PROJECT EULER #44

Link to Project Euler problem 44

Pentagonal numbers are generated by the formula, P_n=n(3n-1)/2. The first ten pentagonal numbers are:

1, 5, 12, 22, 35, 51, 70, 92, 117, 145, ...

It can be seen that P₄ + P₇ = 22 + 70 = 92 = P₈. However, their difference, 70 - 22 = 48, is not pentagonal.

Find the pair of pentagonal numbers, P_j and P_k, for which their sum and difference is pentagonal and D = |P_k - P_j| is minimised; what is the value of D?

using System;
using System.Collections.Generic;

namespace project_euler
{
class Program
{
 static void Main()
 {
     //Problem 44
     DateTime start = DateTime.Now;
     int diff;
     double n;
     List<int> pent = new List<int>(10000);
     for (int i = 0; i < 10000; i++)
     {
         //add pentagonals to list
         pent.Add((i+1)*(3*(i+1) - 1)/2);
         for (int j = 0; j < i; j++)
         {
             diff = pent[i] - pent[j];
             //check if diff is pentagonal
             n = (Math.Sqrt(24*diff + 1) + 1)/6;
             //if n is an integer
             if (n == Math.Floor(n))
             {
                 n = (Math.Sqrt(24*(diff + pent[i]) + 1) + 1)/6;
                 if (n == Math.Floor(n))
                     Console.WriteLine(pent[j]);
             }
         }
     }
     TimeSpan time = DateTime.Now - start;
     Console.WriteLine("This took {0}", time);
     Console.ReadKey();
 }
}
}