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Number theory Difficulty 6.8 National olympiad Prove it Russia

Four distinct integers are written on the board. It is known that the absolute value of each of them is greater than 10610^6, and there is no positive integer greater than 11 which divides all these numbers. Pete wrote on a paper six pairwise sums of the numbers from the board. Then he partitioned the obtained six numbers into three pairs and multiplied the numbers in each pair. May it happen that all three obtained products are equal?

На доске написаны четыре попарно различных целых числа, модуль каждого из которых больше миллиона. Известно, что не существует натурального числа, большего 11, на которое бы делилось каждое из четырех написанных чисел. Петя записал в тетрадку шесть попарных сумм этих чисел, разбил эти шесть сумм на три пары и перемножил числа в каждой паре. Могли ли все три произведения оказаться равными?

Solution

Yes.

Two possible examples are
(N23N+1, N2N+1, 3N2+3N1, N2+N1) (N^2 - 3N + 1,\ N^2 - N + 1,\ -3N^2 + 3N - 1,\ N^2 + N - 1)
and
(N3N2+1, N33N2+2N1, N3+N22N+1, N3+N2+2N1), (N^3 - N^2 + 1,\ N^3 - 3N^2 + 2N - 1,\ -N^3 + N^2 - 2N + 1,\ -N^3 + N^2 + 2N - 1),
where NN is an integer greater than 10610^6.

First solution. For example, the numbers
x=N23N+1, y=N2N+1, z=3N2+3N1, t=N2+N1, x = N^2 - 3N + 1,\ y = N^2 - N + 1,\ z = -3N^2 + 3N - 1,\ t = N^2 + N - 1,
where NN is a natural number greater than a million.

It is easy to see that these numbers are also greater than a million in absolute value and pairwise distinct. Their pairwise sums are
x+y=2(N1)2, x+z=2N2, y+z=2N(N1), z+t=2(N1)2, y+t=2N2, x+t=2N(N1), x + y = 2(N - 1)^2,\ x + z = -2N^2,\ y + z = -2N(N - 1),\ z + t = -2(N - 1)^2,\ y + t = 2N^2,\ x + t = 2N(N - 1),
with the products
(x+y)(x+z)=(y+t)(z+t)=(x+t)(y+z)=4N2(N1)2. (x + y)(x + z) = (y + t)(z + t) = (x + t)(y + z) = -4N^2(N - 1)^2.

It remains to check that the numbers x,y,z,tx, y, z, t are coprime as a set. If they have a common natural divisor dd, then dd also divides x+y=2(N1)2x + y = 2(N - 1)^2 and y+t=2N2y + t = 2N^2; thus, dgcd(2N2,2(N1)2)=2d \le \gcd(2N^2, 2(N - 1)^2) = 2. The case d=2d = 2 is impossible since y=N(N1)+1y = N(N - 1) + 1 is odd. Therefore, d=1d = 1, as required.

Second solution. Another example is
x=N3N2+1, y=N33N2+2N1, z=N3+N22N+1, t=N3+N2+2N1, x = N^3 - N^2 + 1,\ y = N^3 - 3N^2 + 2N - 1,\ z = -N^3 + N^2 - 2N + 1,\ t = -N^3 + N^2 + 2N - 1,
where NN is a natural number greater than a million. Again, these numbers are greater than a million in absolute value and pairwise distinct. Their pairwise sums are
x+y=2N(N1)2, x+z=2(N1), y+z=2N2, z+t=2N2(N1), y+t=2(N1)2, x+t=2N, x + y = 2N(N - 1)^2,\ x + z = -2(N - 1),\ y + z = -2N^2,\ z + t = -2N^2(N - 1),\ y + t = -2(N - 1)^2,\ x + t = 2N,
and they can be grouped into three pairs with equal products:
(x+y)(x+t)=(x+z)(z+t)=(y+z)(y+t)=4N2(N1)2. (x + y)(x + t) = (x + z)(z + t) = (y + z)(y + t) = 4N^2(N - 1)^2.

The coprimality of x,y,z,tx, y, z, t is checked analogously to the previous solution.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.