# Primitive Pyth. triples: Solutions of x^2 + y^2 = 2 z^2

• kingwinner
In summary: I'm looking for the set of solutions for the integers.Thank you!In summary, the theorem states that the positive primitive solutions of x^2 + y^2 = z^2 with y even are x = r^2 - s^2, y = 2rs, z = r^2 + s^2, where r and s are arbitrary integers of opposite parity with r>s>0 and gcd(r,s)=1. Using this theorem, all solutions of the equation x^2 + y^2 = 2z^2 can be found by writing the equation in the form (x+y)^2 + (x-y)^2 = (2z
kingwinner
Theorem: The positive primitive solutions of $$x^2 + y^2 = z^2$$ with y even are $$x = r^2 - s^2, y = 2rs, z = r^2 + s^2$$, where r and s are arbitrary integers of opposite parity with r>s>0 and gcd(r,s)=1.

Using this theorem, find all solutions of the equation $$x^2 + y^2 = 2z^2$$
(hint: write the equation in the form $$(x+y)^2 + (x-y)^2 = (2z)^2$$)
==================================

The above theorem characterizes all "PRIMITIVE Pythagorean triples", but what is the statement of the theorem that characterizes ALL "Pythagorean triples" (not necessarily primitive)?

(i) The positive solutions of $$x^2 + y^2 = z^2$$ with y even are precisely $$x = (r^2 - s^2)d, y = (2rs)d, z = (r^2 + s^2)d$$, where d is any natural number, r and s are arbitrary integers of opposite parity with r>s>0 and gcd(r,s)=1.

(ii) The positive solutions of $$x^2 + y^2 = z^2$$ with y even are precisely $$x = r^2 - s^2, y = 2rs, z = r^2 + s^2$$, where r and s are arbitrary integers with r>s>0.

Which one is correct?

I hope someone can help me out. Thank you!

The only difference is a d, which, as it says, represents any natural number.

So I think (i) correctly characterizes ALL Pythagorean triples.

Is (ii) wrong?
i.e. not all Pythagorean triple is of the form $$x = r^2 - s^2, y = 2rs, z = r^2 + s^2$$, r>s>0?
For example, 6,8,10 is a Pythagorean triple, but I think it cannot be written in the above form?

The most important triples are the fundamental or primitative ones. We can always add a constant to all elements in any triple.

If you can read through the proof of (2), which starts from scratch, you will get a better understanding of this.

But can EVERY Pythagorean triple (primitive or imprimitive) be written in the form $$x = r^2 - s^2, y = 2rs, z = r^2 + s^2$$ for some r,s E Z, r>s>0?

kingwinner said:
But can EVERY Pythagorean triple (primitive or imprimitive) be written in the form $$x = r^2 - s^2, y = 2rs, z = r^2 + s^2$$ for some r,s E Z, r>s>0?

No. For example, $9^2 + 12^2 = 15^2$, but 15 cannot be written as the sum of two squares.

Petek

Petek said:
No. For example, $9^2 + 12^2 = 15^2$, but 15 cannot be written as the sum of two squares.

Petek

OK, so (i) is correct and (ii) is wrong.

Back to our original problem...

$$x^2+y^2=2z^2\iff(x+y)^2+(x-y)^2=(2z)^2$$
<=> $$x+y,x-y,2z$$ is a Pythagorean triple (but it's not necessarily primitive)
$$\begin{cases} x+y=(m^2-n^2)d\\x-y=2mnd\\2z=(m^2+n^2)d \end{cases}$$
where d is natural number, m and n are integers of opposite parity with m>n>0 and gcd(m,n)=1.

Now how can we characterize ALL solutions of $$x^2 + y^2 = 2z^2$$? Is there any restriction on "d"?

Thank you!

kingwinner said:
Now how can we characterize ALL solutions of $$x^2 + y^2 = 2z^2$$? Is there any restriction on "d"?

If $d$ is any natural number, then $x+y$, $x-y$, and $2z$ form a Pythagorean triple. Now just follow your "if and only ifs" in reverse to conclude that $x^2 + y^2 = 2z^2$.

But I believe that when we're asked to find all solutions of the equation $$x^2 + y^2 = 2z^2$$, the answer should be of the form
x=...
y=...
z=...

How can we get this?

kingwinner said:
But I believe that when we're asked to find all solutions of the equation $$x^2 + y^2 = 2z^2$$, the answer should be of the form
x=...
y=...
z=...

How can we get this?

Well, you can solve these three equations for x, y, and z:

$$\begin{cases} x+y=(m^2-n^2)d\\x-y=2mnd\\2z=(m^2+n^2)d \end{cases}$$

The first two are a linear system of two equations with two unknowns (x and y) so should be easy to solve. The third one is even easier - just divide both sides by 2.

Is that what you meant?

: Hmm, I guess you will have to impose constraint(s) to ensure that the solutions are integers.

For example:

$$z = \frac{1}{2}(m^2 + n^2)d$$

We're still assuming that $m$ and $n$ have opposite parity, right? That means $m^2 + n^2$ is odd, so $d$ had better be even. You will have to check $x$ and $y$ as well once you have expressions for them.

Last edited:
Sovling for x,y,z, I get:
$$x = d(m^2 - n^2 + 2mn) / 2$$
$$y = d(m^2 - n^2 - 2mn) / 2$$
$$z = d(m^2 + n^2) / 2$$

So d must be even, is this the ONLY restriction?

Last edited:
Yes, that looks like the only restriction.

So I think the final answer is
$$x = d(r^2 - s^2 + 2rs) / 2$$
$$y = d(r^2 - s^2 - 2rs) / 2$$
$$z = d(r^2 + s^2) / 2$$
where d is any EVEN natural number, r and s are any integers of opposite parity with r>s>0 and gcd(r,s)=1?
Does this include ALL solutions of $$x^2 + y^2 = 2z^2$$?

Last edited:
OK, now I've thought about it more, and I think that are serious problems in my formula above. We're actually missing A LOT of solutions. For example, x=y=z is a solution for any integer (e.g. x=y=z=6 is a solution), but it's definitely not a Pythagorean triple, the formula above misses all of those. Also, all the negative solutions are missing from the above formula because by definition a Pythagorean triple is positive while solutions to the equation can be positive or negative.

So the above formula certainly does not include all solutions, how can we fix this problem? How can we correctly write down a formula that includes all the solutions?

Can someone kindly explain how we can summarize our final answer that includes all solutions into a compact formula?
Thanks a million!

Last edited:
x = y = z isn't a solution to $x^2 + y^2 = z^2$ but it is a solution to $(x +y)^2 + (x - y)^2 = (2z)^2$. That is,

$$(x + x)^2 + (x - x)^2 = (2x)^2$$

Petek

If I want to solve $$x^2 + y^2 = 2z^2$$ in the integers (rather than in the natural numbers), i.e. x,y,z E Z, what would be the answer? How can we fully describe the set of ALL integer solutions? Right now, I'm having a lot of trouble trying to describe ALL integer solutions without missing any. Does anyone happen to know the correct answer?

Thanks!

kingwinner said:
If I want to solve $$x^2 + y^2 = 2z^2$$ in the integers (rather than in the natural numbers), i.e. x,y,z E Z, what would be the answer? How can we fully describe the set of ALL integer solutions? Right now, I'm having a lot of trouble trying to describe ALL integer solutions without missing any. Does anyone happen to know the correct answer?

The solutions in integers are just
(x, y, z)
(-x, y, z)
(x, -y, z)
(-x, -y, z)
(x, y, -z)
(-x, y, -z)
(x, -y, -z)
(-x, -y, -z)
for every solution (x, y, z) in positive integers, together with (0, 0, 0).

## 1. What are primitive Pythagorean triples?

Primitive Pythagorean triples are sets of three positive integers (x,y,z) that satisfy the equation x^2 + y^2 = z^2. They are called "primitive" because the numbers in the triple share no common factors.

## 2. How many solutions are there for x^2 + y^2 = 2 z^2?

There are infinitely many solutions for this equation. Every primitive Pythagorean triple is a solution, and by multiplying each term in a triple by a constant factor, we can obtain another solution.

## 3. Can all Pythagorean triples be generated from primitive Pythagorean triples?

Yes, all Pythagorean triples can be generated from primitive Pythagorean triples. This can be done using the formula a = 2mn, b = m^2 - n^2, and c = m^2 + n^2, where m and n are positive integers with m > n and no common factors.

## 4. Are there any patterns or relationships between primitive Pythagorean triples?

Yes, there are some patterns and relationships between primitive Pythagorean triples. For example, all primitive Pythagorean triples where the two smaller numbers (x and y) are consecutive odd numbers can be generated by the formula x = n, y = (n+1), and z = (n+1) + 1, where n is a positive integer.

## 5. What is the significance of primitive Pythagorean triples?

Primitive Pythagorean triples have been studied for thousands of years and have many important applications in mathematics, physics, and computer science. They also have connections to other areas of mathematics, such as number theory and geometry.

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