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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 7 |
| 3 | 48 |
| 4 | 327 |
| 5 | 2221 |
| 6 | 15060 |
| 7 | 102012 |
| 8 | 690519 |
| 9 | 4671819 |
| 10 | 31596447 |
| 11 | 213633696 |
| 12 | 1444131108 |
| 13 | 9760401756 |
| 14 | 65957919496 |
| 15 | 445671648228 |
| 16 | 3011064814455 |
| 17 | 20341769686311 |
| 18 | 137412453018933 |
| 19 | 928188965638464 |
| 20 | 6269358748632207 |
| 21 | 42343731580741821 |
[0,1,7,48,327,2221,15060,102012,690519,4671819, 31596447,213633696,1444131108,9760401756, 65957919496,445671648228,3011064814455, 20341769686311,137412453018933,928188965638464, 6269358748632207,42343731580741821] |