private static long SumOfAmicableNumbersUnder(int ceiling) =>
StrangeEnumerable.Range(1, ceiling)
.Where(a =>
{
long b = a.GetFactorsNotIncluding().Sum();
return(a.IsAmicablePairOf(b));
})
.Sum();
private static long LongestCollatzSequenceUnder(int ceiling) => StrangeEnumerable.Range(1, ceiling) .OrderByDescending(n => n.CollatzSequenceLength()) .FirstOrDefault();
private static long SumOfNumbersNotSumOfTwoAbundantNumbers(int ceiling, IEnumerable <long> sumsOfTwoAbundantNumbers) => StrangeEnumerable.Range(1, ceiling) .Except(sumsOfTwoAbundantNumbers) .Sum();
private static List <long> AbundantNumbersUnder(int count) => StrangeEnumerable.Range(1, count).Where(n => n.IsAbundant()).ToList();