Идентификатор не определён

#include <iostream>
#include <math.h>
using namespace std;
double func(double x)
{
    return x * x;
}
double ln(double x)
{
    return log(x) - 1;
}
double zero(double, double, double, double (*)(double));
int main()
{
    float а, b; double e = 0.000001;
    typedef double(*FUNC)(double);
    FUNC f[5] = { sin, cos, ln, func, tan };
    cout.precision(6);
    for (int i = 0; i < 5; ++i)
    {
        cout<<" Enter limits а &  b: "; 
        cin >> a >> b;
        cout<< " Enter precision : ";
        cin>> e;
        cout<< " Solution for current function is" << zero(а, b, e, f[i])<< endl;
    }
    return 0;
}
double zero(double а, double b, double e, double (*fp)(double x))
{
    if (b < a) { cout<< " Wrong limits!\n "; return 0; }
    if (fp(a) * fp(b) > 0) { cout<<" No solution!\n"; return 0; }
    char rise_flag = fp(a) < 0 ? 1 : 0;
    for (double mid = (b - a) / 2; mid > e; mid = (b - a) / 2)
    {
        if (fp(a + mid) < 0)
            if (rise_flag) a = a + mid; else b = a + mid;
        else if (fp(a + mid) > 0)
            if (rise_flag) b = a + mid; else a = a + mid;
        else return a + mid;
    }
    return a + mid;
}