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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 1 |
| 3 | 2 |
| 4 | 8 |
| 5 | 40 |
| 6 | 236 |
| 7 | 1648 |
| 8 | 13125 |
| 9 | 117794 |
| 10 | 1175224 |
| 11 | 12903874 |
| 12 | 154615096 |
| 13 | 2007498192 |
| 14 | 28075470833 |
| 15 | 420753819282 |
| 16 | 6726830163592 |
| 17 | 114278495205524 |
| 18 | 2055782983578788 |
| 19 | 39039148388975552 |
| 20 | 780412763620655061 |
| 21 | 16381683795665956242 |
| 22 | 360258256118419518680 |
| 23 | 8283042472303599966974 |
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