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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 0 |
| 3 | 9 |
| 4 | 18 |
| 5 | 60 |
| 6 | 1350 |
| 7 | 9072 |
| 8 | 65520 |
| 9 | 984960 |
| 10 | 11627280 |
| 11 | 135883440 |
| 12 | 2109317760 |
| 13 | 33214821120 |
| 14 | 529403146272 |
| 15 | 9536973415200 |
| 16 | 182108114697600 |
| 17 | 3599078480524800 |
| 18 | 76130266179974400 |
| 19 | 1701744508586747520 |
| 20 | 39652022068801632000 |
| 21 | 970411293528131750400 |
[1,0,0,9,18,60,1350,9072,65520,984960,11627280, 135883440,2109317760,33214821120,529403146272, 9536973415200,182108114697600,3599078480524800, 76130266179974400,1701744508586747520, 39652022068801632000,970411293528131750400] |