# Project Euler 9 Solution: Special Pythagorean triplet

A Pythagorean triplet is a set of three natural numbers, `a` < `b` < `c`, for which,

`a`

^{2}+

`b`

^{2}=

`c`

^{2}

For example, 3^{2} + 4^{2} = 9 + 16 = 25 = 5^{2}.

There exists exactly one Pythagorean triplet for which `a` + `b` + `c` = 1000.

Find the product `abc`.

## Solution

A problem like this is perfect for Haskell, since we can transform the mathematical description par for par into a programm:

solution n = head [a * b * c | c <- [1..n], b <- [1..c], a <- [1..b], a^2 + b^2 == c^2, a + b + c == n]

However, it takes quite some time to find the solution with this brute force attempt. One improvement is to use the fact that \(a+b+c=n\) which lets us construct the variable c as \(c=n - a - b\) (we could construct c as \(c=\sqrt{a^2+b^2}\), but this is computational much more expensive and of course we could construct a and b but this would require another check if the result becomes negative). This minimal change reduces the execution time tremendously:

solution n = head [a * b * c | b <- [1..n], a <- [1..b], let c = n - a - b, a^2 + b^2 == c^2]

In order to keep the relation \(a<b<c\), we would need to add a check of \(b<c\), but as \(a^2+b^2=c^2 \Rightarrow b<c\) for \(a>0\), we can skip this test. But based on the fact that \(a<b<c\), we can conclude even more, namely that \(a<n/3\) and \(a<b<n/2\). With this, we can further improve the search space:

solution n = head [a * b * c | a <- [1..quot n 3], b <- [a..quot n 2], let c = n - a - b, a^2 + b^2 == c^2]

We learned quite a lot already, but the search space is still too large, so back to the drawing board.We know \(a+b=n-c\) and \(a^2+b^2=c^2\), which implies that \(2ab = (n-c)^2 - c^2\). When subtracting the two equations, we get \(a^2 - 2ab + b^2 = c^2 - (n-c)^2 + c^2 = c^2 - n^2 + 2nc\). Since \((a-b)^2 = c^2 - n^2 + 2nc\), we know the right part must be square. We achieved, that we completely removed a and b! As we constructed the triplet this way, we find a solution if the integer square root exists and the result will be maximized as \(abc = n(n/4)^2 - n(c - n/4)^2 \).

solution n = head [ a * b * c | c <- [1 + quot n 3 .. quot n 2], let sqa_b = c * c - n * n + 2 * n * c, let a_b = floor(sqrt(fromIntegral sqa_b)), let b = quot (n - c + a_b) 2, let a = n - b - c, a_b * a_b == sqa_b]

Or the same implemented in JavaScript:

function solution(n) { for (var c = (n / 3 + 1) | 0; c < n / 2; c++) { var sqa_b = c * c - n * n + 2 * n * c var a_b = Math.floor(Math.sqrt(sqa_b)); if (a_b * a_b == sqa_b) { var b = (n - c + a_b) / 2; var a = n - b - c; return a * b * c; } } return -1 } solution(1000);