number theory algorithm

* Check if the given number n is prime.
```
#include <iostream>
using namespace std;

bool isPrime(int n) {
    if(n<2) return false;
    for(int i=2; i*i<=n; i++)
        if(n%i==0)
            return false;
    return true;        
}

int main() {
    cout<<"57 is"<<(isPrime(57)?"":" not ")<<"a prime."<<endl;
    return 0;
}
```
* Constructs a vector that contains the prime factorisation of n.
```
#include <iostream>
#include <vector>
using namespace std;

vector<int> getFactors(int n) {
    vector<int> f;
    for(int i=2; i*i<=n; i++)
        while(n%i==0) {
            f.push_back(i);
            n/=i;
        }
    if(n>1) 
        f.push_back(n);
    return f;    
}

int main() {
    vector<int> f=getFactors(57);
    for(auto it=f.begin(); it!=f.end(); it++)
        cout<<*it<<" ";
    return 0;
}
```
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