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Number theory Difficulty 5.8 AIME, harder Prove it North Macedonia

Find all the triples of integers (a,b,c)(a,b,c) such that the number
N=(ab)(bc)(ca)2+2 N = \frac{(a-b)(b-c)(c-a)}{2} + 2
is a power of 20162016.
(A power of 20162016 is an integer of the form 2016n2016^n, where nn is a non-negative integer).

Solution

Let aa, bb, cc be integers and nn be a positive integer such that
(ab)(bc)(ca)+4=22016n. (a-b)(b-c)(c-a) + 4 = 2 \cdot 2016^n.
We set ab=xa-b = -x, bc=yb-c = -y and we rewrite the equation as
xy(x+y)+4=22016n.(1) xy(x+y) + 4 = 2 \cdot 2016^n. \tag{1}
If n>0n > 0, then the right hand side is divisible by 77, so we have that
xy(x+y)+40(mod7)(2) xy(x+y) + 4 \equiv 0 \pmod{7} \tag{2}
or
3xy(x+y)2(mod7)(3) 3xy(x+y) \equiv 2 \pmod{7} \tag{3}
or
(x+y)3x3y32(mod7).(4) (x+y)^3 - x^3 - y^3 \equiv 2 \pmod{7}. \tag{4}
Note that, by Fermat's Little Theorem, for any integer kk the cubic residues are k31,0,1(mod7)k^3 \equiv -1, 0, 1 \pmod{7}.
It follows that in (1) some of (x+y)3(x+y)^3, x3x^3 and y3y^3 should be divisible by 77.
But in this case, xy(x+y)xy(x+y) is divisible by 77 and this is a contradiction.
So, the only possibility is to have n=0n = 0 and consequently, xy(x+y)+4=2xy(x+y) + 4 = 2, or, equivalently,
xy(x+y)+4=2.(7) xy(x+y) + 4 = -2. \tag{7}
The solutions for this are (x,y){(1,1),(2,1),(1,2)}(x, y) \in \{(-1, -1), (2, -1), (-1, 2)\},
so the required triples are (a,b,c)=(k+2,k+1,k)(a, b, c) = (k+2, k+1, k), kZk \in \mathbb{Z}, and all their cyclic permutations.

Alternative version:
If n>0n > 0 then 99 divides (ab)(bc)(ca)+4(a-b)(b-c)(c-a) + 4, that is, the equation
xy(x+y)+40(mod9) xy(x+y) + 4 \equiv 0 \pmod{9}
has the solution x=bax = b-a, y=cby = c-b.
But then xx and yy have to be 11 modulo 33, implying xy(x+y)2(mod9)xy(x+y) \equiv 2 \pmod{9}, which is a contradiction.
We can continue now as in the first version.

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