How to do a arithmetic sequence with roots using a loop

Question

What I want to do is by using a for loop, calculate the sum of roots under roots (not a math guy, don't know how you call that).

The sequence should go like this:

Sqrt(2 + Sqrt(2 + ... + Sqrt(2))), where n > 0.

For example, if n = 3 on the for loop (while i = 1, i++), it should make this arithmetic sequence:

sum += Math.Sqrt(2 + Math.Sqrt(2 + Math.Sqrt(2)));

and if n = 4:

sum += Math.Sqrt(2 + Math.Sqrt(2 + Math.Sqrt(2 + Math.Sqrt(2))));

So the problem is I don't know how to loop trough n by adding roots under roots until the loop ends.

My code template

        public static double GetSumOfRoots(int n)
        {
            double sum = 0;
            for (int i = 1; i <= n; i++)
            {
                sum += ...;
            }

            return sum;
        }

Please tell me if I am not clear on my description, I will do my best, thank you.

Answer 1

Unwrap the loop, and have a look on what's going on:

  n : formula
  -------------------------------------------------------
  0 : 0                           = 0    
  1 : Sqrt(2)                     = Sqrt(2 + item(0))
  2 : Sqrt(2 + Sqrt(2))           = Sqrt(2 + item(1)) 
  3 : Sqrt(2 + Sqrt(2 + Sqrt(2))) = Sqrt(2 + item(2))
  ....
  n : Sqrt(2 + ...)               = Sqrt(2 + item(n - 1))
  ....

Having math done, you can write corresponding code:

    public static double GetSumOfRoots(int n)
    {
        if (n < 0)
            throw new ArgumentOutOfRangeException(nameof(n)); 

        double sum = 0;
    
        for (int i = 1; i <= n; i++)
        {
            // Note assignment = instead of +=
            sum = Math.Sqrt(2 + sum);
        }

        return sum;
    }

You can simplify the routine with a help of Linq:

    using System.Linq;

    ...

    public static double GetSumOfRoots(int n)
    {
        if (n < 0)
            throw new ArgumentOutOfRangeException(nameof(n)); 

        return Enumerable
          .Range(0, n)
          .Aggregate(0.0, (s, a) => Math.Sqrt(2 + s));
    }

Demo:

  var report = string.Join(Environment.NewLine, Enumerable
    .Range(0, 10)
    .Select(n => $"{n} : {GetSumOfRoots(n)}"));

  Console.Write(report);

Outcome:

0 : 0
1 : 1.4142135623730951
2 : 1.8477590650225735
3 : 1.9615705608064609
4 : 1.9903694533443939
5 : 1.9975909124103448
6 : 1.9993976373924085
7 : 1.999849403678289
8 : 1.9999623505652022
9 : 1.9999905876191524