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Number Theory Show equation has no solution

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Poirot

Banned
Feb 15, 2012
250
Us the fact that 2 is not a quadratic residue of 3 to show that there are no integer solutions to $y^2=x^3-x+5$.
 

Bacterius

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MHB Math Helper
Jan 26, 2012
644
Re: show equation has no solution

Take the equation modulo $3$, and you get:
$$y^2 \equiv x^3 - x + 5 \equiv x^3 - x + 2 \pmod{3}$$
Now FLT tells us that $x^3 \equiv x \pmod{3}$, so we get:
$$y^2 \equiv x - x + 2 \equiv 2 \pmod{3}$$
Can you finish?
 
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Poirot

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Feb 15, 2012
250
Re: show equation has no solution

no such y exist because 2 is a quadractic non-residue mod 3.