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Trying to prove pells equation doesn't work with square numbers

Hello, I am trying to prove that for pells equation "x2ny2=1x^2 - ny^2 = 1" when n is a perfect square the equation has no solutions. Given that all x, y and n are Natural numbers not including zero.
So far I have got this:
x2m2y2=1x^2 - m^2y^2 = 1
x2(my)2=1x^2 - (my)^2 = 1
(x+my)(xmy)=1(x + my) (x - my) = 1 (difference of two squares)
(x+my)=1(xmy)(x + my) = \frac{1}{(x - my)}

So from here am I right in saying that the left hand side is going to be 2 or higher and the right surely must be 1 or less? I feel like I'm missing a final step or possibly need to word my statement better, any ideas?
Reply 1
Original post by iAustinMark
Hello, I am trying to prove that for pells equation "x2ny2=1x^2 - ny^2 = 1" when n is a perfect square the equation has no solutions. Given that all x, y and n are Natural numbers not including zero.
So far I have got this:
x2m2y2=1x^2 - m^2y^2 = 1
x2(my)2=1x^2 - (my)^2 = 1
(x+my)(xmy)=1(x + my) (x - my) = 1 (difference of two squares)


We have two cases: x+my=xmy=1x+my = x-my = 1 or x+my=xmy=1x+my = x -my = -1.
(edited 8 years ago)
Original post by iAustinMark
Hello, I am trying to prove that for pells equation "x2ny2=1x^2 - ny^2 = 1" when n is a perfect square the equation has no solutions. Given that all x, y and n are Natural numbers not including zero.
So far I have got this:
x2m2y2=1x^2 - m^2y^2 = 1
x2(my)2=1x^2 - (my)^2 = 1
(x+my)(xmy)=1(x + my) (x - my) = 1 (difference of two squares)
(x+my)=1(xmy)(x + my) = \frac{1}{(x - my)}

So from here am I right in saying that the left hand side is going to be 2 or higher and the right surely must be 1 or less? I feel like I'm missing a final step or possibly need to word my statement better, any ideas?


You are home and dry on the second to last line: you have the product of two integers equal to one. What are the possible solutions?
Reply 3
Original post by Gregorius
You are home and dry on the second to last line: you have the product of two integers equal to one. What are the possible solutions?


So (x + my) and (x - my) would have to both equal plus or minus 1? That not being possible if all x,y and m are integers, is that proof enough in itself?
Original post by iAustinMark
So (x + my) and (x - my) would have to both equal plus or minus 1? That not being possible if all x,y and m are integers, is that proof enough in itself?


You now have two sets of equations (one each for +1 , -1) in two unknowns. Solve them. x+my=1 and x-my=1 implies x=? and y=?
Reply 5
Original post by Gregorius
You now have two sets of equations (one each for +1 , -1) in two unknowns. Solve them. x+my=1 and x-my=1 implies x=? and y=?


I suppose it would imply x=1 and y=0 but that's not possible because y can't be 0
Reply 6
Original post by iAustinMark
I suppose it would imply x=1 and y=0 but that's not possible because y can't be 0


Yes.

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