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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 100 |
| 5 | 587 |
| 6 | 3660 |
| 7 | 23640 |
| 8 | 157076 |
| 9 | 1066281 |
| 10 | 7363620 |
| 11 | 51568732 |
| 12 | 365369868 |
| 13 | 2614235293 |
| 14 | 18862816112 |
| 15 | 137096744232 |
| 16 | 1002785827620 |
| 17 | 7376023180645 |
| 18 | 54525165453672 |
| 19 | 404858512190316 |
| 20 | 3018190533410664 |
| 21 | 22581907465905018 |
[1,1,4,18,100,587,3660,23640,157076,1066281, 7363620,51568732,365369868,2614235293,18862816112, 137096744232,1002785827620,7376023180645, 54525165453672,404858512190316,3018190533410664, 22581907465905018] |