Today we were messing around with a further restriction: anti-knight where givens may only be placed on the edges of the grid. The best I was able to come up with was 10c (one example below, I found a number of them), and we believe this is minimal but haven't yet proven it conclusively. I found a 9c with 2 solutions, but only 7 distinct digits, and the best we've done with 8 distinct digits is 18 solutions. [edit]Now proven by brute force search; 18 solutions is the best with 9c 8 distinct.[/edit]
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+-------+-------+-------+
| . . 1 | 2 3 . | . . . |
| . . . | . . . | . . 4 |
| . . . | . . . | . . 5 |
+-------+-------+-------+
| . . . | . . . | . . 6 |
| . . . | . . . | . . . |
| . . . | . . . | . . . |
+-------+-------+-------+
| 5 . . | . . . | . . . |
| 7 . . | . . . | . . . |
| . . . | . . . | 1 8 . |
+-------+-------+-------+
..123............4........5........6..................5........7..............18. anti-knight