# Number TheoryTriplets x^2 + y^2 =5z^3

#### tda120

##### New member
Hello!
Could anyone give me a hint to solve this diophantine equation?
Show how to construct an infinite amount of triples x, y and z e N with (x,y)=1 and x^2 + y^2=5z^2?

#### mathbalarka

##### Well-known member
MHB Math Helper
Re: triplets x^2 + y^2 =5z^3

You have to verify what your intended equation was. Your title says $$\displaystyle x^2 + y^2 = 5z^3$$ whereas your thread material says $$\displaystyle x^2 + y^2 = 5z^2$$.

For the later one, invoke a rational transformation to convert it into $$\displaystyle X^2 + Y^2 = 5$$ over $$\displaystyle \mathbb{Q}[X, Y]$$. Now try to seek a nontrivial solution to this and then use transformations $$\displaystyle P = x - x_0$$ and $$\displaystyle Q = y - y_0$$ for the initial solution $$\displaystyle (x_0, y_0)$$ -- this is the basic line of thought to approach all the Pythagoras-like forms over $$\displaystyle \mathbb{Z}[x, y, z]$$

Balarka
.

#### tda120

##### New member
Re: triplets x^2 + y^2 =5z^3

O sorry; my mistake! The title is right! x^2 + y^2 =5z^3..?.

#### Klaas van Aarsen

##### MHB Seeker
Staff member
Re: triplets x^2 + y^2 =5z^3

Hello!
Could anyone give me a hint to solve this diophantine equation?
Show how to construct an infinite amount of triples x, y and z e N with (x,y)=1 and x^2 + y^2=5z^2?
First find 1 solution.
Let's call it $(x_0, y_0, z_0)$.

Then pick $x$ and $y$ by multiplying $x_0$ and $y_0$ by some $a^k$ or something like that, such that it pans out.
That is, pick $x = a^k \cdot x_0$ and $y = a^k \cdot y_0$.
Can you find an $a$ and corresponding $z$ such that the equation is satisfied?

#### topsquark

##### Well-known member
MHB Math Helper
Re: triplets x^2 + y^2 =5z^3

First find 1 solution.
Let's call it $(x_0, y_0, z_0)$.

Then pick $x$ and $y$ by multiplying $x_0$ and $y_0$ by some $a^k$ or something like that, such that it pans out.
That is, pick $x = a^k \cdot x_0$ and $y = a^k \cdot y_0$.
Can you find an $a$ and corresponding $z$ such that the equation is satisfied?
Coolerino! As I've never seen the method I'll ask the obvious question: Does this generate all possible solutions?

-Dan

#### tda120

##### New member
Re: triplets x^2 + y^2 =5z^3

I stumble upon the very first hint; I cannot even find one solution!.. Usually (x,y) isn’t 1 then… Am I missing something simple?

#### mathbalarka

##### Well-known member
MHB Math Helper
Re: triplets x^2 + y^2 =5z^3

I didn't look much at I like Serena's solution, but an $(x, y, z)$ can easily be found. Try $(1, 2, 1)$.

What I worked out last night was a heuristic analysis, i.e., Note that there are heuristically $\frac{N}{\log N}$ numbers which can be written as $x^2 + y^2$ for $(x, y) \leq (N, N)$. Hence, the n-th such number should be heuristically $\gg \frac{N}{(\log N)^\epsilon}$, implying that the total number of such integers, upto a correct error, is $\gg \frac{N}{(\log N)^\epsilon}$ for some $\epsilon \geq 1$, therefore, there should be asymptotically more or less $\frac{\log(N)}{N^{2/3}}$ such triple satisfying the given Diophantine equation.

#### tda120

##### New member
Re: triplets x^2 + y^2 =5z^3

Ok, thank you. I tried so many triples but of course missed this one But, I like Serena, if you multiply both x0 and yo with the same factor a^k, then x and y aren't relatively prime anymore?

#### Klaas van Aarsen

##### MHB Seeker
Staff member
Re: triplets x^2 + y^2 =5z^3

Coolerino! As I've never seen the method I'll ask the obvious question: Does this generate all possible solutions?

-Dan
I'm afraid not.
It only generates some of the solutions that are multiples of $x_0$ and $y_0$.
And for instance it does not generate any of the multiples of with $x_0$ and $y_0$ reversed (assuming they are distinct).

Ok, thank you. I tried so many triples but of course missed this one But, I like Serena, if you multiply both x0 and yo with the same factor a^k, then x and y aren't relatively prime anymore?
Oops. You're right.
I missed that condition.

#### Klaas van Aarsen

##### MHB Seeker
Staff member
Re: triplets x^2 + y^2 =5z^3

But, I like Serena, if you multiply both x0 and yo with the same factor a^k, then x and y aren't relatively prime anymore?
Your problem looks a bit like a pythagorean triplet.
Let's see if we can make it one.

Suppose we pick $z=5^m$, or rather $z=5^{2k+1}$.
Then we get:
$$x^2 + y^2 = 5\left(5^{2k+1}\right)^3$$
$$x^2 + y^2 = \left(5^{3k+2}\right)^2$$

Hmmm... not sure yet if this goes anywhere though...

#### Opalg

##### MHB Oldtimer
Staff member
Re: triplets x^2 + y^2 =5z^3

Your problem looks a bit like a pythagorean triplet.
Let's see if we can make it one.

Suppose we pick $z=5^m$, or rather $z=5^{2k+1}$.
Then we get:
$$x^2 + y^2 = 5\left(5^{2k+1}\right)^3$$
$$x^2 + y^2 = \left(5^{3k+2}\right)^2$$

Hmmm... not sure yet if this goes anywhere though...
Yes, I think that does go somewhere, in combination with the fact that the product of a sum of two squares is a sum of two squares: $$(a^2+b^2)( c^2+d^2) = (ac+bd)^2 + (ad-bc)^2 = (ac-bd)^2 + (ad+bc)^2.$$ Start with $5=2^2+1^2$ and $5^3 = 125 = 11^2 + 2^2$. Using the above formula, you then get $$5(5^3) = 5^4 = 24^2+7^2,$$ $$5(5^2)^3 = 5^7 = 278^2 + 29^2,$$ $$5(5^3)^3 = 5^{10} = 3116^2 + 237^2,$$ $$\ldots\,.$$

In this way, you inductively construct $x_n$, $y_n$ so that $x_n^2 + y_n^2 = 5(5^n)^3$. The only other thing you need to do is to ensure that $x_n$ and $y_n$ are relatively prime. But their only possible common prime factor is $5$, so you want to choose the $\pm$ signs in the formula $(ac\pm bd)^2 + (ad\mp bc)^2$ in such a way as to avoid that. That is always possible, since (in the inductive construction) it cannot happen that both $ac+bd$ and $ac-bd$ are multiples of $5$.

#### tda120

##### New member
Re: triplets x^2 + y^2 =5z^3

Thank you both. Amazing how you think of this!!!

I get it until your last sentence: 'That is always possible, since (in the inductive construction) it cannot happen that both ac+bd and ac−bd are multiples of 5 .'
Can you please give me some explanation on this?

#### Opalg

##### MHB Oldtimer
Staff member
Re: triplets x^2 + y^2 =5z^3

I get it until your last sentence: 'That is always possible, since (in the inductive construction) it cannot happen that both ac+bd and ac−bd are multiples of 5 .'
Can you please give me some explanation on this?
In more detail, the inductive hypothesis is "There exist co-prime integers $x_n,y_n$ such that $x_n^2+y_n^2 = 5^{3n+1}$." The inductive step then takes the form $5^{3n+4} = 125(5^{3n+1}) = (11^2 + 2^2)(x_n^2+y_n^2) = (11x_n\pm 2y_n)(2x_n\mp 11y_n)$. But neither $x_n$ nor $y_n$ can be a multiple of $5$, and therefore $4y_n$ is also not a multiple of $5$. Suppose that $11x_n+2y_n$ is a multiple of $5$. Then $11x_n-2y_n = (11x_n+2y_n) - 4y_n$ is not a multiple of $5$. So by a suitable choice of $+$ and $-$ we can define $x_{n+1} = 11x_n\pm 2y_n$ and $y_{n+1} = 2x_n\mp 11y_n$ so that $x_{n+1}^2 + y_{n+1}^2 = 5^{3n+4}$ and $x_{n+1}$ is not a multiple of $5$. It then follows that $y_{n+1}$ is also not a multiple of $5$. But $x_{n+1}$ and $y_{n+1}$ cannot have any other common prime factor and are therefore coprime.

#### mathbalarka

##### Well-known member
MHB Math Helper
Re: triplets x^2 + y^2 =5z^3

Very nice, Opalg. This particular superfermat form was much harder, although Cohen reckons that there should be infinitely many solutions for any spherical superfermat coordinate.

Now, it is a very interesting thing to discuss with this nice little diophantine form. There are lots of problem related to this one, for example, is there any (at least partial) parameterization for this?* For another, is there any genuine lower bound for the solutions?** If so, then what is the order of magnitude?

(*) : I think there is a way by analysis in $$\displaystyle \mathbb{Z}$$. I hope someone can bring this one down, I am too busy with the incoming examinations to do anything with it.

(**) : Note that a couple of posts back, my analysis partially settles the matter. A better treatment of the sum of squares function and a probabilistic approach should show that there are only finitely many N exceeding $$\displaystyle \delta N^\epsilon \log N$$ for some doable $$\displaystyle \delta, \epsilon \geq 1$$.

#### Klaas van Aarsen

##### MHB Seeker
Staff member
Re: triplets x^2 + y^2 =5z^3

Note that a couple of posts back, my analysis partially settles the matter.
Perhaps. But I for one did not understand your analysis.
Could you clarify?

#### Opalg

##### MHB Oldtimer
Staff member
Re: triplets x^2 + y^2 =5z^3

Now, it is a very interesting thing to discuss with this nice little diophantine form. There are lots of problem related to this one, for example, is there any (at least partial) parameterization for this?* For another, is there any genuine lower bound for the solutions?** If so, then what is the order of magnitude?

(*) : I think there is a way by analysis in $$\displaystyle \mathbb{Z}$$. I hope someone can bring this one down, I am too busy with the incoming examinations to do anything with it.

(**) : Note that a couple of posts back, my analysis partially settles the matter. A better treatment of the sum of squares function and a probabilistic approach should show that there are only finitely many N exceeding $$\displaystyle \delta N^\epsilon \log N$$ for some doable $$\displaystyle \delta, \epsilon \geq 1$$.

I conjecture that a necessary and sufficient condition for the problem to have a solution (with $x$ and $y$ coprime) is that all the prime factors of $z$ should be congruent to $1$ mod $4$.

The first case where $z$ has a prime factor other than $5$ is when $z=13$. We then have $5(13)^3 = 10985 = 101^2 + 28^2.$

#### mathbalarka

##### Well-known member
MHB Math Helper
Re: triplets x^2 + y^2 =5z^3

Perhaps. But I for one did not understand your analysis.
Could you clarify?
Never mind my previous post, I did a mistake. It should have been $$\displaystyle N^{4/3}/\log N$$. For my defense, it was 3:40 am here when I was posting it.

My approach was probabilistic. The usual way to show a weaker result is that the number of integers of the form $$\displaystyle x^2$$ inside the interval $$\displaystyle [-N, N]$$ is at most $$\displaystyle \mathcal{O}\left (N^{1/2}\right )$$. Similarly, integers of the form $$\displaystyle y^2$$ and $$\displaystyle -5z^3$$ are $$\displaystyle \mathcal{O}\left (N^{1/2}\right )$$ and $$\displaystyle \mathcal{O}\left (N^{1/3}\right )$$, respectively.

So, the sum of them to be equal to 0, a doable heuristic would be that there are $$\displaystyle \mathcal{O} \left ( N^{1/2 + 1/2 + 1/3} \right )$$ or, $$\displaystyle \ll N^{4/3}$$ solutions smaller than N.

My approach was to think of $$\displaystyle x^2 + y^2$$ and $$\displaystyle -5z^3$$ rather than each term individually. So that we get $$\displaystyle \mathcal{O} \left ( N^{4/3} / \log N\right )$$

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#### mathbalarka

##### Well-known member
MHB Math Helper
Re: triplets x^2 + y^2 =5z^3

I conjecture that a necessary and sufficient condition for the problem to have a solution (with $x$ and $y$ coprime) is that all the prime factors of $z$ should be congruent to $1$ mod $4$.
Since $x$ and $y$ has opposite parity, it's quite straightforward that your conjecture holds.

Balarka
.

#### Opalg

##### MHB Oldtimer
Staff member
Re: triplets x^2 + y^2 =5z^3

I conjecture that a necessary and sufficient condition for the problem to have a solution (with $x$ and $y$ coprime) is that all the prime factors of $z$ should be congruent to $1$ mod $4$.
Since $x$ and $y$ has opposite parity, it's quite straightforward that your conjecture holds.
It will take a lot more than that to prove the conjecture. First, you have to show that if $z$ has any prime factor of the form $4k+3$ then either no solution exists or, if it does, then in every such solution $x$ and $y$ must have a nontrivial common factor. Then you also have to show that if all the prime factors of $z$ are of the form $4k+1$ then a solution does exist with $x$ and $y$ coprime.

#### mathbalarka

##### Well-known member
MHB Math Helper
Re: triplets x^2 + y^2 =5z^3

It will take a lot more than that to prove the conjecture. First, you have to show that if $z$ has any prime factor of the form $4k+3$ then either no solution exists or, if it does, then in every such solution $x$ and $y$ must have a nontrivial common factor. Then you also have to show that if all the prime factors of $z$ are of the form $4k+1$ then a solution does exist with $x$ and $y$ coprime.
Oh, I see what you're asking. Right, then I have merely proved that z is 1 modulo 4.

For the other part of the conjecture, that it is sufficient to have z in the 1 mod 4 congruence class, I expect this to be almost surely false.

#### Klaas van Aarsen

##### MHB Seeker
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Re: triplets x^2 + y^2 =5z^3

(*) : I think there is a way by analysis in $$\displaystyle \mathbb{Z}$$. I hope someone can bring this one down, I am too busy with the incoming examinations to do anything with it.

Given the original problem with $x^2+y^2=5z^3$ and gcd(x,y)=1 and x,y,z positive.

We can factorize the left hand side in $\mathbb Z$ as follows.
$$(x+iy)(x-iy)=5z^3$$
Let d be the gcd(x+iy, x-iy).
Then d divides the sum and the difference: d|2x and d|2iy. Therefore d is one of 1, 1+i, 2 up to a unit.
Only d=1 pans out, so (x+iy) and (x-iy) are co-prime in $\mathbb{Z}$.

We can factorize 5 into (1+2i)(1-2i) which are also co-prime.
So we might construct x+iy as $(1+2i)(a+ib)^3$ and x-iy as$(1-2i)(a-ib)^3$.

Multiplying this out, and separating the real and imaginary parts gives us x and y, and from their choice also z:
\begin{aligned}
x&=a^3-6a^2b-3ab^2+2b^3 \\
y&=2a^3+3a^2b-6ab^2-b^3 \\
z&=a^2+b^2
\end{aligned}

That leaves how to find a and b such that x and y are co-prime and positive.
I still have some trouble finding those.

#### mathbalarka

##### Well-known member
MHB Math Helper
Re: triplets x^2 + y^2 =5z^3

Finding a parameterization for the general case is easy, the problem occurs when one wants (x, y) = 1.

#### Klaas van Aarsen

##### MHB Seeker
Staff member
Re: triplets x^2 + y^2 =5z^3

Finding a parameterization for the general case is easy, the problem occurs when one wants (x, y) = 1.
I think it should suffice if $x+iy=(1+2i)(a+ib)^3$ and $x-iy = (1-2i)(a-ib)^3$ are co-prime.

And to get that, it should suffice that $\gcd(a+ib, 1-2i)=\gcd(a+ib, a-ib)=1$.
And finally, we don't have to find any $a$ and $b$, only that there are infinitely many.

I guess I'll have to practice a bit with gaussian integers.

#### mathbalarka

##### Well-known member
MHB Math Helper
Re: triplets x^2 + y^2 =5z^3

Probably true, perhaps. You shouldn't pay much attention to my "baseless" heuristics and criticism now, since I am having a very less time of doing math outside of the syllabus since my examinations are coming (a day to go).

And by the way, I think Darmon & Granville must have something to say about these particular equations. I believe there exists a general theorem for all spherical genrealized Fermat-Catalan equations, a theorem on infinitude of the solutions. A similar theorem concerning the finitude of parabolic ones (appearing in the same paper) has been explicitly worked out there, as I have heard, and a general construction of parametric solution is given also.

Balarka
.

#### Klaas van Aarsen

##### MHB Seeker
Staff member
Re: triplets x^2 + y^2 =5z^3

Gotcha! Let $N(u+iv)$ be the gaussian integer norm. That is, $N(u+iv)=u^2+v^2$.

Now suppose $d=\gcd(a+ib, 1-2i)$.
Then d|1-2i and N(d)|N(1-2i)=5.
So we can pick a and b such that $5\not|a^2+b^2$ to make sure a+ib and 1-2i are co-prime.

Suppose $e=\gcd(a+ib, a-ib)$.
Pick a and b co-prime.
Then e|2a and e|2ib, therefore e=1, 1+i, or 2.
Also pick $a \not\equiv b \pmod 2$, then we get that e=1.

There are infinitely many a and b such that $\gcd(a,b) =1, \quad 5 \not| a^2 + b^2, \quad a \not\equiv b \pmod 2$.
With such a and b, the chosen x+iy and x-iy are co-prime.
It follows that x and y are co-prime, because if they have a common divisor > 1, that divisor would also divide both x+iy and x-iy.

Therefore $x^2+y^2=5z^3$ with $\gcd(x,y)=1$ and $x,y,z \in \mathbb N$ has infinitely many solutions. $\qquad \blacksquare$