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This question is inspired by the problem https://projecteuler.net/problem=748

Consider the Diophantine equation $$\frac{1}{x^2}+\frac{1}{y^2}=\frac{k}{z^2}$$ $k$ is a squarefree number. $A_k(n)$ is the number of solutions of the equation such that $1 \leq x+y+z \leq n$, $x \leq y$ and $\gcd(x,y,z)=1$. This equation has infinite solutions for $k=1$ and $k>1$ that can be expressed as sum of two perfect squares.

Let $$A_k=\lim_{n \to \infty}\frac{A_k(n)}{\sqrt{n}}$$

\begin{array}{|c|c|c|c|c|} \hline k& A_k\left(10^{12}\right)& A_k\left(10^{14}\right)& A_k\left(10^{16}\right)& A_k\left(10^{18}\right)& A_k \\ \hline 1& 127803& 1277995& 12779996& 127799963& 0.12779996...\\ \hline 2& 103698& 1037011& 10369954& 103699534& 0.1036995...\\ \hline 5& 129104& 1291096& 12911049& 129110713& 0.129110...\\ \hline 10& 90010& 900113& 9000661& 90006202& 0.0900062...\\ \hline 13& 103886& 1038829& 10388560& 103885465& 0.103885...\\ \hline 17& 86751& 867550& 8675250& 86752373& 0.086752...\\ \hline \end{array}

From these data, it seems that these limits converge. I wonder if it is possible to write them in terms of known constants or some sort of infinite series. For Pythagorean triples, there are about $\frac{n}{2\pi}$ triples with hypotenuse $\leq n$.

The Python code to calculate $A_1(n)$:

def gcd(a, b):
   if b == 0:
       return a
   return gcd(b, a % b)

N = 10 ** 14

cnt = 0
for a in range(1, 22000):
   a2 = a * a
   for b in range(1, a):
       if (a + b) % 2 == 1 and gcd(a, b) == 1:
           b2 = b * b
           x = 2 * a * b * (a2 + b2)
           y = a2 * a2 - b2 * b2
           z = 2 * a * b * (a2 - b2)
           if x + y + z > N:
               continue
           cnt += 1
print(cnt)

The Python code to calculate $A_2(n)$:

def gcd(a, b):
   if b == 0:
       return a
   return gcd(b, a % b)

N = 10 ** 14

cnt = 1     # (1, 1, 1)
for a in range(1, 22000):
   a2 = a * a
   a4 = a2 * a2
   for b in range(1, a):
       if gcd(a, b) == 1:
           b2 = b * b
           b4 = b2 * b2
           x = 2 * a * b * (a2 + b2) - (a4 - b4)
           y = 2 * a * b * (a2 + b2) + (a4 - b4)
           z = 6 * a2 * b2 - (a4 + b4)
           if x > 0 and y > 0 and z > 0 and x + y + z <= N and gcd(x, gcd(y, z)) == 1:
               cnt += 1
           x = (a4 - b4) - 2 * a * b * (a2 + b2)
           y = 2 * a * b * (a2 + b2) + (a4 - b4)
           z = (a4 + b4) - 6 * a2 * b2
           if x > 0 and y > 0 and z > 0 and x + y + z <= N and gcd(x, gcd(y, z)) == 1:
               cnt += 1
print(cnt)
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  • $\begingroup$ Do you know how to get the solution of the problem 748 (in the link you provided)? $\endgroup$
    – NN2
    Feb 24 '21 at 17:49
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This is a partial answer. Consider the case $k=1$. The parametric solutions are given by $(x,y,z)=\left(2ab(a^2+b^2),a^4-b^4,2ab(a^2-b^2)\right)$, where $\gcd(a,b)=1$, $a>b>0$ and $a+b$ is odd. In the triples $(x,y,z)$ generated by this parametrization, it is not necessarily always $x\leq y$. For example, $(20,15,12)$ is a triple generated by this parametrization, and $(15,20,12)$ will not be generated. This parametrization generates only the unique solutions. So we can have two possible cases, either $0<a^2-b^2\leq 2ab$ or $0<2ab\leq a^2-b^2$.

The sum of the triples = $x+y+z=4a^3b+a^4-b^4 \leq n$.

Let us consider the first case. Denote the number of solutions by $A_1^1(n)$ and $A_1^1=\lim_{n \to \infty}\frac{A_1^1(n)}{\sqrt{n}}$.

$$0<a^2-b^2\leq 2ab \implies \sqrt{2}-1\leq\frac{b}{a}=x<1$$ Number of pairs $(a,b)$ satisfying above constraints and also $4a^3b+a^4-b^4 \leq n$ can be approximated by \begin{align} \begin{split} p(n)&\approx\int_{\frac{b}{a}=\sqrt{2}-1}^{1}\int_{4a^3b+a^4-b^4 \leq n}a \ da\ db\\ &=\int_{x=\sqrt{2}-1}^{1}\int_{0<a^4\leq \frac{n}{1+4x-x^4}}a \ da\ dx \\ &=\frac{\sqrt{n}}{2}\int_{x=\sqrt{2}-1}^{1}\frac{1}{\sqrt{1+4x-x^4}}\ dx \end{split} \end{align}

The natural density of coprime numbers is $\frac{6}{\pi^2}$. The condition $gcd(a,b)=1$ will contribute a multiplication factor of $\frac{6}{\pi^2}$. Among all the coprime pairs $(a,b)$ such that $a>b>0$, fraction of which have odd value of $a+b$ is $\frac{2}{3}$. We have to multiply this factor.

\begin{align} \begin{split} A_1^1 &=\lim_{n \to \infty}\frac{A_1^1(n)}{\sqrt{n}}\\ &= \lim_{n \to \infty} \frac{2}{3} \cdot \frac{6}{\pi^2} \cdot\frac{p(n)}{\sqrt{n}}\\ &= \frac{2}{\pi^2} \int_{x=\sqrt{2}-1}^{1}\frac{dx}{\sqrt{1+4x-x^4}} \\ \end{split} \end{align} Similarly, it can be shown that for the other case exactly the same limit exists. So, $$A_1=2A_1^1=\frac{4}{\pi^2} \int_{x=\sqrt{2}-1}^{1}\frac{dx}{\sqrt{1+4x-x^4}} \approx 0.1277999513464289...$$

Using a similar approach it can be shown that \begin{align} \begin{split} A_2&=\frac{2}{\pi^2} \left[ \int_{x=0}^{\sqrt{2}-1}\frac{dx}{\sqrt{3-6x^2-x^4}} +\int_{x=\sqrt{2}-1}^{1}\frac{dx}{\sqrt{4x+6x^2+4x^3-1-x^4}} \right]\\ & \approx 0.1036994744684913... \end{split} \end{align} \begin{align} \begin{split} A_{13}&=\frac{52}{7\pi^2} \int_{x=(\sqrt{13}-3)/2}^{(\sqrt{26}-1)/5}\frac{dx}{\sqrt{20x+36x^2-5-7x^4}}\approx 0.1038855856479065... \end{split} \end{align}

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  • $\begingroup$ It seems you are missing some conditions on $a$ and $b$ in you parametrization; for $(a,b)=(2,1)$ you get $(x,y)=(20,17)$, so $x\not\leq y$. And in for $(a,b)=(1,2)$ you get $y=-17$ which is not positive. $\endgroup$ Feb 25 '21 at 19:41
  • $\begingroup$ Also, I can't follow anything that you write after $A_1=...$. Where do these integrals come from? And why do you multiply them by these factors? $\endgroup$ Feb 25 '21 at 19:43
  • $\begingroup$ @Servaes I added more details. I hope everything is clear now. $\endgroup$
    – piepie
    Feb 26 '21 at 4:18
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To solve the Diophantine equation.

$$ \frac{ 1 }{ x^2 } +\frac{ 1 }{ y^2 } = \frac{ q }{ z^2 } $$

It is necessary to use the solution of the following equation.

$$a^2+b^2=qc^2$$

There are solutions when the coefficient can be represented as the sum of squares. $q=t^2+k^2$

$$a=-tp^2+2kps+ts^2$$

$$b=kp^2+2tps-ks^2$$

$$c=p^2+s^2$$

And the solution itself can be presented in this form.

$x=ac$

$y=bc$

$z=ab$

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  • $\begingroup$ This answer would be greatly improved by explaining where these expressions come from, and showing why these statements are true (if they are). $\endgroup$ Feb 25 '21 at 19:45

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