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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 21 |
| 4 | 137 |
| 5 | 1068 |
| 6 | 9663 |
| 7 | 99249 |
| 8 | 1137858 |
| 9 | 14373531 |
| 10 | 198031153 |
| 11 | 2951536030 |
| 12 | 47270242621 |
| 13 | 808917666365 |
| 14 | 14720125466652 |
| 15 | 283667520561633 |
| 16 | 5768057979319853 |
| 17 | 123364873473674732 |
| 18 | 2767400573883314755 |
| 19 | 64950007415991458989 |
| 20 | 1591227433994704322322 |
[1,1,4,21,137,1068,9663,99249,1137858,14373531, 198031153,2951536030,47270242621,808917666365, 14720125466652,283667520561633,5768057979319853, 123364873473674732,2767400573883314755, 64950007415991458989,1591227433994704322322] |