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How to prove that the Diophantine equation $$(x+y)(x+y+2)=10xy\quad (1)$$ has no positive integer solutions

First attempt as below is wrong :is to rewrite this equation as $$y^2 + y(2-8x) + x^2 + 2x = 0$$. The discriminant is $\Delta = 4(15x^2 - 10x + 1)$. Therefore, $y = 4x - 1 \pm \sqrt{15x^2 - 10x + 1}$. If a solution of the equation (1) exists for some integers $x, y > 0$, then $15x^2 - 10x + 1$ must be a perfect square. However, its discriminant is not zero, hence the equation (1) has no positive integer solutions for $x$ and $y$.

Second adempt : $15x^2 - 10x + 1$ must be a perfect square, Wolfram gives the solution of the equation $15x^2 - 10x + 1=m^2$. We can remark that all $x$ solutions are <0, which shows that no positive solution can exist. But can we retrieve Wolfram's results? I don't know.

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    It could still be possible for $P(n)$ to be the square of some integer despite there not being a polynomial $Q\in\Bbb C[x]$ such that $P=Q^2$. For instance, $P(x)=x^2+5$ does that for $x=2\lor x=-2$. – Sassatelli Giulio Apr 10 '24 at 03:01
  • @SassatelliGiulio You are right – Fox meulder Apr 10 '24 at 03:09
  • FYI, with your second attempt, i.e., by using Wolfram Alpha, its results look like they come from solving a Generalized Pell's equation. However, even if so, since that subject is something I don't know much about, I can't offer any particular details. – John Omielan Apr 10 '24 at 17:57
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    Thank you and maybe someone who knows these Pell Fermat equations well will tell us one more – Fox meulder Apr 10 '24 at 20:27
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    @JohnOmielan, you can find the origin of this question in this thread: [https://mathoverflow.net/questions/468844/enigma-involving-number-theory-find-the-content-of-the-box]. However, it's closed – Fox meulder Apr 11 '24 at 17:46
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    new link https://math.stackexchange.com/questions/4897488/enigma-involving-number-theory-find-the-content-of-the-box?noredirect=1#comment10451482_4897488 – Fox meulder Apr 11 '24 at 23:57
  • I deleted thé theard of. New link also After down vote – Fox meulder Apr 13 '24 at 12:31

2 Answers2

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The Diophantine equation to check on is

$$(x + y)(x + y + 2) = 10xy \tag{1}\label{eq1A}$$

Note that, by inspection, I got $(x, y) \in \{(0, 0), (0, -2), (-2, 0)\}$ being integer solutions, but not positive ones. Since there are integer solutions, doing things like factoring and checking for a perfect square discriminant will succeed (e.g., with $15x^2 - 10x + 1$, it's $1$ for $x = 0$ and $81 = 9^2$ for $x = -2$, with your Wolfram Alpha results show other, only negative, results)). One way to specifically show there are no positive integer solutions is to assume they exist but then use positive divisibility constraints (which assume all of the factors are non-negative), such as with my \eqref{eq4A}, \eqref{eq5A} and \eqref{eq6A} below, to show there are no valid results.


First, since $x + y$ and $x + y + 2$ have the same parity and $10xy$ is even, then both of the LHS factors of \eqref{eq1A} must be even. Thus, $x$ and $y$ have the same parity. They can't both be odd since the LHS has at least $2$ factors of $2$ but the RHS would have only $1$ factor of $2$. This means $x$ and $y$ must both be even. As such, there's a positive integer $d$ where

$$\gcd(x, y) = 2d \;\;\to\;\; x = 2dx_1, \; y = 2dy_1, \; \gcd(x_1, y_1) = 1 \tag{2}\label{eq2A}$$

Substituting this into \eqref{eq1A} and dividing both sides by $4d$ gives

$$(x_1 + y_1)(dx_1 + dy_1 + 1) = 10dx_{1}y_{1} \tag{3}\label{eq3A}$$

Since $d \mid (x_1 + y_1)(dx_1 + dy_1 + 1)$, but $\gcd(d, dx_1 + dy_1 + 1) = 1$, we have

$$d \mid x_1 + y_1 \tag{4}\label{eq4A}$$

Also, $x_1 + y_1 \mid 10d(x_{1}y_{1})$, but $\gcd(x_1, y_1) = 1$ means that $\gcd(x_1 + y_1, x_{1}y_{1}) = 1$, so

$$x_1 + y_1 \mid 10d \tag{5}\label{eq5A}$$

Note that \eqref{eq4A} and \eqref{eq5A}, along with \eqref{eq3A}, means there are positive integers $a$ and $b$ with

$$x_1 + y_1 = ad, \;\; d(x_1 + y_1) + 1 = bx_{1}y_{1}, \;\; ab = 10 \tag{6}\label{eq6A}$$

Substituting the LHS of \eqref{eq6A} into the middle part gives

$$\begin{equation}\begin{aligned} d(ad) + 1 & = b(ad - y_1)y_1 \\ ad^2 + 1 & = (ab)dy_1 - by_1^2 \\ by_1^2 - 10dy_1 + (ad^2 + 1) & = 0 \end{aligned}\end{equation}\tag{7}\label{eq7A}$$

Treating this as a quadratic in $y_1$, the discriminant, i.e.,

$$100d^2 - 4b(ad^2 + 1) = 100d^2 - 4(10)d^2 - 4b = 60d^2 - 4b = 4(15d^2 - b) \tag{8}\label{eq8A}$$

must be a perfect square. Thus,

$$15d^2 - b \tag{9}\label{eq9A}$$

must also be a perfect square. From the RHS of \eqref{eq6A}, we get these cases to check for $b$:

  1. Since $15d^2 - 1 \equiv 2\pmod{3}$, it's not a perfect square. In addition, if $d$ is odd, then $15d^2 - 1 \equiv 7(1) - 1 \equiv 6 \pmod{8}$, while if $d$ is even, then $15d^2 - 1 \equiv 3 \pmod{4}$, with neither being possible.

  2. With $15d^2 - 2 \equiv 3 \pmod{5}$, it can't be a perfect square. Another method is to note that $d$ can't be even since it would have only $1$ factor of $2$, so $d$ is odd. However, then $15d^2 - 2 \equiv 7(1) - 2 \equiv 5 \pmod{8}$.

  3. Checking $15d^2 - 5$, we get $d$ can't be even because then $15d^2 - 5 \equiv 3 \pmod{4}$. However, $d$ being odd means $15d^2 - 5 \equiv 7(1) - 5 \equiv 2 \pmod{8}$, which is also never true for perfect squares. Alternatively, $15d^2 - 5 = 5(3d^2 - 1)$, so $5 \mid 3d^2 - 1$. However, $d^2 \equiv 0, 1, 4 \pmod{5}$, with $5 \nmid 3d^2 - 1$ for each congruence.

  4. Finally, $15d^2 - 10 \equiv 2 \pmod{3}$, so this can't be a perfect square either. Another way to show this is that $d$ can't be even because $15d^2 - 10$ would have only one factor of $2$, but $d$ being odd means that $15d^2 - 10 \equiv 7(1) - 2 \equiv 5 \pmod{8}$.

Since these all result in a non-perfect square discriminant, there are no positive integer solutions for $d$ and $y_1$ in \eqref{eq7A} and, thus, also for $x$ and $y$ in \eqref{eq1A}.

John Omielan
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    Thank you for this very detailed proof. I will study this solution. I'll come back later to accept it or ask for clarification if I get stuck on any obscure points. – Fox meulder Apr 10 '24 at 08:33
  • @Noname You're welcome. Note I've updated my answer, including adding a few more details, that you may find helpful. – John Omielan Apr 10 '24 at 17:53
  • @Noname Easier way: use descent on integer points on the conic ("Vieta jumping"), cf. my comment on the question. – Bill Dubuque Apr 13 '24 at 12:24
  • Hello,

    @Bill Dubuque 1 , could you please share your solution to let readers of the thread learn from your approach? Additionally, if someone asks me (now that I understand how to solve a generalized Pell-Fermat equation), I can explain the results from Wolfram.

    – Fox meulder Apr 13 '24 at 13:41
  • @Noname Ok, I will try to find the time to post an answer. – Bill Dubuque Apr 13 '24 at 15:05
  • Ok thank you so much
    Bill Dubuque
    – Fox meulder Apr 13 '24 at 17:07
  • @BillDubuque In your comment, you didn't seem sure if you'll find the time to post an answer. Thus, I took the liberty of writing & posting a solution, using your suggestion of "Vieta jumping", to prove using that technique that are no integer solutions. However, if you would rather post your own answer instead, please let me know so I can then delete my new one to avoid duplication. – John Omielan Apr 13 '24 at 18:02
  • Thanks for elaborating on the (Vieta) descent method hinted in my comment. For many years there has been an entry on my (infinite) todo list to vividly graphically illustrate descent by reflections on conics ("Vieta jumping"). I though this might be a good opportunity to do so if only I could find the time (but, alas, time is in short supply now due to health issues - battling cancer). – Bill Dubuque Apr 14 '24 at 05:32
  • @BillDubuque You're welcome. I'm sorry to hear about your health issues, i.e., battling cancer. You have my hopes and wishes that you get better soon. – John Omielan Apr 14 '24 at 05:35
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    I'm also saddened to hear about your health issue. I wish you strength and courage to fight your illness. – Fox meulder Apr 14 '24 at 17:29
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The resulting Diophantine equation is

$$y^2 + y(2 - 8x) + x^2 + 2x = 0 \tag{1}\label{eq1B}$$

As indicated in Bill Dubuque's comment, we can use Standard Vieta jumping to prove there are no positive integer solutions. Similar to what's shown there with the example Problem #$6$ at IMO $1988$, let $(X,Y)$ be a solution in positive integers to \eqref{eq1B}, i.e.,

$$Y^2 + Y(2 - 8X) + X^2 + 2X = 0 \tag{2}\label{eq2B}$$

such that $X + Y$ is minimized and, due to symmetry, WLOG let $Y \ge X$. Actually, since $Y = X$ in \eqref{eq2B} leads to $X = 0$ or $X = \frac{2}{3}$, we can instead have $Y \gt X$. Even more strongly, as I explained in my other answer, due to parity and # of factors of $2$, both $X$ and $Y$ are even, so we then get

$$Y \ge X + 2 \;\to\; \frac{1}{Y} \le \frac{1}{X + 2} \tag{3}\label{eq3B}$$

Fixing $X$, replace $Y$ with the variable $z$ in \eqref{eq2B} to get

$$z^2 + z(2 - 8X) + X^2 + 2X = 0 \tag{4}\label{eq4B}$$

We know one root of this equation is $z_1 = Y$. Using the standard properties of quadratic equations, the other root satisfies

$$z_2 = -(2 - 8X) - Y, \;\; z_2 = \frac{X^2 + 2X}{Y} \tag{5}\label{eq5B}$$

The first expression shows that $z_2$ is an integer, while the second indicates it's positive. Using the second expression, and \eqref{eq3B}, we have

$$z_2 = \frac{X^2 + 2X}{Y} = X(X + 2)\left(\frac{1}{Y}\right) \le X(X + 2)\left(\frac{1}{X + 2}\right) = X \tag{6}\label{eq6B}$$

However, then

$$z_2 \le X \;\;\to\;\; z_2 \lt Y \;\;\to\;\; X + z_2 \lt X + Y \tag{7}\label{eq7B}$$

contradicting the minimality of $X + Y$ (since $(x, y) = (X, z_2)$ is another solution in positive integers to \eqref{eq1B}). This contradiction proves there are no positive integer solutions to \eqref{eq1B}.

John Omielan
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    +1 This is essentially the algebra behind the method I hinted in comments. I encourage readers to geometrically visualize the descent by graphing it - draw a zigzag line between each pair of points in the descent, generated by the reflections $,(x,y) \to (y,x),$ and the "conjugate root" reflection $,(x,y)\to (x,\bar y),,$ where $,\bar y = 8x!-!2-y = (x^2!+2x)/y,,$ by Vieta. For general theory see the literature I cite in this answer. – Bill Dubuque Apr 14 '24 at 05:45
  • To elaborate, while the descent on integer points on the positive branch is only hypothetical in the proof (since there are no positive integer points), it actually does exist on the lower negative branch, e.g. $(-18,-144)\to(-18,-2)\to(-2,-18)\to(-2,0)\to(0,-2)\to(0,0)$. It is instructive to analyze how this differs from the positive branch, and how it can be remedied by scaling $x,y$ by $3$ so our positive fixed point $(2/3,2/3)$ becomes integral $= (2,2).,$ Now the positive branch of our scaled curve $,x^2-8xy+y^2+6x+6y=0$ has integer points e.g. $(8,56)\to(8,2)\to(2,8)\to(2,2)\ \ $ – Bill Dubuque Apr 14 '24 at 06:15
  • If someone is handy with graphic software it would be very helpful to readers to add graphs illustrating these points. – Bill Dubuque Apr 14 '24 at 06:19