nth-fibonacci.cs

public long FibonacciNumber(long n)
{
	if (n > 92)
		throw new ArgumentOutOfRangeException("n", "Fib(>92) will cause a 64-bit integer overflow.");
	if (n < -92)
		throw new ArgumentOutOfRangeException("n", "Fib(<-92) will cause a 64-bit integer overflow.");

	var sqrt5 = Math.Sqrt(5);

	var bigPhi = (sqrt5 + 1) / 2;
	var miniPhi = bigPhi - 1;

	return Convert.ToInt64((Math.Pow(bigPhi, n) - Math.Pow(-miniPhi, n)) / sqrt5);
}