public void Problem21()
{
Func <int, dynamic> getDivisorsInfo = n =>
{
var uniqueDivisors = Sequences.UniqueDivisors(n).ToList();
return(new
{
Number = n,
AmicableSum = uniqueDivisors.Sum() - n
});
};
var divisors_cache = Enumerable.Range(1, 9_999)
.Select(getDivisorsInfo)
.ToList();
var d_cache = divisors_cache
.ToDictionary(x => x.Number, x => x.AmicableSum);
Func <int, int> d = (n) =>
{
if (d_cache.ContainsKey(n))
{
return(d_cache[n]);
}
else
{
var divisorsInfo = getDivisorsInfo(n);
divisors_cache.Add(divisorsInfo);
d_cache.Add(divisorsInfo.Number, divisorsInfo.AmicableSum);
return(divisorsInfo.AmicableSum);
}
};
var amicableNumbersUnder10000 = Enumerable.Range(1, 9_999)
.Where(a =>
{
var b = d(a);
return(a == d(b) && a != b);
})
.ToList();
var answer
= amicableNumbersUnder10000.Sum();
Assert.That(answer, Is.EqualTo(31_626));
}
public void Problem23()
{
var abundantNumbers = new SortedSet <int>();
for (int n = 12; n <= 28_123; n++)
{
var uniqueDivisors = Sequences.UniqueDivisors(n);
var sum = uniqueDivisors.Sum() - n;
if (sum > n)
{
abundantNumbers.Add(n);
}
}
var numbersThatAreNotSumOfTwoAbundantNumbers = new SortedSet <int>(
Enumerable.Range(1, 28_123)
.Select(x => x)
.ToList()
);
var abundantNumberList = abundantNumbers.ToList();
for (int i = 0; i < abundantNumberList.Count; i++)
{
var iNum = abundantNumberList[i];
for (int j = i; j < abundantNumberList.Count; j++)
{
var jNum = abundantNumberList[j];
var sum = iNum + jNum;
if (numbersThatAreNotSumOfTwoAbundantNumbers.Contains(sum))
{
numbersThatAreNotSumOfTwoAbundantNumbers.Remove(sum);
}
}
}
Assert.That(numbersThatAreNotSumOfTwoAbundantNumbers.Sum(), Is.EqualTo(4_179_871));
}
