c-sharp-is-prime.cs

       

       /// <summary>
       /// 簡單判斷是不是質數，反正在10以內，不可能太久
       /// </summary>
       /// <param name="n"></param>
       /// <returns></returns>

        static bool IsPrime(int n)
        {
            if (n < 2) return false;
            for (int i = 2; i * i <= n; i++)
                if (n % i == 0)
                    return false;
            return true;
        }

c-sharp-math-2025102.cs

        
           //計時器，非必要
           var sp = new Stopwatch();
           sp.Start();

           Parallel.For(1, 10, a =>
           {
               Parallel.For(1, 10, b =>
               {
                   Parallel.For(1, 10, c =>
                   {
                       Parallel.For(1, 10, d =>
                       {
                           // 檢查 c 是否是質數
                           if (!IsPrime(c))
                               return;

                           int left = 25870 + a;
                           int right = (int)Math.Pow(2, b) * c * d * d * 11; // 2^b * c * d² * 11

                           if (left == right)
                           {
                               lock (Console.Out)
                                   Console.WriteLine($"a={a}, b={b}, c={c}, d={d}, sum={left}");
                           }
                       });
                   });
               });
           });
           Console.WriteLine(sp.Elapsed);
           
           
           //a=2, b=4, c=3, d=7, sum=25872
           //00:00:00.0189032