4 squares theorem, C#

4 squares theorem

//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester{
    public class Program{
        public static void Main(string[] args){

int upto = 2000;
            float circles = 0;
            string[] erg = new string[upto+1];
            Stopwatch sw = new Stopwatch();
            sw.Start();
            List<int> test = new List<int>();
            for(int i = 1;i<=upto;i++)
                test.Add(i);
            
            Task[] Tasks = new Task[Environment.ProcessorCount];
            
            for(int t = 0;t<Tasks.Length;t++)
                Tasks[t] = new Task(()=> {
            while(test.Count != 0){
                
                int n,a,b,c,d,na,nb,nc,nd,tmp = 0;
                bool solved = false;
                n = test[0];
                test.Remove(n);
                
                for(a = 0 , na = n ;a < na && !solved;a++){
                    for(b = 0 , nb=na-a*a;b < nb&& !solved;b++){
                        for(c = 0 , nc = nb -b*b;c < nc&& !solved;c++) {
                            tmp++;
                            nd = nc -c*c;
                            d = (int)Math.Sqrt(nd);
                            if(d*d == nd){
                                solved = true;
                                circles += tmp;
                                erg[n-1] = n + " = " + a +"^2 + "+b+"^2 + "+c+"^2 + "+d+"^2     Circles: "+tmp ;
                            }
                        }
                    }
                }
            }});
            foreach(Task item in Tasks)
                item.Start();

Task.WaitAll(Tasks);
            
            sw.Stop();
            
            erg[erg.Length-1] = circles +" Cricles in "+sw.Elapsed+" seconds";
            
            foreach(string item in erg)
                Console.WriteLine(item);
        }
    }
}


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