Problem:
Problem B: Monday-Saturday Prime Factors

Chief Judge's log, stardate 48642.5.
We have decided to make a problem from elementary number theory.
The problem looks like finding all prime factors of a positive integer,
but it is not.

A positive integer whose remainder divided by 7 is either 1 or 6 is called
a 7N+{1,6} number.
But as it is hard to pronounce,
we shall call it a Monday-Saturday number.

For Monday-Saturday numbers a and b,
we say a is a Monday-Saturday divisor of b
if there exists a Monday-Saturday number x
such that ax = b.
It is easy to show that
for any Monday-Saturday numbers a and b,
it holds that a is
a Monday-Saturday divisor of b
if and only if
a is a divisor of b in the usual sense.

We call a Monday-Saturday number a Monday-Saturday prime
if it is greater than 1 and has no Monday-Saturday divisors
other than itself and 1.
A Monday-Saturday number which is a prime in the usual sense
is a Monday-Saturday prime
but the converse does not always hold.
For example, 27 is a Monday-Saturday prime
although it is not a prime in the usual sense.
We call a Monday-Saturday prime
which is a Monday-Saturday divisor of a Monday-Saturday number a
a Monday-Saturday prime factor of a.
For example, 27 is one of the Monday-Saturday prime factors of 216,
since 27 is a Monday-Saturday prime
and 216 = 27 × 8 holds.

Any Monday-Saturday number greater than 1
can be expressed as a product of one or more Monday-Saturday primes.
The expression is not always unique
even if differences in order are ignored.
For example,

216 = 6 × 6 × 6 = 8 × 27

holds.

Our contestants should write a program that outputs
all Monday-Saturday prime factors
of each input Monday-Saturday number.

Input

The input is a sequence of lines each of which contains a single
Monday-Saturday number.
Each Monday-Saturday number is greater than 1
and less than 300000 (three hundred thousand).
The end of the input is indicated by a line
containing a single digit 1.

Output

For each input Monday-Saturday number,
it should be printed, followed by a colon `:'
and the list of its Monday-Saturday prime factors on a single line.
Monday-Saturday prime factors should be listed in ascending order
and each should be preceded by a space.
All the Monday-Saturday prime factors should be printed only once
even if they divide the input Monday-Saturday number more than once.

Sample Input

205920
262144
262200
279936
299998
1

Output for the Sample Input

205920: 6 8 13 15 20 22 55 99
262144: 8
262200: 6 8 15 20 50 57 69 76 92 190 230 475 575 874 2185
279936: 6 8 27
299998: 299998

Solution in C# (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Contest
{
    class Program
    {
        static int Main(string[] args)
        {
            new Program().Process();

            return 0;
        }

        private CommonInput cin;

        private void Process()
        {
            cin = new CommonInput();

            int x, max;

            // ??????????´???°??§????????¨?????? true
            // ????????\?????? false
            max = 300000;
            bool[] prime = new bool[max];
            List<int> primeList = new List<int>();

            for (int i = 2; i <= max; i++)
            {
                if (i % 7 == 1 || i % 7 == 6)
                {
                    for (int j = 6; i * j < max; j++)
                    {
                        prime[i * j] = true;
                    }
                }
                else
                {
                    prime[i] = true;
                }
            }

            while (true)
            {
                x = cin.GetNextToInt();

                if (x == 1)
                    break;

                primeList.Clear();

                for (int i = 2; i < max; i++)
                {
                    if (!prime[i] && x % i == 0)
                        primeList.Add(i);
                }

                Console.Write("{0}:", x);

                for (int i = 0; i < primeList.Count; i++)
                {
                    Console.Write(" {0}", primeList[i]);
                }
                Console.WriteLine();
            }
        }
    }

    public class CommonInput
    {
        public CommonInput()
        {
            this._s = new string[0];
            this._i = 0;
        }

        public string Line { get; set; }
        private string[] _s;
        private int _i;
        private char[] _sep = { ' ' };

        public void GetLine()
        {
            this._i = 0;
            do
            {
                this.Line = Console.ReadLine();
            } while (this.Line == "");

            if (this.Line != null)
                this._s = this.Line.Split(this._sep, StringSplitOptions.RemoveEmptyEntries);
        }

        public string GetNext()
        {
            if (this._i < this._s.Length)
                return this._s[this._i++];

            this.GetLine();

            return this._s[this._i++];
        }

        public int GetNextToInt()
        {
            return int.Parse(this.GetNext());
        }

        public long GetNextToLong()
        {
            return long.Parse(this.GetNext());
        }

        public double GetNextToDouble()
        {
            return double.Parse(this.GetNext());
        }
    }
}