# Counting Pythagorean Triples

Calculate the number of Pythagorean triples whose hypotenuses $(=c)$ are less than or equal to $N$.

For example for $N = 15$ there are four Pythagorean triples: $$(3,4,5), \quad (5,12,13),\quad (6,8,10),\quad (9,12,15)$$

For any $m,n$ with $\gcd(m,n)=1$ and $m>n>0$, the expressions

$a=m^2-n^2 \\ b = 2mn\\ c=m^2+n^2\\$

give a primitive Pythagorean triple $(a,b,c)$. So $(m,n)=(2,1)$, for example, gives the standard $(a,b,c)=(3,4,5)$.

Then you can also multiply $(a,b,c)$ by some value $k>1$ to give additional non-primitive triples that will not be generated above.

Using these ideas together you can quickly find all Pythagorean triples to a given limit, expecially since as you can see you need $m^2 < c_{\text{limit}}$

HINT

For any Pythagorean triple we have that $c = k(m^2 + n^2)$ for some whole numbers $k$, $m$, and $n$

• maybe some moreee please :) – Nadfrw Mar 29 '17 at 20:04
• @Nadfrw I don't have much more .. other than to say that this will probably not be a nice function. Basically, for any given $n$, you would have to find all $k$ that divide $n$, and if it does, find all pairs of perfect squares $m^2$ and $n^2$ such that $m^2 + n^2 = \frac{n}{k}$. Your best option is really a computer program, as suggested in the comments. But at least with this equation, the search space will be significantly reduced in comparison to working with $c^2 = a^2 + b^2$ directly. – Bram28 Mar 29 '17 at 20:10