Let x and y be whole numbers such that
( x − 2 ) 4 − x 4 = y 3 .
How many pairs ( x , y ) satisfy the equation?
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I'm not sure I follow this. X=1 leads to both sides becoming zero. Can you elaborate on how you were able to rule out other values of y so easily?
It feels like y=2 and y=4 should yield solutions for example
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Neither do I. For I can see no restriction on the negativity of y and therefore y cube. How come LHS should stay non-negative?
nice solution :)
Just a little add-on:
x 2 − 2 x + 2 = ( x − 1 ) 2 + 1 ≥ 0
To make the LHS positive,
1 − x ≥ 0
1 ≥ x
x = 0 doesn't yield a cube, so the only whole number solution is x = 1
Firts we factorize ( x − 2 ) 2 − x 4
( x − 2 ) 2 − x 4
= ( x − 2 ) 2 + x 2 ) ( ( x − 2 ) 2 − x 2 )
= ( x − 2 + x ) ( x − 2 − x ) ( ( x − 2 ) 2 + x 2 )
= ( 2 x − 2 ) ( − 2 ) ( 2 x 2 − 2 x + 4 )
= ( − 8 ) ( x − 1 ) ( x 2 − 2 x + 2 )
= − ( 2 ) 3 ( x − 1 ) ( ( x − 1 ) 2 + 1 )
Since -8 is already a cube then that means that ( x − 1 ) ( ( x − 1 ) 2 + 1 ) must also be a cube hence
( x − 1 ) ( ( x − 1 ) 2 + 1 ) = k 3
since ( x − 1 ) and ( ( x − 1 ) 2 + 1 ) ∈ Z We can split this into 4 cases:
Case 1
x − 1 = k
( x − 1 ) 2 + 1 = k 2
Case 2
x − 1 = k 2
( x − 1 ) 2 + 1 = k
Case 3
x − 1 = 1
( x − 1 ) 2 + 1 = k 3
Case 4
x − 1 = k 3
( x − 1 ) 2 + 1 = 1
Solving the equations we find that the only possible solution is x = 1 and k = 0 :
( − 8 ( k 3 ) = y 3
( 0 ) = y 3
y = 0
So the only pair that satisfies this equation is ( 0 ; 1 )
If k is not prime, more cases could be included. For example if k is a even number, let k=2t. Then RHS=8t^3, which can be further split into case 5: x-1 =4t , (x-1)^2+1=2t^2, etc. So this proof is erroneous.
( x − 2 ) 4 − x 4 = y 3 We can factor this to give ( − 2 ) 3 ( x − 1 ) ( ( x − 1 ) 2 + 1 ) = y 3
Since we have ( − 2 ) 3 on the left, the other factors must also give a cube, so ( x − 1 ) ( ( x − 1 ) 2 + 1 ) = z 3
But x − 1 and ( x − 1 ) 2 + 1 are clearly coprime, and the only way for coprime values to multiply to give a cube is if they are both cubes themselves. This tells us that
x − 1 = n 3
( x − 1 ) 2 + 1 = m 3
Substituting x − 1 = n 3 into the second equation gives
n 6 + 1 = m 3
And so
1 = m 3 − n 6 = ( m − n 2 ) ( m 2 + m n 2 + n 4 )
Since the two factors on the right are both integers, they must be ± 1 .
However m > 0 by our definition of m , which tells us the rightmost factor is positive and so both factors equal 1 , but since m > 0 this can only only happen when m = 1 , n = 0 . Therefore the other solution is
x − 1 = 0 ⟹ x = 1 .
(x-2)^4 - x^4 = y^3 Or, y^3 = -2 (2x-2)(2x^2 - 4x +4) =-8 (x-1)(x^2 - 2x + 2) =(-2)^3×[(x-1)(x^2 - 2x + 2)] Therefore, for x and y to be whole number (x-1)(x^2 - 2x + 2) has to be perfect cube (x-1)(x^2 - 2x + 2)=(x-1)^3 + (x-1) Therefore (x-1)(x^2 - 2x + 2) to be whole cube x-1 has to be equal to zero as (x-1)^3 is a perfect cube. Therefore x-1=0 x=1 This the only solution for which (x,y) are whole numbers. Therefore there is only 1 solution. (x,y) = (1,0)
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Factorize LHS as Difference of Squares, to obtain the following expression,
8 ( x 2 − 2 x + 2 ) ( 1 − x ) = y 3
As it can be seen, for x , y ∈ W ,
x can only equal 1 so that the LHS stays non-negative ; and consequently y = 0 .
∴ ( 1 , 0 ) is the only solution.