+---+---+---+----+----+----+----+
|   | 1 | 2 |  3 |  4 |  5 |  6 |  <- c
+---+---+---+----+----+----+----+
| 1 | 1 | 1 |  1 |  1 |  1 |  1 |
| 2 | 2 | 3 |  3 |  5 |  6 |  7 |
| 3 | 3 | 5 |  6 |  6 |  7 |  7 |
| 4 | 4 | 7 |  9 | 10 | 10 | 11 |
| 5 | 5 | 9 | 12 | 14 | 15 | 15 |
+---+---+---+----+----+----+----+
  ^
  |
  s

1 1 1 1 ...
2 4 6 8 ...
3 5 7 9 ...