почему выдает ошибку

#include <iostream>
#include <cmath>
 
int main()
{
    const int SIZE=10;
    int a[SIZE+1]={0,1}, b[SIZE+1]={0,1};
    unsigned int factorial[SIZE+1]={0,1};
    unsigned int n;
    double sum=0;
 
    for (int i=2; i<=SIZE; i++)
    {
        a[i] = 3*b[i-1] + 2*a[i-1];
        b[i] = 2*a[i-1] + b[i-1];
        factorial[i] = i*factorial[i-1];
    }
 
    std::cout << "Enter n : ";
    std::cin >> n;
 
    for (unsigned int i=1; i<=n; i++)
        sum = sum + double(std::pow(2.0, i)) / ((1+std::pow(a[i], 2.0)+std::pow(b[i],2)) * factorial[i]);
 
    std::cout << sum << std::endl;
 
    return 0;
}

 sum = sum + double(std::pow(2.0, i)) / ((1+std::pow(a[i], 2.0)+std::pow(b[i],2)) * factorial[i]);

#include <iostream>
#include<math.h>
int main()
{
    const int SIZE=10;
    int a[SIZE+1]={0,1}, b[SIZE+1]={0,1};
    unsigned int factorial[SIZE+1]={0,1};
    unsigned int n;
    double sum=0;
 
    for (int i=2; i<=SIZE; i++)
    {
        a[i] = 3*b[i-1] + 2*a[i-1];
        b[i] = 2*a[i-1] + b[i-1];
        factorial[i] = i*factorial[i-1];
    }
    std::cout << "Enter n : ";
    std::cin >> n;
 
    for (unsigned int i=1; i<=n; i++)
        sum = sum + double(pow(2.0, (double)i)) / ((1+std::pow(a[i], 2.0)+pow(b[i],(double)2)) * factorial[i]);
    std::cout << sum << std::endl;
    system("pause");
    return 0;
}

pow(double,(double)i);