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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 3 |
| 2 | 20 |
| 3 | 148 |
| 4 | 1104 |
| 5 | 8240 |
| 6 | 61504 |
| 7 | 459072 |
| 8 | 3426560 |
| 9 | 25576192 |
| 10 | 190903296 |
| 11 | 1424921600 |
| 12 | 10635759616 |
| 13 | 79386390528 |
| 14 | 592548085760 |
| 15 | 4422839123968 |
| 16 | 33012520648704 |
| 17 | 246408808693760 |
| 18 | 1839220386955264 |
| 19 | 13728127860867072 |
[1,3,20,148,1104,8240,61504,459072,3426560, 25576192,190903296,1424921600,10635759616, 79386390528,592548085760,4422839123968, 33012520648704,246408808693760,1839220386955264, 13728127860867072] |