public class Program
{

    private static void Main(string[] args)
    {
        ParallelFactorial factorial = new ParallelFactorial();

        for (int i = 4; i <= 100; i++)
        {
            Console.WriteLine(factorial.ComputeFactorial(i).Result);
        }
    }
}

public class ParallelFactorial
{
    public async Task<BigInteger> ComputeFactorial(int n)
    {
        int processorCount = Environment.ProcessorCount * 2;

        int delta = n / processorCount;
        List<Task<BigInteger>> tasks = new List<Task<BigInteger>>();
        for (int i = 1; i <= n; i += delta)
        {
            int from = i;
            int to = i + delta - 1;
            tasks.Add(Task.Run(() =>
            {
                BigInteger val = 1;
                for (int k = from; k <= to; k++) val *= k;
                return val;
            }));
        }

        return (await Task.WhenAll(tasks)).Aggregate((x, y) => x * y);
    }
}