The history of this problem will be found in The Canterbury Puzzles (No. 90). Since the publication of that book in 1907, so far as I know, nobody has succeeded in solving the case for that unlucky number of persons, 13, seated at a table on 66 occasions. A solution is possible for any number of persons, and I have recorded schedules for every number up to 25 persons inclusive and for 33. But as I know a good many mathematicians are still considering the case of 13, I will not at this stage rob them of the pleasure of solving it by showing the answer. But I will now display the solutions for all the cases up to 12 persons inclusive. Some of these solutions are now published for the first time, and they may afford useful clues to investigators.
The solution for the case of 3 persons seated on 1 occasion needs no remark.
A solution for the case of 4 persons on 3 occasions is as follows:—
| 1 2 3 4 |
| 1 3 4 2 |
| 1 4 2 3 |
Each line represents the order for a sitting, and the person represented by the last number in a line must, of course, be regarded as sitting next to the first person in the same line, when placed at the round table.
The case of 5 persons on 6 occasions may be solved as follows:—
| 1 2 3 4 5 |
| 1 2 4 5 3 |
| 1 2 5 3 4 |
| 1 3 2 5 4 |
| 1 4 2 3 5 |
| 1 5 2 4 3 |
The case for 6 persons on 10 occasions is solved thus:—
| 1 2 3 6 4 5 |
| 1 3 4 2 5 6 |
| 1 4 5 3 6 2 |
| 1 5 6 4 2 3 |
| 1 6 2 5 3 4 |
| 1 2 4 5 6 3 |
| 1 3 5 6 2 4 |
| 1 4 6 2 3 5 |
| 1 5 2 3 4 6 |
| 1 6 3 4 5 2 |
It will now no longer be necessary to give the solutions in full, for reasons that I will explain. It will be seen in the examples above that the 1 (and, in the case of 5 persons, also the 2) is repeated down the column. Such a number I call a "repeater." The other numbers descend in cyclical order. Thus, for 6 persons we get the cycle, 2, 3, 4, 5, 6, 2, and so on, in every column. So it is only necessary to give the two lines 1 2 3 6 4 5 and 1 2 4 5 6 3, and denote the cycle and repeaters, to enable any one to write out the full solution straight away. The reader may wonder why I do not start the last solution with the numbers in their natural order, 1 2 3 4 5 6. If I did so the numbers in the descending cycle would not be in their natural order, and it is more convenient to have a regular cycle than to consider the order in the first line.