private static IEnumerable <long> GetFactors(this long x) =>
StrangeEnumerable.Range(1, x.GetCeiling())
.Where(n => x.IsEvenlyDivisibleBy(n))
.SelectMany(n =>
n != x / n
? new[] { n, x / n }
: new[] { n });
public static IEnumerable <long> GetPrimeFactors(this long x) =>
StrangeEnumerable.Range(2, x.GetCeiling())
.Where(n => x.IsEvenlyDivisibleBy(n))
.SelectMany(n =>
(x / n).IsPrime()
? n.IsPrime()
? n != x / n ? new[] { n, x / n } : new[] { n }
: new[] { x / n }
: n.IsPrime()
? new[] { n }
: new long[] { });