public BigInteger Solve(BigInteger?input = null)
{
bool[] primesUnderAMillion = PrimeHelper.FindPrimesUnderLimit(1000000);
BigInteger result = 0;
for (int i = 2; i < 1000000; i++)
{
var testNumber = i;
var circular = true;
var testString = testNumber.ToString();
for (int j = 0; j <= i.ToString().Length + 1; j++)
{
if (!primesUnderAMillion[testNumber] || testNumber > 1000000)
{
circular = false;
break;
}
testString = testString.Insert(testString.Length, testString.Substring(0, 1));
testString = testString.Remove(0, 1);
testNumber = int.Parse(testString);
}
if (circular)
{
result++;
}
}
return(result);
}
public BigInteger Solve(BigInteger?input = null)
{
int result = 0;
int primesLimit = 100000;
while (result == 0)
{
BigInteger numberOfPrimes = 0;
bool[] test = PrimeHelper.FindPrimesUnderLimit(primesLimit);
for (int i = 0; i < test.Length; i++)
{
if (test[i])
{
numberOfPrimes++;
if (numberOfPrimes == input)
{
result = i;
}
}
}
primesLimit += 100000;
}
return(result);
}
public BigInteger Solve(BigInteger?input = null)
{
bool[] primesUnderLimit = PrimeHelper.FindPrimesUnderLimit(2000000);
BigInteger total = 0;
for (int i = 0; i < primesUnderLimit.Length; i++)
{
if (primesUnderLimit[i])
{
total += i;
}
}
return(total);
}
public BigInteger Solve(BigInteger?input = null)
{
var primes = PrimeHelper.FindPrimesUnderLimit(28124);
List <int> abundantNumbers = new List <int>();
for (int i = 2; i < 28124; i++)
{
if (DivisorsHelper.SumOfProperDivisors(i, primes) > i)
{
abundantNumbers.Add(i);
}
}
BigInteger result = 0;
bool[] canBeSummed = new bool[28124];
for (int i = 0; i < abundantNumbers.Count; i++)
{
for (int j = 0; j < abundantNumbers.Count; j++)
{
if (abundantNumbers[i] + abundantNumbers[j] >= 28124)
{
break;
}
canBeSummed[abundantNumbers[i] + abundantNumbers[j]] = true;
}
}
for (int i = 1; i < 28124; i++)
{
if (!canBeSummed[i])
{
result += i;
}
}
return(result);
}
public BigInteger Solve(BigInteger?input = null)
{
// b must be prime for the case when n = 0; -- this must make it easier,
// b + a + 1 must be prime -- does this? IT TURNS OUT IT DOES, as long as b isn't 2 a must be odd
// n^2 + an + b = p
var primesUnder2Million = PrimeHelper.FindPrimesUnderLimit(2000000);
int highestIndex = 39, bestABProduct = 41;
// skip even version of a, hope a isn't 2
for (int a = -999; a < 1000; a += 2)
{
int b = 3;
while (b < 1000)
{
// b has to be prime!
if (primesUnder2Million[b])
{
int i = 0;
// iterate to see how deep the rabbit hole goes for this combination of a and b
while (i * i + a * i + b > 0 && primesUnder2Million[i * i + a * i + b])
{
i++;
}
// we have a highestindex
if (i > highestIndex)
{
highestIndex = i;
bestABProduct = a * b;
}
}
b++;
}
}
return(bestABProduct);
}
public BigInteger Solve(BigInteger?input = null)
{
BigInteger result = 0;
bool[] primesUnderLimit = PrimeHelper.FindPrimesUnderLimit(10001);
for (int i = 2; i < 10000; i++)
{
if (primesUnderLimit[i])
{
continue;
}
BigInteger sumOfDivisors = DivisorsHelper.SumOfDivisors(i, primesUnderLimit) - i;
if (sumOfDivisors < 10000 &&
sumOfDivisors != i &&
DivisorsHelper.SumOfDivisors((int)sumOfDivisors, primesUnderLimit) - sumOfDivisors == i)
{
result += i;
}
}
return(result);
}
public BigInteger Solve(BigInteger?input = null)
{
int numberOfDivisors = 0;
int i = 2, triangleNumber = 1;
while (numberOfDivisors < 500)
{
numberOfDivisors = 0;
triangleNumber += i;
int testNumber = triangleNumber;
Dictionary <int, int> divisors = DivisorsHelper.CalculatePrimeDivisors(testNumber, PrimeHelper.FindPrimesUnderLimit(200000000));
numberOfDivisors = divisors.Values.ElementAt(0) + 1;
for (int l = 1; l < divisors.Count; l++)
{
numberOfDivisors *= divisors.Values.ElementAt(l) + 1;
}
i++;
}
return(triangleNumber);
}