public int Problem9()
{
int s = 1000;
int mlimit = (int)Math.Sqrt(s / 2);
var range = Enumerable.Range(2, mlimit - 2);
Func <int, Tuple <bool, int> > action = m =>
{
if ((s / 2) % m == 0)
{
int k = m % 2 == 0 ? m + 1 : m + 2;
while (k < 2 * m && k <= s / (2 * m))
{
if (s / (2 * m) % k == 0 && MathsHelper.gcd(k, m) == 1)
{
int n = k - m;
int d = s / (2 * m * k);
return(Tuple.Create(true, MathsHelper.Power(d, 3) *
((m * m) - (n * n)) *
(2 * m * n) *
((m * m) + (n * n))));
}
k += 2;
}
}
return(Tuple.Create(false, 0));
};
return(range.Select(action).First(x => x.Item1).Item2);
}
private static IEnumerable <int> GetPerimetersOfPrimitivePythagoreanTriplets(int s)
{
//generate coprime m,n with exactly one m,n even
int m = 2;
int n = 1;
int mlimit = (int)Math.Sqrt(s / 2);
while (m <= mlimit)
{
n = m % 2 == 0 ? 1 : 2;
while (n < m)
{
if (MathsHelper.gcd(m, n) == 1)
{
yield return(2 * m * (m + n));
}
n += 2;
}
m++;
}
}