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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 19 |
| 4 | 104 |
| 5 | 614 |
| 6 | 3816 |
| 7 | 24595 |
| 8 | 162896 |
| 9 | 1101922 |
| 10 | 7580904 |
| 11 | 52878654 |
| 12 | 373100272 |
| 13 | 2658188524 |
| 14 | 19096607120 |
| 15 | 138182654595 |
| 16 | 1006202473888 |
| 17 | 7367648586954 |
| 18 | 54214472633064 |
| 19 | 400698865376842 |
| 20 | 2973344993337520 |
| 21 | 22142778865313364 |
[1,1,4,19,104,614,3816,24595,162896,1101922, 7580904,52878654,373100272,2658188524,19096607120, 138182654595,1006202473888,7367648586954, 54214472633064,400698865376842,2973344993337520, 22142778865313364] |