I decided to post the code as a full submission because I think c# often gets dismissed as Java copy-cat, which hasn't been true in a decade.

The code (non-destructive, no side effects) calculates square roots :

    public static void Main()
    {
        const int n = 21;

        Observable.Generate<double,double>(1, x=>true, x=> 0.5 * (x + (n/x)), x=>x )
        .Scan(new {prv = 0d, cur = 0d}, (prev, curr) => new {prv = prev.cur, cur = curr})
        .FirstAsync(_ => Math.Abs(_.cur - _.prv) < 1E-10)
        .Select(_ => _.cur)
        .Subscribe(Console.WriteLine);

        // this is just to compare values, so is not part of the solution
        Console.WriteLine(Math.Sqrt(n)); 
        Console.ReadLine();
    }

Read the Haskell code that inspired both here: https://news.ycombinator.com/item?id=7043943