Rational numbers counting (with inverse)

RationalNumberCounting.cs

public static long RationalNumber(long i, long j)
{
	if (j == 1)
	{
		if (i == 0)
			return 1;
		else if (i == 1)
			return 2;
	}

	long kronDelta = i == 0 ? 0 : 1;
	long jSign = Math.Sign(j);
	long result = ((i + j - 1) * (i + j - 2) + 2 * j + 2 * kronDelta) * (1 + jSign) / 2
		+ ((i - j - 1) * (i - j - 2) - 2 * j + 2 * kronDelta - 1) * (1 - jSign) / 2
		+ 1 - kronDelta;
	return result;
}

public static void RationaNumberFromNumber(long n, out long i, out long j)
{
	if (n == 1)
	{
		i = 0; j = 1;
		return;
	}

	if (n == 2)
	{
		i = 1; j = 1;
		return;
	}

	double sqrt4n11 = Math.Sqrt(4 * n - 11);
	ulong M = (ulong)((sqrt4n11 + 1) / 2);
	i = (long)((M * M + M) / 2 - (ulong)((n - 3) / 2));

	long sign = n % 2 == 0 ? 1 : -1;
	j = sign * Math.Abs((long)((sqrt4n11 + 3) / 2) - i);
}

static void Main(string[] args)
{
	for (long n = 1; n < 5000; n++)
	{
		long i, j;
		RationaNumberFromNumber(n, out i, out j);
		Console.WriteLine("N(i,j) = {0}; i = {1}; j = {2}; N = {3}", n, i, j, RationalNumber(i, j));
	}

	Console.ReadLine();
}