C#

Pex is Automated White box Testing for .NET from Microsoft Research.

Pex for fun on the web is a radically simplified version of the fully featured Pex Power Tool for Visual Studio.

Challenge Fibonacci 2 is a kind of the simplest recursive problem after you find out the duel is actually second order Fibonacci array.

using System;

public class Program {
  public static int Puzzle(int x) {
    // Can you write code to solve the puzzle? Ask Pex to see how close you are.
    if (x <= 1) return x;
    if (x == 2) return 3;

    return Puzzle(x-1)*2 - Puzzle(x-3);
  }
}