Maths Olympiad Prep

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Number theory Difficulty 5.4 AIME, harder Prove it Argentina

Prove that there are infinitely many positive integers nn such that the equation
x2+y11z2022!=n x^2 + y^{11} - z^{2022!} = n
has no solution (x,y,z)(x, y, z) over the integers.

Solution

We claim that if n=23k+20n = 23k + 20 with kk a non-negative integer then the equation has no solution over the integers. Let's assume that there is one and get a contradiction. Indeed, by Fermat's little theorem,
x2+y11n+z2022!20+(0 or 1)20 or 21(mod23) x^2 + y^{11} \equiv n + z^{2022!} \equiv 20 + (0 \text{ or } 1) \equiv 20 \text{ or } 21 \pmod{23}
and hence
x2+(1,0 or 1)20 or 21(mod23) x^2 + (-1, 0 \text{ or } 1) \equiv 20 \text{ or } 21 \pmod{23}
because 1,0-1, 0 and 11 are the only congruence classes modulo 2323 such that its squares are congruent to 00 or 11 modulo 2323.
It follows that
x219,20,21 or 22(mod23) x^2 \equiv 19, 20, 21 \text{ or } 22 \pmod{23}
which is a contradiction because neither of them is a quadratic residue as can be seen by direct inspection.

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