public static long Solution()
{
long solution = 0;
var stopwatch = new Stopwatch();
stopwatch.Start();
var previousTime = 0.0;
var currentTime = 0.0;
var count = 0;
var solutions = new List <long>();
for (int i = 7; count < limit; i++)
{
var order = UtilityFunctions.MultiplicativeOrder(10, 9 * i);
if (order > 0 && (i - 1) % order == 0 && !UtilityFunctions.IsPrime(i))
{
count += 1;
solution += i;
solutions.Add(i);
Console.Write($"{ i}, ");
}
}
currentTime = stopwatch.ElapsedMilliseconds;
Console.WriteLine($"Solution took {currentTime - previousTime} seconds.");
Console.WriteLine(solutions.ToArray());
return(solution);
}
public static long Solution()
{
long solution = 0;
var stopwatch = new Stopwatch();
stopwatch.Start();
var previousTime = 0.0;
var currentTime = 0.0;
for (int i = 1; true; i++)
{
var p = 3 * i * i + 3 * i + 1;
if (p > limit)
{
break;
}
if (UtilityFunctions.IsPrime(p))
{
solution += 1;
// Console.WriteLine($"{i} -> {p}");
}
}
currentTime = stopwatch.ElapsedMilliseconds;
Console.WriteLine($"Solution took {currentTime - previousTime} milliseconds.");
return(solution);
}
public static long Solution()
{
long solution = 0;
var primes = UtilityFunctions.Primes(8000);
var residues = new List <long> {
1, 3, 7, 9, 13, 27
};
for (long num = 10; num < limit; num += 10)
{
if (num % 3 == 0 || (num % 7 != 3 && num % 7 != 4) || num % 13 == 0)
{
continue;
}
bool skipNum = false;
for (int i = 3; i < primes.Count; i++)
{
long p = primes[i];
if (p > num * num)
{
break;
}
var r = num % p;
foreach (var s in residues)
{
if ((r * r + s) % p == 0)
{
skipNum = true;
break;
}
}
if (skipNum)
{
break;
}
}
if (skipNum)
{
continue;
}
var nSquared = num * num;
if (UtilityFunctions.IsPrime(nSquared + 1) &&
UtilityFunctions.IsPrime(nSquared + 3) &&
UtilityFunctions.IsPrime(nSquared + 7) &&
UtilityFunctions.IsPrime(nSquared + 9) &&
UtilityFunctions.IsPrime(nSquared + 13) &&
UtilityFunctions.IsPrime(nSquared + 27) &&
!UtilityFunctions.IsPrime(nSquared + 19) &&
!UtilityFunctions.IsPrime(nSquared + 21))
{
solution += num;
Console.WriteLine(num);
}
}
return(solution);
}
private static bool IsStrongHarshad(long n)
{
var digitSum = UtilityFunctions.DigitSum(n);
if (n % digitSum != 0)
{
return(false);
}
if (!UtilityFunctions.IsPrime(n / digitSum))
{
return(false);
}
return(true);
}
public static long Solution()
{
long solution = 0;
HashSet <long> strongTruncatableHarshads;
var rightTruncatableHarshads = RightTruncatableHarshads(limit, out strongTruncatableHarshads);
foreach (var s in strongTruncatableHarshads)
{
foreach (var i in new[] { 1, 3, 7, 9 })
{
var n = 10 * s + i;
if (UtilityFunctions.IsPrime(n))
{
solution += n;
}
}
}
return(solution);
}
