Project Euler task 46

It was proposed by Christian Goldbach that every odd composite number can be written as the sum of a prime and twice a square.

9 = 7 + 21²
15 = 7 + 22²
21 = 3 + 23²
25 = 7 + 23²
27 = 19 + 22²
33 = 31 + 21²

It turns out that the conjecture was false.

What is the smallest odd composite that cannot be written as the sum of a prime and twice a square?

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#include < iostream >

#include < math.h >

using namespace std;

const int ciMax = 1000000;

const int ciSeqCount = 4;

int iEratArray[ciMax];

bool isRepresentable(int _n) {

if (_n % 2 == 0)

return false;

if (iEratArray[_n] == 0)

return false;

for (int i = 3; i < _n; i += 2) {

if (iEratArray[i]) // select a prime

continue;

int iX = (_n - i) / 2;

double dSqrt = sqrt((double) iX);

if (dSqrt - floor(dSqrt) > 0)

continue;

return true;

}

return false;

}

int main() {

memset(iEratArray, 0, ciMax);

// eratosphene's sieve

iEratArray[0] = 1;

iEratArray[1] = 1;

for (int k = 2; k < ciMax; k++) {

if (iEratArray[k]) // if not prime, continue

continue;

for (int j = 2; k * j < ciMax; j++)

iEratArray[k * j] = 1; // mark as not prime

}

for (int i = 3; i < ciMax; i += 2) {

if (!isRepresentable(i) && iEratArray[i]) {

cout << i << endl;

break;

}

}

return 0;

}