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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 1 |
| 3 | 4 |
| 4 | 21 |
| 5 | 136 |
| 6 | 1045 |
| 7 | 9276 |
| 8 | 93289 |
| 9 | 1047376 |
| 10 | 12975561 |
| 11 | 175721140 |
| 12 | 2581284541 |
| 13 | 40864292184 |
| 14 | 693347907421 |
| 15 | 12548540320876 |
| 16 | 241253367679185 |
| 17 | 4909234733857696 |
| 18 | 105394372192969489 |
| 19 | 2380337795595885156 |
| 20 | 56410454014314490981 |
| 21 | 1399496554158060983080 |
[1,0,1,4,21,136,1045,9276,93289,1047376,12975561, 175721140,2581284541,40864292184,693347907421, 12548540320876,241253367679185,4909234733857696, 105394372192969489,2380337795595885156, 56410454014314490981,1399496554158060983080] |