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Number theory Difficulty 6.5 National olympiad Prove it Saudi Arabia

Find all pairs of integers (x,y)(x, y) such that
y3=8x6+2x3yy2. y^{3} = 8x^{6} + 2x^{3}y - y^{2}.

Solution

We rewrite the equation:
y3+y22x3y8x6=0. y^{3} + y^{2} - 2x^{3}y - 8x^{6} = 0.

Consider this as a cubic in yy:
y3+y22x3y8x6=0. y^{3} + y^{2} - 2x^{3}y - 8x^{6} = 0.

Let us try small integer values for xx.

If x=0x = 0:
y3+y2=0    y2(y+1)=0    y=0 or y=1. y^{3} + y^{2} = 0 \implies y^{2}(y + 1) = 0 \implies y = 0 \text{ or } y = -1.
So (0,0)(0, 0) and (0,1)(0, -1) are solutions.

If x=1x = 1:
y3+y22y8=0. y^{3} + y^{2} - 2y - 8 = 0.
Try y=1y = 1:
1+128=81 + 1 - 2 - 8 = -8.
Try y=2y = 2:
8+448=08 + 4 - 4 - 8 = 0.
So y=2y = 2 works. (1,2)(1, 2) is a solution.
Try y=2y = -2:
8+4+48=8-8 + 4 + 4 - 8 = -8.
Try y=1y = -1:
1+1+28=6-1 + 1 + 2 - 8 = -6.
Try y=4y = 4:
64+1688=6464 + 16 - 8 - 8 = 64.
So only y=2y = 2 works for x=1x = 1.

If x=1x = -1:
y3+y2+2y8=0. y^{3} + y^{2} + 2y - 8 = 0.
Try y=1y = 1:
1+1+28=41 + 1 + 2 - 8 = -4.
Try y=2y = 2:
8+4+48=88 + 4 + 4 - 8 = 8.
Try y=2y = -2:
8+448=16-8 + 4 - 4 - 8 = -16.
Try y=1y = -1:
1+128=10-1 + 1 - 2 - 8 = -10.
Try y=4y = 4:
64+16+88=8064 + 16 + 8 - 8 = 80.
So no integer solution for x=1x = -1.

Try x=2x = 2:
8x6=8×64=5128x^{6} = 8 \times 64 = 512
2x3y=2×8y=16y2x^{3}y = 2 \times 8y = 16y
So:
y3+y216y512=0y^{3} + y^{2} - 16y - 512 = 0
Try y=8y = 8:
512+64128512=64512 + 64 - 128 - 512 = -64
Try y=16y = 16:
4096+256256512=35844096 + 256 - 256 - 512 = 3584
Try y=8y = -8:
512+64+128512=832-512 + 64 + 128 - 512 = -832
Try y=4y = 4:
64+1664512=49664 + 16 - 64 - 512 = -496
Try y=4y = -4:
64+16+64512=496-64 + 16 + 64 - 512 = -496
So no integer solution for x=2x = 2.

Try x=2x = -2:
8x6=5128x^{6} = 512
2x3y=2×(8)y=16y2x^{3}y = 2 \times (-8)y = -16y
So:
y3+y2+16y512=0y^{3} + y^{2} + 16y - 512 = 0
Try y=8y = 8:
512+64+128512=192512 + 64 + 128 - 512 = 192
Try y=8y = -8:
512+64128512=1088-512 + 64 - 128 - 512 = -1088
Try y=4y = 4:
64+16+64512=36864 + 16 + 64 - 512 = -368
Try y=4y = -4:
64+1664512=624-64 + 16 - 64 - 512 = -624
So no integer solution for x=2x = -2.

Now, for large x|x|, the term 8x68x^{6} dominates, so y38x6y^{3} \approx 8x^{6}, so y2x2y \approx 2x^{2}.
Try y=2x2y = 2x^{2}:
y3=8x6y^{3} = 8x^{6}
2x3y=4x52x^{3}y = 4x^{5}
y2=4x4y^{2} = 4x^{4}
So:
8x6=8x6+4x54x48x^{6} = 8x^{6} + 4x^{5} - 4x^{4}
0=4x54x40 = 4x^{5} - 4x^{4}
0=4x4(x1)0 = 4x^{4}(x - 1)
So x=0x = 0 or x=1x = 1.

If x=0x = 0, y=0y = 0 (already found).
If x=1x = 1, y=2y = 2 (already found).

Try y=2x2y = -2x^{2}:
y3=8x6y^{3} = -8x^{6}
2x3y=4x52x^{3}y = -4x^{5}
y2=4x4y^{2} = 4x^{4}
So:
8x6=8x64x54x4-8x^{6} = 8x^{6} - 4x^{5} - 4x^{4}
16x6+4x5+4x4=0-16x^{6} + 4x^{5} + 4x^{4} = 0
4x4(x+14x2)=04x^{4}(x + 1 - 4x^{2}) = 0
So x=0x = 0 or x=1x = -1 or x2=1/4x^{2} = 1/4 (not integer).
If x=0x = 0, y=0y = 0 (already found).
If x=1x = -1, y=2y = -2.
Try x=1x = -1, y=2y = -2 in the original equation:
(2)3=8(1)6+2(1)3(2)(2)2(-2)^{3} = 8(-1)^{6} + 2(-1)^{3}(-2) - (-2)^{2}
8=8+2(1)(2)4=8+44=8-8 = 8 + 2(-1)(-2) - 4 = 8 + 4 - 4 = 8
So 8=8-8 = 8 is false.
So x=1x = -1, y=2y = -2 is not a solution.

Try y=kx2y = kx^{2} for small integer kk.
Try y=xy = x:
y3=x3y^{3} = x^{3}
8x6+2x3yy2=8x6+2x4x28x^{6} + 2x^{3}y - y^{2} = 8x^{6} + 2x^{4} - x^{2}
So x3=8x6+2x4x2x^{3} = 8x^{6} + 2x^{4} - x^{2}
Try x=0x = 0: 0=00 = 0
Try x=1x = 1: 1=8+21=91 = 8 + 2 - 1 = 9
Try x=1x = -1: 1=8+21=9-1 = 8 + 2 - 1 = 9
So only x=0x = 0, y=0y = 0 (already found).

Try y=1y = -1:
(1)3=8x6+2x3(1)(1)2(-1)^{3} = 8x^{6} + 2x^{3}(-1) - (-1)^{2}
1=8x62x31-1 = 8x^{6} - 2x^{3} - 1
0=8x62x30 = 8x^{6} - 2x^{3}
2x3(4x31)=02x^{3}(4x^{3} - 1) = 0
So x=0x = 0 or x3=1/4x^{3} = 1/4 (not integer).
So x=0x = 0, y=1y = -1 (already found).

Therefore, the only integer solutions are (x,y)=(0,0),(0,1),(1,2)(x, y) = (0, 0), (0, -1), (1, 2).

Final answer:
All integer solutions are (x,y)=(0,0),(0,1),(1,2)(x, y) = (0, 0), (0, -1), (1, 2).

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