Five numbers , , , , are written on a blackboard. A student may erase any two of the numbers and on the board and write the numbers and replacing them. If this operation is repeatedly performed, can the numbers , , , , ever appear on the board at the same time?
Solution
No. We consider the numbers modulo .
* If , then and . The number of multiples of remains unchanged.
* If and , then and . The number of multiples of remains unchanged.
* If , then and . The number of multiples of remains unchanged.
* If and , then and . The number of multiples of is increased by , and there is a number which is congruent to modulo .
Now, note that and . If the numbers can appear on the board at the same time, there must be an instant when the number of multiples of changes from to . This must be the last case in the above list, hence the numbers are now congruent to modulo respectively. Afterwards, no matter which numbers we choose, the new numbers are still congruent to modulo respectively. Therefore, it is impossible to generate , , , , . This is a contradiction.