NaN при табуляции функции

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
namespace ConsoleApp5
{
    class Program
    {
        static void Main(string[] args)
        {
            string temp;
            double y,a, x, k,max,dx;
            StreamReader streamReader = new StreamReader("C:/Users/User/Documents/Visual Studio 2017/Projects/ConsoleApp5/ConsoleApp5/date.txt");
            temp = streamReader.ReadLine();
            a=Convert.ToDouble(temp);
            temp = streamReader.ReadLine();
            k=Convert.ToDouble(temp);
            temp = streamReader.ReadLine();
            x =Convert.ToDouble(temp);
            temp = streamReader.ReadLine();
            max =Convert.ToDouble(temp);
            temp = streamReader.ReadLine();
            dx = Convert.ToDouble(temp);
            StreamWriter streamWriter = new StreamWriter("./result.txt");
            for (double i = x; i < max; i+=dx,x+=dx)
            {
                if (Math.Log(a * x)>0 )
                {
                    y = (Math.Pow(Math.Cos(a * x), 1.0/3.0)) + (k * Math.Cos(a * x) / Math.Log(a * x));
                    streamWriter.Write("\t {0:N3}", y);
                }
            }
            streamWriter.Close();
            streamReader.Close();
            Console.ReadKey();
        }
    }
}

Данные в файл записаны вот так (в столбик):

2,3
15,2
11,13
22,23
0,5

Вывод в файле:

5,150 NaN NaN NaN 2,248 5,401 2,823 NaN NaN NaN 4,409 4,504 NaN NaN NaN 2,465 5,016 2,301 NaN NaN NaN 4,337 4,048

Откуда берётся NaN?