C#-Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Diagnostics;

namespace trainYourProgrammer
{
	class MainClass
	{
		private static int[] CalcSigma(int limit) { //Summe der Teiler für jede Zahl <= limit
			int[] result = new int[limit + 1];
			for (int i = 2; i <= limit; i++) {
				result [i] = 1;
			}
			for (int i = 2; i * 2 <= limit; i++) {
				for (int j = 2 * i; j <= limit; j += i) {
					result [j] += i;
				}
			}
			return result;
		}

		private static List<List<int>> SocialNumbers(int limit) {
			List<List<int>> result = new List<List<int>> ();
			int[] sigma = CalcSigma (limit);
			for (int i = 2; i <= limit; i++) {
				List<int> chain = new List<int> (5);
				int tmp = i;
				while (tmp > 0 && tmp <= limit && !chain.Contains(tmp)) {
					chain.Add (tmp);
					tmp = sigma [tmp];
				}
				if (chain.Count > 2 && tmp == chain [0]) { //mache >2 zu == 2 für befreundete Zahlen bzw. == 1 für vollkommene Zahlen
					result.Add (chain);
				}
				if (tmp == chain [0] || tmp == 0 || tmp > limit) {
					foreach (int j in chain) { //eine Zahl kann nicht in 2 verschiedenen Ketten vorkommen
						sigma [j] = 0;
					}
				}
			}
			return result;
		}

		public static void Main(string[] args) {
			const int limit = (int)1e7;
			Stopwatch sw = Stopwatch.StartNew ();
			List<List<int>> numbers = SocialNumbers (limit);
			foreach (List<int> chain in numbers) {
				Console.WriteLine (String.Join (", ", chain));
			}
			sw.Stop ();
			Console.WriteLine ("\nbenötigte Zeit: " + sw.ElapsedMilliseconds/1000.0 + " s");
		}
	}
}