Возникает inf в бесконечном ряду делал с рекурсией

#include <iostream>
#include <cmath>
using namespace std;
double xNull(int a){
    double x0;
    if (a < -5){
        x0 = log(fabs(2 / a));
    }else if (-5 <= a && a < 5){
        x0 = pow(a, 2) + sqrt(a + 5);
    }else if (a >= 5){
        x0 = log(a + 2);
    }
    return x0;  
}
double recursion(int n, int a){
    if (n == 0){    
        return xNull(a);
    }else{
        return sqrt(fabs(recursion(n - 1, a))) + log(fabs(recursion(n - 1, a) + 5));
    }
}
double woRecursion(int n, int a){
    double xn;
    xn = xNull(a);
    for (int i = 0; i < n; i++){
        xn = sqrt(fabs(xn)) + log(fabs(xn + 5));
    }
    return xn;
}
int main(){
    int n = 9, a;
    a = 0;
    cout << "Finding the " << n << " member of the sequence at a = " << a << ": " << recursion(n, a) << endl;
    a = -7;
    cout << "Finding the " << n << " member of the sequence at a = " << a << ": " << recursion(n, a) << endl;
    a = 20;
    cout << "Finding the " << n << " member of the sequence at a = " << a << ": " << recursion(n, a) << endl << endl;
    
    a = 0;
    cout << "Finding the " << n << " member of the sequence at a = " << a << ": " << woRecursion(n, a) << endl;
    a = -7;
    cout << "Finding the " << n << " member of the sequence at a = " << a << ": " << woRecursion(n, a) << endl;
    a = 20;
    cout << "Finding the " << n << " member of the sequence at a = " << a << ": " << woRecursion(n, a);
}

#include <iostream>
#include <cmath>
double fS(double x)
{
    return 1 / sqrt(1 + x);
}
double S(double x)
{
    double eps = 1e-7;    // Точность
    double u = 1.0, res = 1.0;
    int n = 0;
    while (u < -eps || eps < u)
    {
        n++;
        u *= (1 - 2 * n)*x / (2 * n);
        res += u;
    }
    return res;
}
int main()
{
    double x;
    do
    {
        std::cout << "x = ";
        std::cin >> x;
    } while (x <= -1 || 1 <= x);
    std::cout << "S = " << S(x) << std::endl;
    std::cout << "1/sqrt(1+" << x << ") = " << S(x) << std::endl;
    return 0;
}