| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 4 |
| 3 | 20 |
| 4 | 56 |
| 5 | 336 |
| 6 | 1056 |
| 7 | 6864 |
| 8 | 22880 |
| 9 | 155584 |
| 10 | 537472 |
| 11 | 3762304 |
| 12 | 13312768 |
| 13 | 95091200 |
| 14 | 342328320 |
| 15 | 2481880320 |
| 16 | 9051563520 |
| 17 | 66378132480 |
[1,2,4,20,56,336,1056,6864,22880,155584,537472, 3762304,13312768,95091200,342328320,2481880320, 9051563520,66378132480] |