Error: "an object reference is required for the non-static field, method or property..." [duplicate]

You can't access non-static members from a static method. (Note that Main() is static, which is a requirement of .Net). Just make siprimo and volteado static, by placing the static keyword in front of them. e.g.:

 static private long volteado(long a)

Create a class and put all your code in there and call an instance of this class from the Main :

static void Main(string[] args)
{

    MyClass cls  = new MyClass();
    Console.Write("Write a number: ");
    long a= Convert.ToInt64(Console.ReadLine()); // a is the number given by the user
    long av = cls.volteado(a);
    bool isTrue = cls.siprimo(a);
    ......etc

}