv -1 \pmod 4. But then c21(modp)c^2 \equiv -1 \pmod p, which is impossible since 1-1 is not a quadratic residue modulo pp.

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