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anemone
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Find all integer solutions of the equation $(x^2-y^2)^2=1+16y$
anemone said:Find all integer solutions of the equation $(x^2-y^2)^2=1+16y$
mente oscura said:Hello.
Missing me "thank you" button, anemone
1º) [tex]3|y[/tex]
Demonstration:
1.1) [tex]If \ 3|x,y \rightarrow{}3|(16y+1) \cancel{=}3k \ / \forall{} k \in{\mathbb{Z}}[/tex]. Absurdity.
1.2) [tex]If \ 3|x \ or \ 3|y \ or \ [/tex][tex]3 \cancel{| } \ x,y[/tex][tex] \rightarrow{} (x^2-y^2)^2 \equiv{1 } mod(3)[/tex]
Opalg said:[sp]I found solutions $(x,y) = (\pm4,3),\ (\pm4,5)$. I will look at mente oscura's solution to see if it can be adapted to show that these are the only solutions.[/sp]
Further thoughts:
[sp]The left side of the equation $(x^2-y^2)^2 = 16y+1$ is positive, so $y$ must be positive. Let $x-y = k$. Then the equation becomes $k^2(2y-k)^2 = 16y+1.$
Case 1: $k>0$. If two positive integers have a given sum (which in this case will be $2y$) then their product is minimised by taking one of them equal to $1$ and the other to be $2y-1$. So the minimum value of $k(2y-k)$ is $2y-1$, and therefore $k^2(2y-k)^2 \geqslant (2y-1)^2$. Thus $ (2y-1)^2 \leqslant 16y+1$. That simplifies to $y^2 \leqslant 5y$, so that $y \leqslant 5$.
Case 2: $k<0$. In this case, let $r=-k$, so that the equation becomes $r^2(2y+r)^2 = 16y+1$. This time, $r$ is a positive integer, so that $r(2y+r) > 2y$ and hence $r^2(2y+r)^2 > 4y^2$. Thus $4y^2 < 16y+1$ from which it follows that $y\leqslant4.$
This shows that there are no solutions with $y>5$.
BUT ...
in a PM, anemone pointed out that I overlooked the solutions $(x,y) = (\pm1,0)$. So altogether there are six solutions $(\pm1,0), (\pm4,3), (\pm4,5)$.[/sp]
Opalg said:[sp]I found solutions $(x,y) = (\pm4,3),\ (\pm4,5)$. I will look at mente oscura's solution to see if it can be adapted to show that these are the only solutions.[/sp]
Further thoughts:
[sp]The left side of the equation $(x^2-y^2)^2 = 16y+1$ is positive, so $y$ must be positive. Let $x-y = k$. Then the equation becomes $k^2(2y-k)^2 = 16y+1.$
Case 1: $k>0$. If two positive integers have a given sum (which in this case will be $2y$) then their product is minimised by taking one of them equal to $1$ and the other to be $2y-1$. So the minimum value of $k(2y-k)$ is $2y-1$, and therefore $k^2(2y-k)^2 \geqslant (2y-1)^2$. Thus $ (2y-1)^2 \leqslant 16y+1$. That simplifies to $y^2 \leqslant 5y$, so that $y \leqslant 5$.
Case 2: $k<0$. In this case, let $r=-k$, so that the equation becomes $r^2(2y+r)^2 = 16y+1$. This time, $r$ is a positive integer, so that $r(2y+r) > 2y$ and hence $r^2(2y+r)^2 > 4y^2$. Thus $4y^2 < 16y+1$ from which it follows that $y\leqslant4.$
This shows that there are no solutions with $y>5$.
BUT ...
in a PM, anemone pointed out that I overlooked the solutions $(x,y) = (\pm1,0)$. So altogether there are six solutions $(\pm1,0), (\pm4,3), (\pm4,5)$.[/sp]
mente oscura said:Hello.
Opalg, actually, I have an error.(Headbang)
[tex]3 \cancel{| } \ x,y[/tex] \rightarrow{} (x^2-y^2)^2 \equiv{0 } mod(3)[/tex]
It also fulfils the conditions:
[tex]x= \pm 4 \ and \ y=5[/tex]
Regards.
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Correct.
An integer solution is a set of values for the variables in a mathematical equation that result in whole numbers as the solution. In other words, the values of the variables must be integers (positive and negative whole numbers) in order for the equation to be satisfied.
The equation $(x^2-y^2)^2=1+16y$ is a quadratic equation in two variables, x and y. It is a special type of quadratic equation known as a "binomial squared" equation, where the binomial (x^2-y^2) is squared.
This equation has an infinite number of integer solutions. This is because for any integer value of y, there will be a corresponding integer value of x that satisfies the equation. Therefore, the equation has an infinite number of solutions.
Some examples of integer solutions for this equation are (0,1), (3,4), (-3,-4), (4,3), (-4,-3), (5,6), (-5,-6), etc. Essentially, any pair of integers that satisfy the equation can be considered an integer solution.
One way to find integer solutions for this equation is by using guess and check. Start by picking a value for y and plugging it into the equation. Then, solve for x. If the resulting values for x and y are both integers, then you have found an integer solution. If not, try a different value for y and repeat the process until you find a pair of integers that satisfy the equation.