Friday, December 18, 2015

Diophantine Equation Without Using Fermat's Last Theorem



I'm having trouble with a problem. The problem asks me to solve the equation (x+1)4(x1)4=y3 in integers. I found out that the only integer solution is (0,0). I found this answer by setting x as a3 and x2+1 as b3. After doing that, I got the equation a6+1n=b3, which assures me that there is no other solution than (0,0) by Fermat's Last Theorem. However, I just realized that I am not supposed to use Fermat's Last Theorem. So far, I have simplified the equation to 8x3+8x=y3. Please help me prove that the only integer solution is (0,0) without using Fermat's Last Theorem.


Answer



Consider y3=8x(x2+1). Then y must be even (say y=2z). Then we get z3=x(x2+1). Then we have
z3x3=x.
OR

(zx)(z2+zx+x2)=x
But the factor |x2+zx+z2||x|. So for this to be true x=0=z.


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