There are men sitting around a circular table with equal distances and cards with numbers on them are dealt among them. It is possible that one has no cards and the other has more than one. In each step one can give one of his cards to his adjacent individual if the card number has the following property: before and after this stage, the places of the cards with numbers , and are not the vertices of an acute-angled triangle. (card is the same as card , and card is the same as card ) At the beginning the cards to are dealt among them counter-clockwise. (everyone has exactly one card) Prove that there will never be a man who has all the cards.
Solution
First divide the table into equal arcs. Now if the cards be on the points on the table, we define the distance between these two cards as the number of arcs between on the table (the smaller one). For example, the distance between is in the following figure:

Now, after each step we sum the distances between every two cards with consecutive numbers. Easily it can be proved that after each step this sum either remains invariant or varies by . Because if the place of card is between and , this sum doesn't change, otherwise it changes by . So the parity of this sum remains invariant. At first it is odd (), and if they are all in one place this sum would be even, so it is impossible.
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