public static ListEntry Multiplication(ListEntry pol_1, ListEntry pol_2)
        {
            if (pol_1 != null && pol_2 != null)
            {
                // перемножение членов
                ListEntry ans  = new ListEntry();
                ListEntry list = ans;
                while (pol_1 != null)
                {
                    ListEntry help_pol_2 = pol_2;
                    while (help_pol_2 != null)
                    {
                        list.Next  = new ListEntry(pol_1.Degree + help_pol_2.Degree, pol_1.A * help_pol_2.A);
                        list       = list.Next;
                        help_pol_2 = help_pol_2.Next;
                    }
                    pol_1 = pol_1.Next;
                }
                ans  = ans.Next;
                list = ans;

                // приведение подобных
                while (list != null)
                {
                    ListEntry help_list      = list.Next;
                    ListEntry help_list_last = list;
                    while (help_list != null)
                    {
                        if (help_list.Degree == list.Degree)
                        {
                            list.A += help_list.A;
                            help_list_last.Next = help_list.Next;
                            help_list           = help_list.Next;
                        }
                        else
                        {
                            help_list_last = help_list_last.Next;
                            help_list      = help_list.Next;
                        }
                    }
                    list = list.Next;
                }

                // удаление нулевых членов
                while (ans.A == 0)
                {
                    ans = ans.Next;
                }
                if (ans != null)
                {
                    list = ans.Next;
                    ListEntry last_list = ans;
                    while (list != null)
                    {
                        if (list.A == 0)
                        {
                            last_list.Next = list.Next;
                            list           = list.Next;
                        }
                        else
                        {
                            last_list = list;
                            list      = list.Next;
                        }
                    }
                }
                return(ans);
            }
            else
            {
                return(null);
            }
        }